"DF Hunting Theory"                


This article describes the fine art of direction finding (also known as "DF'ing") and how it comes into play on the CB band.  DF'ing is a term describing the process of homing in on a radio signal in order to trace its origin. This is best accomplished through the use of directional antennas and signal reduction techniques. There are some sophisticated systems which electronically switch antenna phasing to determine direction. While these are nice, they are also somewhat expensive.  Most of us had to do it the old fashioned way, with loops (sometimes homemade) or some other innovative manual directional antenna setup. 


Why the need to DF? Almost everyone who has spent any time in the CB hobby, has run across the "undesirable element". Those people who like to disrupt other people's fun for either kicks and grins, or due to serious psychological issues.  But before you can deal with the problem though, you have to first be able to identify it. Most troublemakers are empowered by their relative anonymity which allows them to behave in an anti-social manner without any accountability.  But once their identities become known, they lose their advantage and, in most cases, they will slide back under the rocks that they crawled out from.  DF'ing gives you the means to locate those troublemakers.


I got my first taste of DF'ing in the mid 70's, when we started having a problem with a few dedicated agitators who were trying to disrupt our home channel.  I soon found out that simply driving around in random directions while watching the radio's "S" meter was a large waste of time.  You had 360 degrees worth of directions in which to decide to head out in. It would be much easier, and more expedient, if you knew which direction to go right from the onslaught. There had to be a better way to cut the odds.  Well, as luck would have it, the answer hit me like a ton of bricks when I got the chance to participate in a ham radio "fox hunt".  A "fox hunt" is an organized event where one guy hides somewhere and transmits a continuous signal, and the rest of the people race to be the first to find him.  Every contestant in this "hunt" was equipped with some sort of directional antenna. Some were simple loops, and some were multi-element yagi's.  Since this event was held on 2 meter VHF, the size of a multi-element yagi mounted to a mobile was not a problem. But such setups would never be practical for mobile use on CB.  But there were some techniques which translated fairly well between VHF and CB.


So what is a directional antenna anyway? And why are they important for homing in on unwanted signals? In a nutshell, a directional antenna is one that concentrates its signal in primarily one (or two) direction only.  Since it transmits and receives in primarily one direction, you must point it in the direction that you want to communicate.  By turning the antenna and peaking for the strongest received signal, you can determine which direction that signal is coming from.  Typically, you will find this type of antenna on a base station, usually in the form of a multi-element yagi, or a quad.  In a mobile, it's more difficult on 11 meters to make a directional antenna, due to the need for multi-elements, and the sheer size of them.  However, you can make a mobile setup somewhat directional by simply placing a magnet mount antenna in the center of the rearmost point of the vehicle.  This works because the largest part of the reflective ground plane  (the car's body) is in front of the antenna, and it will concentrate the majority of its signal in that direction.  Spin the car around in a parking lot, while a signal is being transmitted and, at some point, the signal will peak. Drive in the direction of the strongest signal, and you're on your way.  Another signal hunting method involves the use of a special purpose "loop" antenna.  A loop is, as the name implies, a circular antenna which works like a bi-directional dipole. The loop is rotated while listening to the signal, and rather than looking for the highest signal, you look for the point where the signal nulls (minimum level) , and that is the direction where you want to go. The advantage of using a loop is that the null is very sharp and will yield a very accurate direction.  The drawback to the loop is that it is bi-directional, meaning that the signal can come from either one of 2 directions which are 180 degrees apart.  When using a loop, you need to remain conscious of this fact, and try to keep the signal in the 180 degree section of the compass that's "in front" of you. If you happen to drive by the location while hunting, you can easily become confused. That's why it is often advantageous to combine the loop with another directional technique like the mobile antenna on the back of the trunk. Use the trunk mount antenna to get a rough direction, and use the sharper loop to fine tune your heading.


