Mitigating the Drone/RDF Threat – Part 1, by Tunnel Rabbit


This is an extension to my recent SurvivaBlog article titled Advanced Field Telephone Techniques, yet it examines and details the topic in the context of a specific threat. It often pays to reiterate and reinforce. While partly an intellectual pursuit, this discussion is grounded in decades of real world experience, sans actual battlefield experience, or military training. With this disclaimer stated, we can rest assured because the method of remotely operating transceivers via field phones was once SOP (Standard Operating Procedure) for U.S. military forces. It is well-proven on the battlefield as method of avoiding RDF, and subsequent direct attack by enemy forces.

It is also a method that can be used as a substitute for a radio repeater. However, in this modern age, we should be aware that the best that we can attempt is mitigation, and should not expect that we can totally avoid the threat. At any time that there is RF generated by any device to include, for example, the charge controller of our PV array, there exists RF (Radio Frequency) energy that could be detected. Total RF silence at a base station should be in effect for best results. I, therefore, use relatively RF-quiet charge controllers, the Morningstar TS-45/60 with the PWM switched off to lower my RF footprint. Typically, many MPPT-type charge controllers are relatively RF noisy.

JWR Adds:  Even the local oscillator within a radio receiver produces detectable RF energy  — but minuscule, compared to what you’d produce in operating a transmitter.

Understanding the Threat

Drone technology is here to stay and drones will increasingly become problems in many ways. They can be used to gather intelligence visually, day and at night. Thermal imaging can be used both during the day and at night. And now affordable drones have Radio Direction Finding (RDF) capabilities. Drones are increasingly used for intelligence gathering, and kinetic (offensive) measures. An ability to use doppler RDF technology, allows drones to locate a potential target. Learning how to deal with such a growing threat will be an ongoing process. As technology advances, we can adopt low-tech practices that can help us conduct a more secure communications plan. To better understand the threat and the measures proposed, we need to understand how drones are used in RDF operations. There is an excellent video on how RDF using drones works. Please first view this instructional video from S2 Underground, and then come back to this article:

Radio Direction Finding: AKA How “They” Can Find You

A drone does not necessarily need to have RDF capability to be a threat. Rudimentary direction-finding methods involving a yagi or a moxon antenna, and a handheld receiver with a strength meter, or simply a good ear, and a map of the area, can give the user a rough bearing as to where he should fly his drone to pinpoint a retreat location visually. If the location has enough electrical power to run a radio, then it is of interest as it likely has other desirable or essential supplies to attract looters.

Remote Transceiver Operation Using Field Phones

We begin with the most effective method that is available, one that may work for most people without much background in the subject. Field phones, and the devices mentioned, are relatively simple devices, devices so simple, that as a child, I was ‘messing with this stuff’. If I can, then you can. I would encourage anyone with any understanding of electrical circuits to use these methods. The terms mentioned in this article might be new and strange to the reader. But do not be discouraged, since this stuff is not rocket science. Anyone who can connect two wires to a field phone could be in business. And that is the beauty and strength of field phones, rugged simplicity that is sustainable low technology that can defeat the highest tech, and cutting edge of surveillance means and methods. It is a 25-cent solution to a million-dollar problem.

While drones will become an increasing threat, as a part of a tiered communications plan, we will use radio communications in addition to field phones. In a previous two-part article Advanced Field Telephone Techniques, operating transceivers remotely was discussed. We can also use commercial phones to remotely operate a transceiver with a VOX (Voice Operated) transmission function. This could be a handheld, or a HF set. My 1980s Kenwood 830S, as old as it is, has VOX.

With the purchase of these two items, one can easily connect two commercial phones and operate a radio as far away as 2 to 3 miles when using military surplus wire, WD1A. WD1A is 22AWG, yet only has a few steel strands that are copper-washed. Not all WD1A has copper wash wire strands, however. 24 AWG gauge solid copper wire that is used by telephone companies, would extend this estimated distance several fold. Depending on the alloy, copper is 8 times more conductive than steel.

Historically, when field phones were used extenively by the military, field phones needed a phone-repeater on the line about every 1 mile or so, to maintain the strongest and best quality voice signal. To avoid the necessity that makes this repeater mandatory, is why we should use the best operating field phones available. Even the venerable and popular TA-312 can suffer from aging transmitters in the handset. Sound-powered sets, such as the wonderful TA-1, do not use a battery-powered amplifier, and therefore are reliant upon the sensitivity of both the transmitter and receiver in the handset. If either or both, the TA-312 or TA-1 can be found at an affordable price. Purchase your pair(s) from a reputable vendor. The TA-1 is a superior choice when the distance is less than one mile, since no batteries are needed. The unit is lightweight and designed to be hand-carried They come with waterproof plastic shell cases. Given variations of the type and quality wire used, the quality of the wire splices, or connections made, and the types, or particular phones used, your mileage will vary. If good quality field phones are not available, then we can also use commercial phones.

