How Far Will It Transmit ?
What determines the transmission range of surveillance and communications equipment, why you may not want longer range, and how to improve range if you do.
To the 33 officers on our mailing list in Oklahoma City, some of whom certainly lost friends in the bombing, we share your grief. We never know when our time will come. We should be ready always, both in our affairs and in our spiritual position.
It was nice to meet new friends and see several old ones at the Secret Service Combat Pistol Championship last week. Thanks for stopping by and saying hello. I shot OK, although things have been going downhill steadily since I was half of the first place two man team in 1980. The laurels are drying up and starting to crumble after fifteen years . . . This year things did not seem as busy as in previous years although the squadding seemed as full as ever. This may have been due to the NRA being excommunicated from the shoot and the NRA asking a lot of vendors to sympathetically boycott the show. I wonder how this will affect future tournaments? Who will maintain classification records? Will other tournament sponsors follow suit, or was last week just a knee jerk reaction to send a message? Nobody from the NRA was evident to give an official position.
A bit sobering was the auction to benefit the families of the Secret Service agents who lost their lives in the Oklahoma City bombing. It was nice to see the troops rally 'round and take care of our own.
OK on to the matter at hand. Most of us in police work use radio transmitters of various sorts. Everyone uses two way radios, and many use surveillance transmitters. Obviously we want this equipment to work as well as it is capable of working, because in some cases our lives may depend on our radios.
The question we hear the most regarding the communications and surveillance equipment we sell is, "How far will it transmit?". There is no pat answer to this question. Because there are a lot of misconceptions and a certain degree of mystique surrounding radio equipment, this article will discuss factors that contribute to range. So the next time you ask (or are asked) this question, you will understand more about what goes into the answer.
We will break our discussion down to cover two areas: traditional two way radio communications, and surveillance work. They have two different sets of operational requirements although the technology affecting the two is very similar. There are both common and unique factors in two way communications being used for traditional officer to dispatcher messages and two way radios used in a surveillance scenario. It is important to understand the differences between them.
With traditional two way communications between an officer and the dispatcher, in most cases we do want the maximum range possible. The only times I can think of where this would not be the case is on low band (30 50 megacycles) and to a lesser extent on VHF systems. With these lower frequencies there are times when an atmospheric phenomenon known as "ducting" (frequently erroneously called "skip") results in signals from distant areas being received strongly in your area. This can be disconcerting as many times you will be picking up another police agency using similar calls and procedure to your own. Ducting is most common during the summer evenings, and affects lower frequencies far more than higher. A good indicator of conditions being favorable for ducting is when you get crummy television reception on lower channels.
In God's country where my home is (between Baltimore and Philly, closer to Baltimore), I can barely get the Baltimore stations even when things are normal. When ducting is occurring, the Philly stations come in equally as strong (or weak) as the Baltimore stations I am trying to get, and as a result neither is watchable. The Philly stations are stronger because of ducting, and their signal strengths are as string or stronger than the Baltimore stations being received via the normal "ground wave" path.
When ducting is happening, stations hundreds of miles away can be as strong as your local stuff. The problem has been severe enough to cause many agencies to move their radio systems up to UHF or 800 megacycles where the phenomenon is much less likely to occur. On CB frequencies (27 megacycles), ducting is a very common occurrence, to the delight of the good buddies infesting those channels. A $39 CB radio easily can communicate with another one halfway across the country. The range usually is limited by the crowded channels rather than atmospherics.
Because of ducting, potent fixed station or mobile transmitters on the affected frequencies may have their coverage deliberately limited as a courtesy to the other jurisdictions sharing the same frequencies. Even handhelds, though, can carry hundreds of miles if conditions are right.
In some very populated areas there are just not enough frequencies available to give every user their own channel. Therefore the same channel also may be used by groups not really far enough away to prevent interference even during normal times. In these situations, proper system engineering may deliberately restrict coverage to keep peace on the airwaves. Co-channel problems tend to affect business users more than law enforcement as there are many more commercial users than law enforcement agencies vying for the limited number of frequencies available.
The two situations mentioned above are the only two I can think of offhand where coverage would deliberately be restricted. I am sure there are others.
The most important things affecting transmit range is the "path" This is the distance between the transmit and receive antennas, and everything in between that determines how well the signal can get from here to there unimpeded. One must specify path conditions to give an accurate answer to "how far will it transmit?". Because path conditions are extremely variable, the industry must pick a standard to use as a reference. This standard is called "Line of Sight", or "LOS", which means the transmit and receive antennas are looking at each other, with nothing in between other than air. No buildings, trees, hilltops, no lids, kids or space cadets, or planes flying overhead causing reflections.
A line of sight path rarely exists in the real world. Almost anything can work well line of sight. Home satellite dishes get decent signals (more or less) from satellites approximately 22,000 miles in space. As satellites are powered by energy gathered from solar cells, their transmitters are relatively pipsqueek. The transmitters used on the moon's surface for communications back to earth (approximately a quarter of a million miles) were only a few watts. The lunar transmitters may have been repeated through the lunar module or the orbiting capsule I do not know. But in any case they were low powered signals. They got back to earth largely because the path was line of sight, and to a lesser extent because the very high frequencies (high frequencies mean small antennas) allowed for efficient high gain antennas at both ends. But if even one building, or perhaps even a tree in full bloom, had been blocking a receive antenna, I doubt that communication would have been possible.