Another technique which works well is called "triangulation".  This is normally used when you have multiple base stations equipped with beam antennas, but it can also be used with multiple mobiles.  Using this technique can greatly shorten your searching time.  By coordinating your "hunt" with a few base stations with beam antennas before heading out,  you can narrow down the search area to less than a 1/2 mile radius if everyone is accurate in their headings. If each base station points their antenna at the carrier thrower, and a line is drawn on a map out from each particular base station in that direction,  at the point where all the lines cross, is the place where the signal is originating from.  Depending how far apart the base stations are, this technique can be very accurate, and will greatly reduce hunting time.  Another variation on this technique, while not as accurate, can also assist you by reducing mobile hunting time.  You only need one base station with a beam antenna, but the base station operator also has to be keenly aware of the surrounding local terrain and its affect on signal characteristics, as well as knowing what a normal signal should be from those various areas within a given set of assumptions.  For example, if I found that my beam heading was SW, and the signal was giving me an "S9" reading, and I knew it was another base station, I could assume that it was about 3 miles away, assuming stock power and a "normal" antenna. This technique works best with a radio which has a fairly linear and sensitive "S" meter such as a Hy-Gain 623.  You can also employ triangulation techniques while mobile if you have more than one hunter or using the loop to take readings from 2 or 3 different locations. But coordinating several mobiles is difficult if you don't want your target to know you are hunting, and is the most effective if you have another frequency band (such as 2 meter ham or FRS) to use for coordination.  I've also found that taking multiple directional readings from a single hunter usually takes more time than just heading straight toward the signal.

Collectively, these techniques allow you to eliminate a lot of wasted time heading in the wrong directions, and help to reduce the search area (and time) when you are finally ready to hit the streets in the mobile.


As you have probably guessed by now, having a mobile directional antenna readily available, or having the capacity to determine your initial search area is probably the most important element for a successful DF hunt.  But it is not the only important element to consider.  Once you get into the area where the interfering signal is originating, you also need to be able to narrow down your search to the exact building or spot.  Sometimes, the only building on the street with a CB antenna becomes the obvious choice.  But in many cases (especially in the "boom" days in the late 70's) there might be several CB antennas clustered on the same block.  You will then need to determine which antenna is the right one.  At this point, a directional antenna may not be of much help, if the signal is too strong to determine a single direction from.  What you need is a radio with good dynamic range, a sensitive, easily read, and somewhat linear "S" meter, along with some means to attenuate the signal.  Most CB radio "S" meters are neither accurate or linear throughout their total range. It is common for strong signals to be compressed into the upper end of the meter's range, where small signal changes do not show up readily. But most radio meters will have some area (Usually in the low to middle area) of its range, where minute changes in signal can be readily seen. Thus illustrates the necessity for having a method to attenuate signals. If we can take a very strong signal, and attenuate it down to a level where the radio's meter is more sensitive to changes, you can then better determine where the exact origin of the signal is coming from. There are a few methods for bringing a signal down to a usable level.  Some work better than others. The easiest method is simply to disconnect the coax cable from the back of the radio.  A variation on this method, is to use a simple switch box.  This can then be used to switch instantly between the regular antenna, the DF loop, and a 50 ohm dummy load.  Another signal reduction method is to use a radio with an R.F. gain control or a Local/Distance switch. Use care with this method however, as some radios use the RF Gain control to manually override the AGC voltage to bias the R.F. transistor in order to reduce the gain of the front end of the radio. This AGC voltage is usually where the "S" meter is tied into as well. In some cases, this combination can cause strange results and not the meter sensitivity that you need.  Yet another signal reduction method is to use a 50 ohm step attenuator. This method allows you to specifically tailor the amount of signal reduction in order to keep the radio within its most sensitive meter range. But you must use care to not transmit through it, or you may smoke the resistors inside, if it is not rated to handle serious power.

While we are on the subject of radios and "S" meters, I guess it goes without saying that the larger the meter, the easier it will be to read.  Radios with "Bar graph" meters should be avoided as they do not have the resolution for fine signal work.

Once you are close to the target, it is often only necessary to drive down the street and watch the signal peak and then fall off again.  Where the signal peaks is the likely source of your target. A visual confirmation of the target's antenna will seal the deal.   If there is more than one antenna on the same block, you can use the loop antenna and attenuator to "null" the correct signal and determine which is the correct location.


Here are some additional tricks which can improve your accuracy when DF'ing:

Good luck in your hunting endeavors!