Commercial Phones as a Field Expedient Substitute

You can use commercial phones as ersatz Field Phones with this device: The Phone Line Simulator Model: DLE-200B. This provides full functioning. Simply pick up the handset, and the other commercial phone rings. At a purchase price of about $150, we have in effect, two field phones at a price of about $75 each, plus the price of each phone. These can provide a sufficiently high level of audio as compared with aging field phones. However, genuine military field phones would be my first choice for a list of reasons detailed in an earlier article. If we cannot obtain this device, simply connect 9 to 18 vdc in series on the phone line anywhere between two commercial phones. This makes a circuit that can talk to each other, yet one cannot ring the other. Installing a 12vdc led lamp in series partly solves the problem, as when one phone is off hook, the LED lights up. We could also use a transceiver to contact the other party and request a conversation via phone.

This device requires alternating current (AC) to operate. A small or larger inverter would be necessary for off-grid applications.

See: “Phone Line Simulator Model: DLE-200B. “Compatible with fax machines and modem speeds up to 28.8 kbps Precise dial tone Switch selectable ring cadence Provides CPC breaks after hang-up on either port Switch select-able audio attenuation Can provide 2 seconds of precise dial tone before ringdown Provides ringback signal to calling device”

Connect a commercial phone directly to a transceiver via the earphone and mic jacks on the transceiver with this device, the VOX-PHONE. Old school TA-312, TA-43, EE8, and Erickson manufactured European military field phones that use D batteries, can also use this device by using the military field phone at the base station and a commercial phone on the receiving end. Simply apply 9 to 18 vdc in series on the line. The higher the voltage, the further the phones will ‘talk’. 2 to 3 miles could be expected, yet is not guaranteed. To conserve resources, limit the number of batteries used. Start with one 9 vdc battery to see if that is adequate. I use 12 vdc from the base station’s ‘solar system’ batteries. Connect two 9 vdc batteries together in series to obtain 18 vdc, or use rechargeable AA or AAA batteries and make a suitable battery pile at the lowest voltage that works. I use a 6 pile or 6 gang AA battery holder, and connect them as one gang, or two in series to obtain the voltage sought. 6 double AA batteries together produce 9 VDC. A fully charged single AA or AAA battery will have 1.5 to 1.6 volts, therefore, 6 batteries x 1.5 vdc = 9 vdc. Off-hook, depending on the voltage applied, the commercial phones draw approximately 200 Ma (200 milliamp hours). We can use two of these 6 gang battery holders to produce up to 18vdc when two gangs are connected in series.

At eBay or another online sales site, search for:  “3×6×1.5V AA 2A CELL Battery Batteries Holder Storage Box 9V Case With Lead Wire“.

Hybrid Field Phone System for Remotely Controlled Transceivers

Often, yet not in every case, the military field phone can ring the commercial phone. Not all commercial phones are compatible with field phones. Electro-mechanical ringers require much more current to ring. If so, then a piezoelectric buzzer, instead of a electro-mechanical ringer, can be used. Or if the commercial phones are equipped with electronic ringers, many more phones can be rung simultaneously by the magneto crank type generator of a military field phone. As it is untested by myself, do not use the TA-1, or other sound-powered phones in this system. Unfortunately, the commercial phone can not ‘call’, that is ‘ring’ the military field phone using the same wire pair that connects these phones.

If the wire runs are short enough, or conversely we have plenty of wire to use, to create a full-functioning hybrid field phone system, then use two wire pairs, and use one wire pair to operate all buzzers on the same line with a momentary switch that delivers at least 6 volts to the line (12vdc piezo electric buzzers). This would be known as a party line where all stations can hear and talk to each other. We can also signal to all parties that a conference call is needed, or to individuals. A Commo Window can also be established —  that is, a scheduled time each day when regular briefings are conducted.

To operate a transceiver remotely, use a military field phone to make the outgoing ‘call’ to a commercial phone that is placed on this device. Ideally, for the best quality audio, and to better manage our resources, it would be better to use two commercial phones in this system. The commercial phones can be made operational as detailed. This configuration allows remote transmission and reception via the phone.

SKU: VOX-PHONE, $14.50

(To be continued tomorrow, in Part 2.)