So line of sight is not really an accurate indicator. As an example we discussed recently, wireless video transmitters are advertised with a line of sight (LOS) range of several miles. (This may or may not be true). But try to go from inside a building to a parking lot a hundred feet away and you likely will be out of luck. The path loss from the building structures is high enough to attenuate the signal to unusable levels.
Almost anything in the way will increase path loss. Generally, the denser the material the more loss. Glass, wood and sheetrock are not too bad. Metal is terrible. If you are in a wood building the range of your portable radio may not be affected much. But if you are trying to use your radio from inside a vehicle, where the antenna is completely surrounded by metal, you may not be able to communicate as far as you can holler.
Keep in mind that the signal is transmitted from (or received by) the antenna, not the radio itself. On a handheld radio the antenna is a part of the radio. On a mobile radio the antenna is outside the vehicle. In a base radio, the radio room may be in the basement, but if the antenna is on the roof, coverage is calculated from the roof, not the physical location of the radio. This seems obvious, but I've seen guys try tests not understanding this.
You can improve transmission by doing anything you can to decrease path loss. In realistic terms, this means try to get the antenna as much in the clear as possible. If you know where the other end is in relation to where you are talking from, you may try to get near a window so you're not punching through the building. If you're in a car (or, especially, a van), hold your portable radio near a window. If you have a speaker mike on your portable and you're in a weak signal area, you can stick the radio out the window as high as your arm will reach and use it that way. (This sometimes is called the "Armstrong" method).
If you're driving in a vehicle, and you don't have an urgent transmission, wait a few seconds until you crest the next hill. My communications habits were developed back in the days when mobile signals were weak (before repeaters were common), and I still instinctively pause when it's my turn to transmit until I pop over the next hill, get out from underneath a bridge or away from tall buildings.
Antennas for mobile radios ideally should be mounted as high on the vehicle as possible, and in the center of the vehicle as a secondary consideration. If your antenna is mounted other than in the center of mass, your transmission and reception will be strongest in the direction of most metal. This means if your antenna is mounted on the left rear fender, your strongest signal will be off towards the right front of the car. If you're stationary in a weak signal area, you might try turning your car to get the best signal. Especially do not mount your antenna directly behind your light bar or something else that may shade the signal.
Cellular phone antennas mounted on the trunk lid or rear fender frequently are the "elevated feed" type. This means the antenna uses a small mast to raise the antenna above the level of the car's cab, so the antenna has a clear field of view in all directions.
It is important, especially with high gain antennas, to consider the signal pattern of the antennas used at each end. You may recall, from our articles in 1992 on antennas, that unity (no gain) antennas have a pattern like a fat donut skewered on the antenna. Signals are radiated in all directions around the antenna, plus a significant amount of signal up into the air (called a "high angle of radiation"). For many two way and cellular applications, mobile antennas with this pattern work well, especially in cities where repeater antennas tend to be up high on top of buildings or towers where God intended them to be.
For longer haul rural communications, a "gain" antenna may a better choice. Gain antennas work by squashing the fat donut pattern down into something more resembling a dinner plate. This results in more of your precious signal being radiated at a low angle, more towards the horizon (and less up into the air where it may not be doing you much good). If you are going for the long haul, a gain antenna is an inexpensive item to try. Most gain antennas for mobile radios cost under $50 for a good one, under $100 for the best. For VHF, a 3 dB gain antenna replacing a unity (no gain, or 0 dB gain) antenna at both ends will double your range, all other factors remaining the same. Gain added anywhere in the system all is totalled together, so a 3 dB gain antenna on the transmitter and the same 3 dB at the remote receiver offers a total system gain of 6 dB. By the way, dB is the industry abbreviation for deciBels, one tenth of a Bel. In an oversimplified explanation, decibels are a relative measure of signal levels. More dBs is more signal. A 6 dB "gain" in a system will give you twice the transmit range, again with all other factors (path loss, etc.) being equal. Therefore replacing a unity gain antenna at each end with 3 dB gain antennas will give you an overall system gain of 6 dB. Adding only 3 dB by replacing just the antenna on your car will give you a 40% increase. As a reference, a no gain ("unity" gain) antenna also is called a 1/4 wave.
Keep in mind that going to higher gain antennas does not give you something for nothing. All antenna gain is at the expense of pattern. For omnidirectional gain antennas, the pattern above and below the plane of the antenna is sacrificed in return for a stronger signal radiated horizontally. If you are in a big city with repeater or base antennas up high, you may want a lower gain antenna with more signal going up into the heavens. But for many other applications a gain antenna may be a very inexpensive method of getting better performance out of your system. To know if you can improve your mobile antenna, you first need to know what band you are using, and if you already have a gain antenna.
Antenna size increases as gain increases, and there are practical maximums. And remember that antennas get smaller as frequency increases. For low band (30 50 megacycles), these low frequencies mean the antennas are large to start with, and gain antennas are not practical for mobile use. In fact, many low band antennas actually are electrically shortened, resulting in an effective negative gain. If your low band antenna is a whip something like 8 feet long with a big spring at the bottom, it is full length with unity (0 dB) gain. If it is a few feet long with a cylindrical coil at the base like a 4 inch long, 1 inch diameter fiberglass covered swelling at the bottom, it is a "coil loaded" antenna and has less than unity gain. Most low band antennas anymore are coil loaded for practicality. There is not much you can do to improve things here. Not a whole lot of agencies still use low band. It is a good place to be for agencies who need wide area coverage, like state police or utility companies. Low band radios tend to be powerful, with 100 watt mobiles common. But the band is capable of providing reliable long range communications. Repeaters rarely are used on low band. Ducting is common here.
