Wireless Video Useful tools? Or Smoke and Mirrors?
Part One of Three
Riddle for the month - What goes, "Clop, clop, clop - BANG! BANG! BANG! - clop, clop, clop?" Answer below.
Lately there have been many new companies popping up offering various wireless video products to law enforcement agencies. While the concept of wireless video transmission has been around for many years and there are indeed a number of genuinely useful products to help you do your job, we need to cover some basic technical, operational and legal information so you can decide if these products are suitable for you.
Video surveillance can be a potent tool for law enforcement. A very basic video system consists of a camera, a video monitor and/or recorder, and some sort of transmission media to connect the camera to the other stuff. Normally coaxial cable is used to transfer the video signal generated by the camera to somewhere else where you can do something with it.
Many applications involve situations where it would be convenient (if not mandatory) to have the camera placed at some location where it's not practical to run a cable from the camera. This might be a camera inside a building monitored from the parking lot (or vice versa), a sting in a hotel room monitored from another room, video needing to be sent between buildings, from covert packages such as briefcases, or the like. Remote cameras also are used in hazmat and tactical situations where it may be dangerous to place an officer right at the scene. The newest applications now have a camera worn on an undercover operative's body, giving you a video body wire similar to the older audio-only units with which we're all familiar.
In the cases mentioned above, a radio signal from a video transmitter is used to send the camera's video output to a companion receiver located in a friendly area. This video transmitter and receiver is what we will be referring to as "wireless video systems" in this article.
We'll assume you are familiar with the basics of working with video. If not, refer to the many excellent articles published previously in Police and Security News for this information.
At first glance, wireless video sounds great. And, if you have a professional system supported by people who can help you get it working, it can be. But, you need to know a few things before pulling out your checkbook or purchase order.
One of the most important points to be aware of is that there are serious legal considerations affecting use of video transmitters. A term we will use frequently is "FCC Type accepted." Basically, the Federal Communications Commission is an agency having jurisdiction over the manufacture, sales and use of transmitters, including our video transmitters.
Any video transmitter advertised, sold or used by any non federal agency in the good old U.S. of A. must be tested and type accepted by the FCC. Type acceptance is the term for an approval of a design and production which has been verified through testing to meet rigorous criteria to permit operation without interfering with other legitimate users of the airwaves.
Type acceptance is granted only to systems operating on radio frequencies assigned for the purpose by the FCC. Transmitters meeting type acceptance standards do not exceed a certain power output (usually very low), have harmonics and other unwanted emissions below a specified level, meet other design criteria such as having a fixed antenna, and other factors such as not drifting off frequency with temperature extremes. In short, type acceptance generally indicates a quality product that will not interfere with other things out there in radio land.
It bears repeating: A non-type-accepted video transmitter is illegal to sell, advertise, use or possess, the only exception being federal agencies who are exempt from type acceptance requirements.
If a non-exempt user in the U.S. possesses a non-type-accepted video transmitter, the illegal equipment is subject to confiscation, and the violator may be subject to "monetary forfeitures." Additionally, any evidence gathered with the illegal equipment will be the "fruit of a poisonous tree" and unusable in legal proceedings.
The reason for all my bleatings about FCC type accepted equipment is that virtually all wireless video transmission systems being marketed to law enforcement are not type accepted and, therefore, *illegal*. Judging from the calls we get, most law enforcement agencies are not aware of the requirement for type acceptance and are dismayed (read: pissed) to find out they've been sold illegal equipment by a spy shop or some such vendor.
For now we'll discuss only legal video transmitters. The matter of the illegal ones (most of the ones advertised) will be covered in a future article.
I will lump legitimate type accepted video transmitters into two basic groups: those operating on 900 MHz and those operating at the higher microwave frequency ranges. We will concentrate primarily on the 900 MHz stuff, since that is the bulk of what is available. The design and implementation of microwave equipment is considerably more difficult than at 900 MHz and therefore generally is manufactured only by truly capable firms.
My address is at the end of the article.) Note that with the exception of Microtek's consumer line, these firms sell their products to law enforcement only, so don't waste their time if you are not government. A government agency is *not* an exporter, dealer or especially a firm who "sells to police."
Unfortunately, 900 MHz type accepted transmitters are very short range devices. One of the requirements to meet FCC criteria is the product cannot interfere with other users of the radio spectrum. The main way this is accomplished is by restricting the field strength (power output and antenna) to a very low level. If the signal is weak enough the chances of interference are very low.
The industry joke is, "If it's type accepted it barely works." FCC specifies a maximum field strength permitted for a transmitter. Field strength is created by a combination of transmitter power output and antenna efficiency. You cannot increase transmitter power output to increase range and still be FCC legal. Likewise, you cannot improve the transmit antenna without also increasing the field strength beyond legal limits. Type accepted transmitters are not allowed to have removable antennas to prevent people from replacing the antenna with one having more gain or efficiency.
The bottom line of all this is a 900 MHz type accepted device, regardless of the manufacturer, is limited to a relatively pipsqueak signal. The typical transmit range of a legal video transmitter on 900 MHz is around 150 feet in open air. If you are trying to punch through an interior wall (sheetrock or wood) you will be lucky to see 100 feet. Forget it if you try to go through any metal, or an outside wall. Won't happen.
Yeah, we all know about using bigger antennas on the receiver to increase the range. These Yagi antennas, or "beams" can indeed help the situation within certain parameters. If you read our antenna article a while back, you will remember that range is increased through antenna gain. This happens by taking the normal omnidirectional pattern of a "unity" gain (no gain) antenna and squashing the round omni pattern into some sort of shape that concentrates the power in a specific direction. You can get better performance from the antenna, but only in the precise direction in which it is pointed. You lose the omnidirectional feature of the unity gain antenna.
