# Letter Re: Using Direction Finding on Looter Bands

Dear Mr Rawles,
I’ve just finished reading your latest book [“How to Survive the End of the World as We Know It: Tactics, Techniques, and Technologies for Uncertain Times”], and let me begin by thanking you for writing it. I have just one small quibble in Chapter 9, Communications and Monitoring. This is something we both missed, and it didn’t occur to me until after reading this chapter.

While I agree with you that looters are unlikely to have the inclination, hardware, and talent to do direction finding (DF) on a retreat’s radio transmissions, the reverse is not necessarily true. If the readers follow your suggestions and get involved in ham radio, it’s quite likely that they may have the means to DF the looters. If you know where the looters are, you also know where they are not, and this could be very useful information.

A retreat with a single DF antenna for CBs may not seem to be particularly useful, but bearings on the looters combined with a maps of the area might serve very well.
In addition, if you either have a large retreat with space for two widely separated antennas, or two retreats working in tandem and communicating on a VHF frequency
or field telephone, it’s possible to get a good fix on where the looters are. Best Regards, – Jeff K.

JWR Replies: I used to do communications intercept and radio direction finding work, when I served as an Army intelligence officer. It is a skill that does take practice, but it isn’t rocket science. As described in my novel “Patriots”, all it takes is at least two intercept sites equipped with loop antennas, compasses, and enough time to get lines of bearing (LOBs) on a groundwave signal before it goes off the air. Those LOBs are plotted on a map. The intersection of two LOBs is called a “cut”, and it takes three or more LOBs to establish an accurate “fix” with a half-way decent circular error probability. (Actually an ellipse, but I won’t bore you with the math.) A single intercept site with a loop antenna cannot effectively do DF in part because there is no expedient way to eliminate the “front/back” loop antenna ambiguity. (You don’t know if your LOB is correct, or if it is off by 180 degrees.) This ambiguity problem was solved by the introduction of later DF rigs such as the AN/PRD-11, that use an H-Adcock antenna array and some clever processing power to do precise time-of-arrival calculations–actually comparing the micro-second difference in time when the speed of light signal strikes the different antenna elements. My great-uncle Albert Michelson would be proud of the designer!

If anyone wants to become adept at DFing in the field, I suggest that they get involved with a Fox and Hound group, organized by their local ARRL-affiliated ham radio club. It is great fun, requires only rudimentary directional antennas, and it will build a very useful skill.