Unity VHF mobile antennas are a single wire about 16 or so inches long (a quarter wave). There is a gain VHF antenna available, called a "5/8 wave". It is about 4 feet long with a small coil on the bottom, and offers 3 dB gain. Most vehicles can handle a 4 foot long antenna. There is a 6 dB VHF antenna available also, called a "collinear", but it is about 7 feet long and not practical for most vehicles. This is unfortunate, as the 6 dB gain from a collinear doubles your range over a simple 1/4 wave. There are some collinears out there, which you can identify by a long slim coil about 1/3 of the way down from the top, and a very heavy whip to support the antenna against the wind.
When you get into UHF, the smaller antennas at the higher frequencies mean you really can do some stuff here. A quarter wave (unity gain) antenna on UHF is about 6 inches long. You will find these on vehicles in big cities where the repeaters are on top of buildings and it is desirable to have signal squirting up high. But outside of the cities you may well want to go to a high gain antenna. A 5 dB gain collinear antenna at UHF is less than 3 feet long, and can be identified by a coil at the base about the size of a golf ball with a flat top and bottom, and another plastic covered coil about halfway up. This 5 dB will nearly double your range over a quarter wave.
On 800 and 900 megacycles (including trunking systems), a 1/4 wave whip is maybe 3 inches long. A collinear gain antenna looks like a cellular antenna, and in fact most 800 two way and cellular antennas are interchangeable (cellular in this country is on 800 megacycles).
With the above information, you can determine what type of antenna you have now and decide if a gain antenna is a good choice to improve your communications. Because many radio systems are bought low bid, it is very common to see an expensive mobile radio coupled to a cheap unity gain antenna. Many times antennas are added as an afterthought, and the cheapest one out of a catalog on the right band is provided. The key to good communications is all in the antennas. A good antenna on a low end radio will give much better results than a crummy antenna stuck on an expensive radio. Too many people get hung up on the radio and ignore the antenna. Better antennas are not expensive and in fact are the first place to look if things are not working as well as you wished. In a large fleet, a savings of $30 per antenna multiplied by hundreds of cars many times is all the profit in a cutthroat contract, so they frequently are compromised. And antennas tend to be a mystery to many less experienced two way providers. Guys who don't understand antennas look at the price column above any other spec.
Antennas of different qualities make a difference, even amongst different brands who claim similar gain. For example, one low end company, MAXRAD, offers an inexpensive line of antennas claiming impressive gain figures. A high end company, DB Products, offers a similar looking and similar specification antenna for over twice the price. Even though the specs are the same, the DB Products antenna works noticeably better than a MAXRAD. Considering how important antennas are, and a good antenna will easily last 5 to 10 years, an extra fifty bucks for the best is money well spent. Antenna Specialists and Larsen are two brands of good middle of the road antennas.
If it is you installing the antenna, remember they need to be tuned to the exact frequency you will be using. If you don't have access to a BIRD Model 43 wattmeter with the appropriate slugs, you can get pretty close using the antenna cutting charts that come with each antenna. Antennas deliberately are manufactured longer than necessary so the installer can trim it to the proper length for any frequency needed. You must trim the antennas or you will suffer significant losses in performance. Approximately half of the police radio systems we take over for maintenance have antennas that were not trimmed to length. It takes about 60 seconds to do that; I guess the original installers were too busy to fit it in.
Next issue we will go into a good bit more on path loss, best frequencies to use for different applications, how transmitter power affects range, transmitter and receiver quality, and quite a bit on surveillance gear. Don't miss it. Please let me know if there are any specific things you would like to see covered.
A while back in one of the wireless video articles, I oversimplified a technical matter relating to the bandwidth of a transmitted signal. While the information I did give was absolutely correct, I did not back it up with a lot of supporting math and engineering theory, which really is of little interest to most Police and Security News readers. One reader who publishes his own spy and countersurveillance newsletter took me to task on my explanation. While he is technically capable of understanding the math behind the bandwidth issue, he did not bother to ask me directly and instead criticized the Police and Security News article in his own publication. I supplied him with the math and theory to back up my statements, but he did not feel it appropriate to publish my supporting information in his newsletter.
This gentleman also took me to task for my position on law enforcement agencies dealing with spy shops. Well, guys, I was just discussing matters that you, the readers of Police and Security News, real cops doing real jobs in the real world, asked me to cover. Therefore the criticism from hobbyists and spy groupies is not against me, it's against you. I got a good number of calls and faxes (probably about 30) from cops basically thanking me for finally speaking their thoughts on the issue of spy shops. And the raid by U.S. Customs on 41 spy shops in 27 cities on 5 April (with inventory and customer lists seized, and proprietors arrested) shows that the Justice Department apparently agreed.
Every column I write for this magazine is reviewed for both accuracy and content by two others, one a degreed and experienced communications engineer, the other a cooperative federal technical surveillance type from a letter agency. I make every effort to relate accurate technical information, but not bury it deep in a lot of dry theory and math. At times I am guilty of oversimplification. I am not perfect, and welcome any corrections or clarifications on anything I print. If you have anything to say about what I share, please be mature enough to fax it to me directly and I will take appropriate action including researching the material or contacting other experts, and publishing corrections if warranted.
