Fun With Near Field Receivers by Thomas Icom A near field receiver is a piece of communications equipment whose purpose is to intercept radio communications in it's immediate vicinity. This "immediate vicinity" can be as far away as two miles depending on the strength of the transmitted signal and the antenna used on the near field receiver. When connected to an antenna and turned on, a near field receiver starts sweeping its frequency coverage. When it hears a signal of adequate strength, it locks on and demodulates the audio. When it loses the signal it continues its sweep until it finds something else to listen to. Most near field receivers are also capable of providing rudimentary trouble shooting data such as signal strength and deviation. Near field receivers are commonly used in field service and counter- surveillance operations. A hand-held near field receiver with the proper features can replace some of the functions of the more expensive and bulky service monitor. When doing a sweep with a near field receiver, the receiver will lock onto the signal of any RF-based surveillance device within its frequency coverage at a greater range than the traditional field strength meter. The Opto' R10 This article will focus on my experiences with the Optoelectronics R10 "Interceptor" near field receiver. The R10 is a high quality, reasonably priced, battery operated unit with a coverage of 30 Mhz. to 2 Ghz. Specifications and product reviews have appeared in other publications for those interested in such information and opinions. The first thing that one should realize with the R10 is that it's operation is different than that of a conventional VHF/UHF "scanner" receiver. That's because its role in RF work is different. With a scanner one either programs in specific frequencies into channel memories and then has the unit go through them, stopping when it encounters one with activity on it, or they program a high and low end of a frequency range and search that range for frequencies which have activity on them. A scanner is also a lot more sensitive than the R10, which enables it to receive radio signals from long distances. The R10 is much less sensitive than a scanner. That's because it's intended to receive radio communications in the near field. Depending on the power, elevation, and antenna of the transmitter in question; this near field range can extend up to a couple miles, but is usually limited to about 2500 feet for a standard 25 watt mobile radio running a unity gain antenna. Instead of having memory channels or a search function, the R10 just sweeps its entire frequency range looking for a transmitter that is of adequate enough signal strength to be considered near field. With the above in mind, the first thing one should remember about the R10 is that it's not a scanner, nor will it replace a scanner. The R10 is a powerful specialized SIGINT tool, but if you want the capabilities a scanner provides you shouldn't be purchasing an R10. The R10 works best in areas that don't have a high RF background level. In high RF background areas one can expect the overall effective range of the R10 to be reduced, and for it to be more likely to lock onto things like broadcast and paging transmitters. It also works best in a stationary, as opposed to a mobile installation. In a mobile installation or when listening to mobile units, one will experience signal loss on fringe signals due to the "picket fencing" effect. Depending on whether or not other signals of adequate strength are in the area, one will either wind up getting a totally different signal acquisition or quickly reacquire the original signal. The R10's unique capabilities and mode of operation take some getting used to. I suggest you spend a few weeks getting used to its peculiarities before actually going out and doing something serious with it. Antennas A lot of individuals seem to think that if they attach a "big" enough antenna to an R10, it will work better. They then get all upset when they turn it on and it constantly locks on a nearby FM or TV station. The R10 was generally not intended for use with a gain antenna. When one uses a broad-banded gain antenna with the R10 not only will the range will be increased, but also the possibility of picking up potentially undesirable signals like continuously transmitting broadcast stations or busy paging transmitters. The best antenna I've found for general use is the telescoping whip. It's length can be adjusted to provide signal attenuation when tracking down surveillance devices and attenuating an undesirable signal, or to provide a resonant length when focusing on a specific frequency range. This antenna type is the one which is sold with the R10. One can also use a yagi antenna with an R10 to great effect. Yagis are directional and tuned to a specific frequency band. These two characteristics help increase the effective range in the target band, and eliminate undesirable signals which originate outside the frequency band and direction of interest. In addition to using the right antenna, the judicious use of filters, particularly bandpass and notch filters, can increase effective range and eliminate undesirable signals. One filter I would recommend getting right away is an "FM trap". This will eliminate the unit locking up on FM broadcast stations. After that, I would recommend bandpass filters for specific frequency ranges of interest. Applications In regard to SIGINT operations, the R10 has proven its utility on many different occasions. It's utility in conducting counter surveillance sweeps for RF-based devices has already been mentioned. Assuming the device in question operates using non-encrypted FM modulation, the R10 will lock onto the device's signal upon walking into the affected room. By walking around the room and gradually attenuating the received signal strength by reducing the length of the telescoping antenna and adjusting the R10's squelch control, one will be able to pinpoint the location of the device . When one gets "on top" of the surveillance device, one will notice a full scale signal strength reading despite the fact that one has set maximum received signal attenuation (i.e.. the squelch control cut all the way back and the telescoping antenna fully collapsed), and a feedback squeal will result from the coupling of the device's microphone with the R10's speaker. The feedback will indicate that one is in the immediate area of the surveillance device. It will also alert the third listening party to the fact that their toy was discovered (keep this in mind). The feedback squeal will only result if the surveillance device's mode of transmission is unencrypted FM voice. One however will still be able to note the device's location by attenuating the received signal's strength and watching the signal strength meter for a full scale reading when one has maximum received signal attenuation. Once one has determined the approximate location by using that method, one should then be able to find the device by conducting a good thorough search and having some knowledge of what surveillance devices look like. The R10 is excellent for cellular surveillance, especially when attached to a cellular band antenna and/or a cellular bandpass filter. Good signal locks have occurred on mobiles up to a half mile away when using a 5/8 wave cellular antenna in a mobile surveillance installation, and up to two or more miles away on cellsites when used with an 800 Mhz. yagi antenna. When surveilling cellular mobile units, the R10 is able to quickly reacquire a mobile when a frequency change occurs due to a hand-off to another cellsite. For quick acquisition tactical signal intercepts, the R10 is excellent. A quick acquisition situation is when you notice or suspect nearby RF communication activity (i.e.. someone who's talking on a handheld radio nearby, being caught in a traffic jam, etc.) and you want to know what the parties are saying. A frequency search may not be possible due to time or manpower considerations. (Ever try to do a frequency search with a scanner while driving down the highway at 55+ MPH?) I just recently used my R10 to good effect while caught in a traffic jam that resulted from a five car accident a half mile down the highway I was traveling on. Within minutes, the R10 had successfully intercepted several transmissions from people who were talking about the accident. The R10 can act as a sensor system to alert you to RF activity in your immediate area. One can implement this application by turning down the sensitivity so that background signals (i.e.. FM and TV broadcast stations, paging transmitters, and other annoyances) are eliminated, and then having a friend with a handheld radio walk around one's immediate area while transmitting in order to get an idea of effective range. Once the range is determined, the R10 is left on and listened to. One could also add a sound activated alarm to the audio output to give a louder alert indication than the received audio alone would provide. When one hears audio one will know that someone is using radio communications within that predetermined range. By listening to the transmissions in question one should be able to determine the general identity and intentions of the radio user(s), and thus be able to generate an appropriate reaction to their presence.