Mitigating the Drone/RDF Threat – Part 3, by Tunnel Rabbit

(Continued from Part 2. This concludes the article.)

A Low Cost, and Simple-to-Operate Simplex Repeater

We can use a simplex repeater such as the Argent Data Systems ADS-1 Simplex Repeater. This is essentially a sophisticated digital voice recorder that is superior in quality to the Surecom, and other cheap Chinese renditions. We must have a reliable system especially when it is relatively difficult to access and a linchpin in importance. Using a simplex repeater in conjunction with a cross-band repeater can confuse the RDF analysis further.

This ‘repeater’, is actually a digital voice recorder that provides many useful functions. For the sake of this article, it rebroadcasts seconds after receiving a transmission. It rebroadcasts on the same frequency as the sending station. This means we can use transceivers that are not repeater capable. Transceivers that are not ‘repeater capable’, such are most GMRS/FRS radios, and all MURS radios (transceivers). Common transceivers, including CBs, can use this kind of repeater system. A repeater for CB is less likely to be necessary as CB propagates much better than UHF in mountainous terrain and uses 4 watts of power. This simplex ‘repeater’, the ADS-SR1, will be used in my AO to ‘connect’ three small mountain communities that will use very low-power GMRS transceivers.

The ADS-SR1 recorder is presumably legal for use on GMRS and MURS, since this device is not actually a repeater. These communities would otherwise not have communications between them. I use this on line tool to conduct a virtual Radio survey to identify a repeater site, or propagation ‘problem’ areas, or RF holes, or ‘dead spots’. The utility of this tool can not be overstated.

There are many video overviews available, showing the use of simplex repeaters.  Here are some examples:


SHTF WROL Simplex Repeater Configuration For Deployment.

SHTF Urban Simplex Radio Repeater Range Test

Selecting a Cross Band Repeater

To have the best equipment, the cost would be prohibitive for most preppers. There are many mobile transceivers that offer a crossband repeater function. These are desirable, yet cost prohibitive if redundancy is a requirement, and do not provide the lowest powered transmissions of 1 or 2 watts, but generally speaking, the lowest power setting is no lower than 5 watts. Mobile transceivers also require, at a minimum of three times greater capacity power supply, or power generation to operate. We would need a battery and a PV array that is roughly three times larger to sustain operations. In an austere environment power generation could be difficult. Most will need to look at lower-cost and power-efficient options as a starting point. And with lower-cost equipment, we can obtain several spares to broaden our capability, and to build redundancy into our plan.

A poor man’s crossband repeater can be put together at home using a simple cable adapter, or we can purchase a low-cost cross-band repeater for only $79 such as this example:

TYT UV8000E V/U 10W HP Cross-Band Repeater FM Two-way Radio Transceiver. (Easily found on eBay.)

A simple and short video demonstration: TYT UV8000E Repeater Function.

Aside from making our own cable to use two transceivers as a homemade cross band repeater, the lowest cost commercially made cable can be purchased for less than $40, also on eBay:

Surecom SR-629 2in1 Duplex Cross Band Radio Repeater Controller with Radio Cable

A video about the Surecom SR-629: Demo #1 SR-629

The Problem with Field Expedient Low Power Cross-Band Repeaters

When attaching two transceivers together to make a cross-band repeater, this method can make the handhelds used as a receiver, ‘deaf’ over time when exposed to transmissions in such close proximity to another. To reduce the odds that this will occur, use low-loss cable such as LMR 400 cable, and separate antennas — separated by many wavelengths in distance and several wavelengths in height, and shield the transceivers from each other, and from other electronics such as a PV system charge controller.

The RF from the transmitter can interfere with the function of other electronics as well. So also use the shielding and other techniques for the simplex type of repeater. Shield the charge controller, and the repeater itself from the radio by installing in a separate metal box or wrapping it in aluminum foil, or use RF reflective material such as Mylar or aluminized HVAC tape on the device and cabling if LMR400 cable is not adequate or if it is not available. If the electronics components are not shielded then there is the very real risk that the transmitting transceiver will leak RF that will disrupt the function of either the charge controller, or the ‘simplex repeater’ microcircuits, and render the repeater inoperable. If one can afford it, then purchase a good cross-band repeater designed for the job. The TYT8000e that I mentioned is a bargain.

Lowering the risk of Interception by Reducing Your RF Footprint

We can use a high-gain directional antenna with a narrow RF footprint, or we can use terrain masking to limit the RF footprint. Or both. If the repeater is located lower down on a mountainside (lower than what is often referred to as a military crest), nearer the bottom instead of the top of the mountain, the rearward direction lobe of the RF is blocked, and can even be reflected back toward the transmitting station. If the F/B ratio of the directional antenna is high, the majority of the RF is limited to the rear of the directional antenna, and limited to the sides, and RDF attempts would have a difficult time establishing either a line of bearing, or a cut, or more importantly, a ‘fix’ that pinpoints a location. If we lack a suitable directional antenna, then we could use an omnidirectional antenna that is shielded by a metal building, or a hillside. But in any case, perform an RF survey to properly assess the actual propogation.

The more narrow the RF footprint of the directional antenna, the higher it’s Front/Back (F/B) ratio, and the lower the Effective Radiated Power (ERP) from the antenna, the more difficult it is to locate it with RDF methods. If the repeater transmits with as much, or more power, as the primary transmission, analysis and identification of the original source of the RF is more likely to be inaccurate, and the base station tranmission ignored. A repeater that broadcasts with a strong signal could be seen as the most interesting source of RF, because Doppler RDF technology on the drone will detect it much easier.

Use the lowest power setting going into the repeater. If need be, using higher power setting through the highest gain omnidirectional antenna at the highest available repeater site would be a kind of ‘drone magnet’ as recievers will key in on the strongest signals, and the omnidirectional antenna would make it easy to triangulate and find the transmitter. Use this method if only necessary, and as a beacon that makes it an easy target for an RDF effort.


By no measure does this article describe all of our options. RF camouflage is an art that is at its best if the artist understands all the possible tools available. I\On the electronic battlefield, there are intangible and vastly creative other possibilities that can be used to distract, disorient, and deny. This is just a part of a defense, much as we would strive to channelize, deny, and otherwise control enemy movement on a physical battlefield.

As we grow older and slower, it is becomes increasingly important that we use our brains as a more than adequate substitute for our failing brawn. A good radio man would be more important than another trigger -puller, if he could excel at his craft. A remotely-operated transceiver is relatively simple and inexpensive to deploy with the tools detailed. But this is only starting point in an integrated and redundant communications plan. Just as we should never stop improving our defenses, we should also continually improve all of our other skills.