Some people have an excess of time on their hands and like to pick apart work done by others. As we like to print something for everybody, occasionally I will say something controversial to keep these people happy, and give them something to snipe about so they have material to use in their own newsletters. If you make NATIA in July, please look us up in booth 12. Betty, Ozzie, Randy and I should be there most of the week although I will be teaching two sessions on wireless video Tuesday and Wednesday mornings. Hope to meet some of you there.
NATIA (NAtional Technical Investigator's Ass'n) was great, and a super job for a volunteer effort. There were some new products introduced that may be the beginning of a new generation of electronic surveillance. Semco had their 2.4 gig video transmitters both in body worn and standalone configurations. This is military quality stuff, well designed and performs adequately for its intended purpose. PR Communications had their cellular intercept systems working, and while I have been patently unimpressed with every single other brand of cell intercept I've seen, the PR stuff definitely got my attention. If you've been burned by the kids playing around with their science fair toys, you will be interested in the PR cellular intercept equipment. What a difference it makes to use equipment designed by someone who really understands both the technical aspects of the job as well as law enforcement operations. If I had a need for cellular intercept, I'd look no further than PR Communications. Actually, as I understand it, these systems are used a lot more for locating a particular cellular phone than they are for actually monitoring the content of a call.
Textron was demonstrating their impressive new digital spread spectrum audio body wires. On top of basically zero probability of intercept, the transmit range and, especially, audio quality of their equipment was fantastic. They use a diversity receiver (I'll explain later), and their recovered (transmitted) audio is as clean as that from a hardwired copper pair. Audio is available from the receiver in digital form for direct recording on DAT (Digital Audio Tape), and any subsequent post processing can be done directly in the digital domain. The equipment not only was space flight quality, it was smaller than virtually any analog body wire I've seen, especially for the range. Price was not out of line. It looks like digital surveillance audio finally has arrived.
Although I didn't see it at NATIA (I saw it in a video magazine), I understand NAGRA has introduced an all solid state (no tape or other magnetic media) surveillance video recorder. From the photo it was not all that tiny (about the size of a cigar box but twice the height), but keep in mind this is essentially first generation equipment. The major advantage, as I see it, of digital video recording is that you will have no generation or conversion loss when you process the video. And as most post processing anymore is done digitally, if your source starts as digital you will be able to do a much better job than if you have to convert back and forth from digital to analog.
Contact information for the above is: SEMCO
I do not have contact info for NAGRA, but if you have a legitimate need for their product you probably already know how to find them.
When you contact these firms, it doesn't really matter whether you mention my name or not as I have no relationship with any of these places and they may not even remember my name. It might be nice to mention you read about them in Police & Security News as a subsequent result of their exposure at NATIA, so they can track their marketing efforts.
These firms cater to law enforcement only, so don't waste their or your time unless you draw a government paycheck and can demonstrate a need to know about their products. I am advised dealer inquiries are not solicited.
NATIA will be in Orlando next August. Call me if you are a law officer interested in attending and I will pass their contact info along. Vendors are present by invitation only. If you manufacture something of interest to the group you undoubtedly will be contacted.
To briefly reopen a topic I covered a while back, this year I taught several sessions on wireless video which have received very nice evaluations. Thanks. I had a chance to inquire of several hundred attendees about the wireless video products their agencies were using. There was a smattering of HDS gear (HDS 703-620-6200) which most users seemed more or less happy with (especially the older HDS equipment apparently, when the former regime was in place). Other than HDS, no one brand of professional wireless video equipment was predominant. But get this: by far the largest number of agencies were using the same type of ham radio (hobby) equipment I've been grousing about in these articles. And not one person in any of the classes was satisfied with the performance. Not one was getting the range or image quality they had been promised. Remember, guys, this was at an association of technical investigators. These guys know what they are doing. If they can't make it work you probably can't either. I hope a lot of these guys process the legal paperwork to get a refund of what they paid for this junk. After the classes a number of attendees looked me up. If as many of them submit complaints to the FCC as they claimed (after what they learned in the class), the FCC will have to staff up to the level of the IRS. Or a welfare agency.
Earlier I mentioned "diversity" receivers. As several of you have asked about this, let's take a moment. You probably have noticed as you use radio transmitting equipment (whether two way or surveillance), that you will find periodic dead spots. Moving either the transmitter or the receiver's antenna a few inches many times will bring your signal strength up from barely readable to perfectly usable. For various technical reasons, there will be dead spots where the signal does not make its way between the antennas as well as we would like. Remember, in many cases there is not a direct (line of sight) path between the antennas. Much radio work depends on reflections and refractions, most of which are variable factors.
But where a signal is weak here, it might well be stronger over there. So a diversity receiver uses two antennas and two receivers. The theory is when one antenna is in a weak signal area, the other will be in a stronger area, and vice versa. Special circuitry compares the outputs of the two receivers and selects the stronger of the two. The end result is a more stable, better overall signal from the system.
The antenna types and placement are critical, as are the receivers and diversity circuitry. Although diversity can be used on many types of communications links, it is most popular, proven and easiest to implement in an FM link. Analog signals such as voice or video do not suffer as badly from dropouts or fading as do digital, because our ear or eye can "fill in" the missing hunks. But when you are transmitting data, a momentary dropout may render the signal totally unusable. Yeah, error correction techniques may fill in a certain amount of missing data, but there is a limit. Diversity reception is an effective technique to get the most from a radio link.