All well and good - or is it? Here's the fact that the junior operators don't understand: In most cases, the additional loss in the longer feedline needed to connect to the gain antenna more than outweighs the gain you pick up by using the better antenna. At 900 MHz, coaxial cable used to carry the signals from the antenna to the receiver is not very efficient. Going through even 10 or 15 feet of even quality coax can kill so much of your signal that you may be worse off than if you just stuck to a unity antenna mounted directly on the receiver. We saw a good example of this recently with equipment we manufacture.
We had a camera and 900 MHz video transmitter concealed on an confidential informant (C.I.). The C.I. was on foot inside a building. We had our primary receiver inside the building feeding a recorder so we had good video from that, but our client agency wanted to monitor activities in real time from the parking lot in case the C.I. got into trouble. We had arranged hand signals the C.I. could give us if things got out of hand and she needed backup.
Fortunately we were able to get our van right up against the outside wall, with the bulk of the activity just on the other side of the wall inside the building. Using our #2055 receiver with a rubber duck antenna mounted directly on the receiver we were able, just barely, to get a usable signal. At this point our receiver was inside the van, of course surrounded completely by metal which was killing what little signal happened to squeak out of the building. We thought if we placed a magnetic mount gain antenna on the roof of the van we could get enough extra signal both from the gain of the antenna and by having the antenna outside the metal van.
Well, when we tried the antenna with 17 feet of feedline our signal was weaker than what we were getting from just the rubber duck. I was using a low loss foam coax with BNC connectors properly installed for minimum SWR, but no dice. Later I even tried Heliax - a flexible hardline that is even less attenuation - with the same negative results. We finally were able to get a good enough signal by positioning the van so the receive antenna was looking through a window of the van instead of the metal side. But the bottom line is that we were worse off with the gain antenna than without. Keep this in mind if a vendor claims larger antennas will give ranges greater than the 150 feet I mentioned earlier for their equipment.
With clear line of sight (meaning nothing between the transmit and receive antennas - almost an impossibility in the real world) it is possible to get a maximum of about 700 feet from a type accepted 900 MHz wireless video system. Keep in mind, though, that this is under optimum conditions which you are very unlikely to enjoy in a surveillance scenario. And to get even this 700 feet requires a large (not covert) gain receive antenna, a low loss feedline of only a few feet, properly installed connectors, and the proper phase of the moon. Your realistic range probably will be significantly less. Do not depend on the transmit ranges claimed in the advertisements when planning your surveillance.
Another thing we need to know regarding 900 MHz video transmitters is that they all operate in the ISM (Industrial, Scientific and Medical) band. This translates to 900 MHz being a garbage dump for all sorts of consumer wireless products ranging from garage door openers to cordless telephones, burglar alarms, toys and household devices. The closer you are to a populated area the greater the chances of other electronic devices operating on the same frequencies as your wireless video system.
Chances are you won't know anything else is there (unless you're clever and check with a spectrum analyzer), and they won't know you're there either - except that you may see a reduced range. Also, operating in a built up area almost guarantees a high level of background RF (radio signals) on all frequencies from DC to light. Even though this background RF is not on your frequency, your receiver will be picking it all up and fighting to filter it out. On inexpensive receivers (most of them), this loud RF will swamp the receiver with the effect of reducing its sensitivity and, therefore, the range of your system.
Again, you probably will not know this is happening and may wonder why you are getting miserable performance when everything looks like it should be working. The problem will be much worse if you start hanging larger antennas on your receiver because the larger antenna will pickup that much more garbage RF.
With antenna systems, using the proper connectors and installing them properly is critical to good performance. When you're dealing with signal levels of a microvolt (a millionth of a volt), you can't afford to waste any. Use only top quality connectors and buy the proper tool to install them properly. My preference is BNC's, and here at SWS we use primarily 3 piece crimp ones. Generally instructions on installing connectors do not come with the connector, and each one is different, so ask your supplier to photocopy you the installation details for your particular connector. Buy the right crimping tool - they're not all the same. Other than BNC's, acceptable connectors are type N, and any of the SMA series.
Absolutely unacceptable are UHF series (PL-259 male and SO-239 female - the connectors found on CB radios), RCA (phono) connectors and any twist on BNC's. My connector and feedline guru is Joel at RF Connection (800-783-2666 or 301-840-5477); call him for excellent technical advice, prices and stock. Unfortunately, most of the problems we see with antenna systems are poor choices for type or quality of connectors and poor installation techniques. Be aware, also, that good low loss feedline (coax) is required (call Joel), and antenna coax is a different type than video coax.
Don't use adapters on antenna lines if you can help it. The higher the frequency the more important antenna feedline and connectors are, and even at the relatively low 900 MHz range it's still critical. You could easily double the range of your wireless system by insuring you have a minimum length of proper feedline and quality connectors properly chosen and installed.
By the way, the typical power output of a type accepted 900 MHz video transmitter (or anything type accepted on 900 MHz) will be in the region of one or two milliwatts. A milliwatt is one thousandth of a watt. Your handheld radio may have a power output five thousand times that of a video transmitter.
We've only scratched the surface of wireless video technology. If you would like to see more on this topic covered in future articles, call Al Menear at Police and Security News magazine (215-538-1240) and let him know. If you'd like any other technical topics covered mention those to him also.
Feel free to call me at 410-879-4035 if you have a wireless video system that isn't performing up to snuff and maybe we can offer some suggestions.
Hope to see some of you at NATIA in July. Come by our booth and say Hi.
Answer to riddle at the beginning of the article: An Amish drive by shooting!
Copyright June 1994 by Steve Uhrig, SWS Security. All rights reserved.