A company whose name I don't recall has been advertising in the commercial two way magazines an add on diversity controller for radio data links. Apparently this can be retrofitted to existing comm systems to improve their performance. So it looks like the technology is becoming more popular and accessible. Diversity used to be almost black magic technology. It still is not for beginners if you see a receiver using diversity, it generally is a sign of a high end product. Swintek and possibly others makes a diversity analog body wire system (Swintek 800-7-SWINTEK or 408-727-4889). I imagine Swintek's receiver could be used with any other manufacturer's VHF transmitter. Might be a way to upgrade your older system without buying a new transmitter.
Now back to the topic How far will it transmit? Last issue we discussed a number of issues relating to this topic. We covered a bit about the transmission "path", what it is, how to deal with it, and the nearly fictitious "line of sight" path used a reference in advertising claims for transmit range. We also learned that it is much easier, cheaper, and usually more satisfactory to get better performance by improving antennas than by increasing transmitter power. We covered what gain and decibels are (in reference to antenna and transmitter performance), and that gain basically is a "measure of goodness", as one author puts it.
One point a reader brought out that I should have mentioned: antenna gain in the catalogs often is given in decibels, or dB (small d, big B). A decibel is a tenth of a Bel. But dB's in themselves are meaningless. A decibel is a ratio, or a reference. You basically can think, "dBs louder than what?" With antennas, gain usually is referenced to a dipole. When this is the case, the gain figure will be listed as dBd (dB referenced to a dipole). And this is the honest way. Several manufacturers, though, in an attempt to mislead and inflate gain figures, will reference their antenna to an "isotropic" source. If this is the case, the expression "dBi" may be used, or merely dB without specifying the reference. Without getting too far off on a tangent, an isotropic source is a theoretical source, and references to them basically are meaningless. There is about a 2 dB difference between an isotropic antenna and a real world dipole. So, if a reference is made to dBi, subtract 2 dB to get the real dBd (industry standard) figure. If just the term "dB" is given, you should ask, "dB in reference to what? Do you mean X dBd or X dBi?" (if it's the more honest figure of dBd, you can bet the manufacturer will so specify.) Flipping through a magazine just now, I see a significant number of ads listing antenna gain in dBi. This is not honest, and now that you understand you won't be fooled. Your best bet is to stick with a reputable company and trust them. Be suspicious of antenna claims (or claims made for any electronics) that seem far out of keeping with the rest of the industry. It is unlikely there has been a breakthrough. More likely somebody has found a way to mess with the specs. Remember the stereos in the 1970's with hundreds or even thousands of watts of power listed? Copywriters learned what Instantaneous Peak Power was, and that it was about 4 times RMS (or average) power. Divide the claimed power ratings by four and you'd have a more accurate figure.
Choosing and installing even an honest high gain antenna does not automatically mean you will realize a lot more range. On paper (or from a salesman) the numbers may say so, but there are practical considerations some of the junior operators don't understand. The radiation pattern of the antenna is a major factor in how it will work in your system.
Remember, antennas don't make more power just because they have gain. Gain antennas give different performance because they take the power from the transmitter and concentrate it in one direction instead of just letting it squirt off in all directions, some of which may or may not be usable.
The higher the gain of a particular antenna, the more concentrated its beamwidth. On a very high gain directional antenna (typically a beam, or Yagi in surveillance operations, looks like a TV antenna on your roof except all elements are about the same length. The staggered length elements on your home TV antenna are different lengths for the different frequencies of the many television channels), the beamwidth may be only a few degrees. This means you may have an antenna pattern as sharp as a pencil. If you have a direct line of sight path from one antenna to another, and if concealment is not a problem, these high gain antennas may well increase your range over smaller antennas by several times. But read on. Beams have a vertical as well as a horizontal pattern. This means you have to point the antennas at each other in both planes. A slight amount of vertical misalignment will shoot the pattern over or under the target. A slight amount of horizontal misalignment will miss the target left or right. The further away you are the worse the problem is. If you cannot see the other end, it is extremely difficult to point a sharp antenna accurately.
For purposes of this discussion, we will assume you are operating in a surveillance scenario where the transmitter is concealed with a small (low or even negative gain) antenna. We also will assume you are operating the listening post where you are allowed to have a large antenna (beam or high gain vertical) mounted on a pole or some such. Whether these are real world scenarios are immaterial; the concepts are the same.
Another factor is that the radiation may leave the antenna at an angle. Professional antennas often are designed with a degree of "downtilt". If intended for use on a high tower, downtilt may be necessary to cover the area of interest. The downtilt keeps the signal from shooting over the area to be covered.
Another factor: as gain rises, it is more and more difficult to predict the actual radiation pattern of the antenna. A lot of physical separation from the mounting hardware and other objects is absolutely necessary. The orientation of the feedline also is a factor. Using a metal pole or being anywhere near metal objects will confuse the pattern issue even more.
Yeah, you can use PVC pipe or 2 x 4 wood to mount your antenna. But a high gain antenna is large. And it will wiggle in the wind. Without a very secure mounting (impractical in a surveillance or temporary installation), that antenna, its pattern and resulting performance will be all over the place. The swaying and misalignment easily can cause a 15 - 20 dB variation (a loss of 90% - 95%) in signal strengths. Much or most of the useful radiated energy may be lost in unwanted directions. Any gain due to the fancier antenna is completely lost.
This means your elaborate, high gain antenna quite possibly will perform worse than a simpler, lower gain antenna. Remember all the above the next time you read an ad for a 90 mile range video transmitter, and the fine print says you need high gain beams at each end to achieve this! The same comments hold true for the 5 mile range video body wire. See previous articles on wireless video.
If you ever get a close up view of a high gain omnidirectional commercial antenna, you will see it enclosed in fiberglass or some similar rugged structure, and mounted on a very sturdy mast or tower. This is not so it can survive earthquakes, it is so the antenna doesn't sway in the wind. Very likely the antenna radiation pattern, together with the design and effects of the mounting arrangement, all have been carefully plotted to give the desired results.
A Yagi antenna consists of a "driven" element, usually one "reflector" element, and a number of "director" elements. The driven element almost always is at the rear of the antenna. Remember this I've seen antennas pointed backwards on many an occasion. If you do point the wrong end at the target you will see a certain amount of performance, but of course nowhere near if it was pointed properly. As a reference, a driven element and a reflector (the minimum elements necessary for a beam) will give you about 3.5 dBd gain. Each director will add 1.35 dBd for the first few elements. Once you get beyond 3 or 4 directors, though, mutual coupling comes into play and extra gain is significantly less as more elements are added. This information is provided so you can examine an unknown antenna (or an antenna in a catalog) and determine approximately what its gain figure should be.
An important but often overlooked advantage to using directional antennas is that they can greatly reduce bad effects from multipath. Multipath, like the name indicates, is the signal from the transmitter's antenna to the receive antenna traveling over many paths. These different paths are due to reflections. The problem is, some of these reflections arrive at a slightly different time ("out of phase") with the main signal, and partially cancel it. If either the transmitter or receiver is moving, multipath tends to average out. A directional antenna looks much more strongly in one direction compared to an omnidirectional antenna which is looking everywhere. This means multipath-induced fading will be less, since the directional antenna is concentrating on the main path and trying to ignore signals coming in from the sides, top and back.
Directional antennas also can be used to favor certain areas and null out signals you don't want to hear.
We're about out of space for this issue. Next time we'll get a little bit more into path loss, an easy way to calculate transmitter range based on antenna height, transmit power versus range, transmitter and receiver quality, and more into surveillance gear. As always, please let me know any particular topics you would like to see covered, either in this series or in future articles. I also appreciate hearing about things we did cover that aren't quite clear. After part one of this article, I got calls or faxes from 4 countries and 12 states (and I was out of town most of the month).
Another electronics newsletter writer dropped me a line the other day. He mentioned the bulk of his writing effort is spent in trying to simplify his information so it can be understood without an engineering or math degree. I can appreciate this. There are a lot of techies out there who can write, but communication is explaining a complex issue in terms an average educated man can understand. Various computer programs are available which analyze your document and give various statistics, including the required education level for the reader to understand it. I use Grammatik in WordPerfect 6.02b (DOS), and it indicates this one and most of my articles are written to the 13th grade level. As these articles include a number of technical terms and police industry jargon which will affect Grammatik's ability to analyze the file, I figure we are just about where we ought to be. Comments? If I am either too technical or seem to talk down, please let me know. The object is to share information, not to prove to each other how smart we are or how many buzzwords we can use.
Any Blue Knights out there?
We're just wrapping up the insanity from the end of the federal government's fiscal year. Same thing every year a rush of last minute spending of whatever end-of-budget-year money happens to be left in the coffers. I suspect (actually I know) that much of the end of year spending is for stuff not really needed, but anything is better than turning the money back in. Hey, when I was a fed back in ought-six I had the same situation. Let me throw a suggestion out on the table: How about, instead of a lot of last minute panic buying, then scraping for what you really need the following year, try this. Issue a vendor an open ended purchase order merely for generic electronics, or some such, to cover the full fiscal year following. Then just call the vendor when you need something and have him charge it against the P.O. As a term of the P.O. the vendor could certify he would hold prices to a certain level or something. With this method the excess funds technically could be "spent" by year end, but the client agency could actually get what they really need when they need it. As the manufacturers come out with new and better stuff, you could buy it from the funds "spent" from the previous fiscal year.
I know this arrangement is similar in some ways to GSA contracts, or to BOA's or BPA's, but those don't commit funds until equipment is needed, which keeps you in the whole budget thing again. With my idea, you "spend" the money ahead of time when you have it, but you don't actually commit to particular equipment until you need it.
I've been tempted for years to cut a deal with my best customers. Spend all your left over end of year money and buy all the antique, trade in and excess crap we have littering the warehouse. Then, during the following year when you need some actual stuff, return the antique crap for "warranty repair", at which time we will replace it with a shiny new video transmitter, intercept system, ICOM radio or whatever you need. Under "warranty". This would satisfy the auditors, not be dishonest, and would get youse guys on the street the equipment you really need, instead of whatever flotsam we happen to have laying around to sell you when you call on September 29th, and needing it shipped and invoiced by the 30th . . .
Comments on any of the above?
I made a mistake in the last issue. The manufacturer of the digital body wire system I was so impressed with is TEKTRON, not Textron as I put it. Sorry. Tektron is available at 410-850-4200. Ask for Bill Meyn or Bill Heineman. The TIMES regrets the error.
It's a small world (after all). Recently I learned that my dad and Dave Yaw's dad (Dave is the publisher of this esteemed publication) have been friends for some years. They get together periodically but for some reason neither mentioned what their respective sons did for a living. One day they did, and both dads learned to their surprise that their sons knock heads on this magazine. What a coincidence.
Industry News ICOM has announced they are discontinuing the following two way radios effective immediately: U16, H16, U8, H8, U28 and H28 portables, U400 and V100 mobiles. There still is some inventory of these items left in the warehouse, but when they're gone, that's it. The older portables have been replaced by the F10/20 and F30/40/LT, as reviewed in these pages back in May. The mobiles will be replaced by new units compatible with the F30/40/LT portables, but not yet ready for distribution. Also the venerable R7000 receiver is history. It has been replaced by the R7100. If you are a professional and active ICOM end user and would like to receive a monthly ICOM UPDATE, drop me a line and request to be put on the mailing list. No calls, please. Please indicate your law enforcement agency affiliation and include a work address and phone. Normally I wouldn't mention such things here, but ICOM two way radios and receivers are extremely popular law enforcement products and I'm sure you appreciate a heads up when your favorite radios will be disappearing shortly. When did you last see a surveillance van that did not have an ICOM R7000 or R100 nestled somewhere?
Are our cultural values changing? Yesterday the pope visited Baltimore, and he was greeted by a Boys II Men concert held in his honor. I would have thought something like Vivaldi would have been more appropriate, but that's Baltimore for you. Then again, if it were up to me I would have picked up the pope at the airport in a '64 Mustang instead of the limo. Or maybe on my hog.
In part 1 and part 2 of this series of articles we discussed various factors affecting the transmit range of two way and surveillance transmitters. There was a lot of valuable information in those two articles, so if you're a new reader and interested in the subject please contact the editor of this magazine, Al Menear, at 215-538-1240 for back issues. I no longer work for the Department of Redundancy Department, so I will not repeat the information covered in the previous articles.
The information we learn here is based largely on input from you guys in the real world, as well as my own observations working with you on the street. Thanks to all who call and write with questions or clarifications on these articles. The feedback is important.
One thing I see more and more as a cost savings is entire departments using only portable radios, even in the cars. In my own county here in Maryland, the sheriff's department uses mostly portables. The multi-site receiver and voting system here works quite well and UHF portables cover most of this rural county effectively. Using the portables in the cars, though, demands some special considerations. The main thing is that a "public safety" microphone must be used. A public safety mike is the one where the radio's rubber duck antenna is mounted on the speaker mike. This gets the transmitter and receiver's antenna up to where there is some chance of signal getting into and out of the portable radio. If one merely were to try to use a portable radio on one's belt while in the car, without a remote antenna, performance would be dismal due to the body and metal of the car blocking most of the signal.
It's not a good idea to try to use a portable radio when you should be using a larger mobile. There are reasons mobile radios are larger than portables. One is because mobile radios usually have more powerful transmitters, which requires real estate for heatsinks as well as for the larger power transistors. Another, and much more important, reason is because mobile radios have better receivers. It is now possible to build super tiny, very sensitive FM receivers. For the most part, an entire FM receiver now can be had on one chip, with very little external support circuitry required. Look at pagers, for example. We've all seen the industry standard Bravotm pagers from Motorola, and they're mighty small. But have you seen the pager built into a pen? Or a wristwatch? These truly are marvels of technology, and extremely affordable due to the economies of scale in large volume production.
What makes a better receiver? Well, it's no big trick to make a sensitive receiver. Sensitivity means how much signal in from the antenna the receiver needs to produce a usable output. Pagers are good examples of modern sensitive receivers. They have essentially no antenna, but still work well inside buildings and cars, even when up against your body where most of the signal is blocked anyway. Voice is a different animal than data, though. For an FM voice radio, a typical sensitivity spec might be 0.25 microvolts for 20 dB quieting. Don't worry about the technical form of how of the spec is written; it just means a good receiver needs only a quarter of a millionth of a volt of signal at the antenna to output an essentially near-perfect signal. Note well: with sensitivity specs, a lower number is better, and there are two standards for sensitivity specs for FM radios. One is the 20 dB quieting, and the one most old timers instinctively quote. A newer spec is 12 dB SINAD which probably is a more useful method of quoting. At some point I will go into the details of these two methods of measuring, but for now be aware there are two standards and be sure you are using the same standards if comparing the sensitivity claims of different radios, because the numbers are different for the two methods. But sensitivity is not the big thing. Sensitive receivers have been around for decades. The factor anymore, especially in large metropolitan areas with lots of stray RF floating around, is selectivity.
Selectivity is the ability of a receiver to separate the desired signal from the thousands of unwanted signals. Remember any antenna can pick up any signal, although with miserable efficiency unless the antenna is appropriately designed to work at a particular frequency and with a particular type of radio. Whenever any transmitted signal (called "RF") hits any piece of metal, a current is produced. When the guys on the moon transmit, they are inducing a signal in your bedsprings, although probably too weak to be useful!
So the rubber antenna on your portable is picking up not only your own agency's transmissions (the desired signals), but also RF from all the bozos on adjacent channels in the same band (all adjacent channel users are bozos regardless of who or what they are), strong signals from broadcast radio and TV stations, ham and CB stuff, foreign broadcast on shortwave, and a myriad of other things intentional and unintentional. It is the not insignificant job of the receiver to pull out the signal you want from the jungle and ignore the stuff you don't want. Remember, many times the signal we want may be only a few millionths of a volt, and it easily is possible for there to be a hundred thousand times that level of unwanted signals being picked up by the antenna at the same time.
So how does the receiver do it? Mostly with filters. Filters are either in the "front end" (electrically near the antenna) or in the "I.F." (Intermediate Frequency stage of the receiver, roughly halfway electrically between the antenna and the speaker). The better the filters, the better the job the long-suffering receiver can do separating the desired signal, amplifying it and outputting it in some useful form. And the better the filters, the better job the receiver can do rejecting unwanted garbage (anything unwanted is considered garbage, even if it is transmitted from a million dollar TV station).
Filters are the thing. And filters take up real estate. The better the filter, the physically bigger it is. Receiver filtering stages are designed by reasonably competent Pacific Rim engineers with several things in mind: how much desired signal is expected to be available (available meaning presented to the receiver's circuitry by the antenna), how much garbage is expected, the practical size and weight of the receiver, how rugged the receiver must be, and how much you're willing to pay for the thing. Cost is a major factor, as there can be a 50:1 difference in cost between barely adequate and superb filters for the same radio. Size is the other predominant factor in modern communications equipment. A portable radio contains filtering suitable for the expected use working within a narrow range of frequencies with the pipsqueek signals picked up by the whip or rubber duck antenna provided with the radio.
This last gets back to the original point I am trying to make. The rubber duck on your portable is tuned approximately to the frequencies you will be trying to use. A rubber duck is an inefficient antenna and will not pick up a whole lot of signal at your or any other frequencies. The filtering in the portable is adequate considering this. The small amount of garbage picked up by the rubber duck can be rejected by the tiny and relatively impotent filters in the portable, leaving enough of the desired signal for the receiver to process adequately. And the peasants rejoiced.
But, and here's the moral to the story: When you try to make your portable do double duty as a mobile by using an external antenna mounted on your vehicle (whether a mag mount in an undercover car on a surveillance, or a temporary radio in a rental car), the receiver falls apart. All of a sudden this larger antenna is cramming a whole lot more garbage down the receiver's throat. Remember any antenna picks up any signal. There isn't enough filtering in the small receiver to handle all this extra garbage, and performance suffers.
Mobile radios have much better receivers primarily because they have room for much bigger and more effective filters. A mobile radio is designed to work with a larger antenna, knowing a lot more garbage will be picked up also.
When you operate outside the equipment's design parameters, all bets are off. All sorts of weird symptoms pop up. One common symptom is hearing a lot of broken and very sporadic crap coming out of your speaker. This usually is adjacent channel interference working its way through your receiver. Adjacent channel problems may come and go as you or the other transmitter moves around, with the time of day as certain type of users get more or less busy, with reflections and atmospheric conditions, and other factors. The problem may be worst in certain parts of town where you are near high powered transmitters (many towns have areas known to the radio guys as "intermod alley" due to the concentration of transmitters in a high area). Another very serious problem happens when adjacent channel crap picked up by your receiver is so strong it confuses the receiver into thinking some of that signal is what the receiver is supposed to be hearing. As most modern receivers have "AGC" (Automatic Gain Control), the receiver faithfully cuts down its sensitivity thinking it has plenty of signal to work with. The problem with this is that the receiver is wrong. But when the receiver cuts back its sensitivity due to too much signal being picked up, it kills its own performance on what it is supposed to be hearing. So the symptoms look very much like your desired signal is too weak, causing you to put on an even larger antenna, go to more transmitter power or whatever, trying to get the signal level you think you're supposed to have. These efforts only make the problem worse. It most cases where this is happening, the only symptom you may see is reduced sensitivity. Without a spectrum analyzer in the hands of someone who knows how to use it, you won't know what's happening. I've seen even competent technicians be fooled by this. You think you don't have enough signal when, in fact, you have too much.
And it's all due either to having a crappy receiver or using more antenna than the receiver is designed for. Or both.
This problem is very common in surveillance installations for the following reasons:
1) Most surveillance transmitters are low powered.
I've said many times, and it bears repeating: if you are getting crummy performance from your surveillance system where everything seems as if it should be working OK, try a smaller antenna first.
I just got back from Los Angeles where we had a multi channel wireless video surveillance package operating. It was one of those super high pressure emergency last minute jobs with no prep or setup time, boogie out the door and read the briefing on the plane, with a very narrow one shot window in which to get everything up and working. Los Angeles is a rough place to be messing with RF, and I didn't have a spectrum analyzer. Our two video body wires just weren't giving us the performance I knew they were capable of. My customer was a knowledgeable and experienced RF guy who kept making noises about using the bigger antennas he saw in my kit. We ended up with 100% usable video and audio by using straightened out paper clips stuck down in the BNC antenna connectors of the video receivers.
All the stray garbage in LA was killing the front end of my receivers. By going to the smaller antennas (paper clips), we reduced the level of garbage to where the receiver could sort it out. This example shows that even high end properly designed receivers can be affected by excessive garbage coming in through the antenna.
An additional factor affecting both two way and surveillance communications is when transmitters or receivers are used in body worn applications. Without getting into dry theory, realize that placing an antenna up against the body will soak up something like 85% of the signal. The body is full of water and minerals that make it a perfect RF soak. A body wire antenna must be specifically designed to work when up against the body, and this is not a job for junior operators.
We'll get into this more in a future article.
In the next issue we'll learn more about path loss and different frequencies, a very easy way to calculate transmit range based on antenna height, and some new information from recent research affecting transmit range in some real world situations.
Holler if you have any comments or suggestions on areas we should cover. How have you solved various technical problems? Drop me a line so you can share your work with your colleagues. You do the talking, I'll do the work of putting it down on paper. Be a published expert and win the love and respect of your fellow man!
Copyright October 1995 by Steve Uhrig, SWS Security. All rights reserved.
Steve Uhrig is the president of SWS Security, a manufacturer of electronic surveillance, communications and intelligence gathering products for law enforcement since 1972. You may contact Steve at