Air-To-Air DF
If you are an aircrew member who want's to try something new, try to locate a buddy aircraft using your Airborne DF unit. Think its a trick? It's not the easiest thing in the world to do, but it can be done. I'll also admit that doing something like this is very unorthodox. It is fun to think about, though!
Admittedly, the real-world applications of air to air direction finding would be very rare. The military uses air-air TACAN (similar to VOR-DME) for formations and aerial refueling. There are possible scenarios, however. Hunting down an airborne ELT is possible, but probably overly aggressive: it is easy enough to wait until the offender full-stops. If a pilot was completely lost but retained communications capability, an airborne DF could find him and lead him home. Someone might suggest giving that person a DF-steer from the nearest Flight Service Station, but this capability is being phased out at many FSSs--the equipment is very old and expensive to upkeep for the limited use it receives. To take this scenario further, imagine the student pilot who gets stuck on top of IMC (clouds) with no way to get home. An aircraft could lead him to VMC (clear weather). To best facilitate this, the aircraft should circle as near as he or she can in one spot. It should be noted that an air to air DF will only give bearing information; no distance (DME) is available. While this seems obvious, it does make the problem more difficult.
There are a few considerations before you begin. First, if you plan to DF another airplane, you need to make prior arrangements with the other aircrew. This is required by the Federal Aviation Regulations for any formation flight. While CAP regulations prohibit formation flight except by special permission, if you remain well-clear (500 feet) of the other aircraft, you are never considered a formation. In any case, brief with the other aircrew. You should establish sanctuary altitudes at least 1000' apart. Have the search aircraft, then, 1000' above or below the target aircraft--a good choice would correspond to your DF antenna installation.
You also need to select a frequency for operation. 121.5 MHz would be an easy choice in an actual emergency, but obviously would also gum up that channel--it is one of CAP's goals to prevent this! Use 121.775. Every DF unit SHOULD have this training frequency, and nearly all aviation transceivers should be able to select this frequency for transmit. Later-model DF units allow the user to select any frequency that the communications radio can receive. If you have such a unit, try 122.75 Mhz (interplane) or some other frequency not in use. You could use 122.9 for practice and 123.1 for actual searches.
The next thing you need as a search aircraft is a signal. Have your target airplane go out and circle at the designated altitude. They should pick a spot on the ground and practice their turns around a point. Have them begin to transmit on your designated frequency. They should attempt to keep a constant signal with a constant amplitude--humming, singing, or reciting limmericks into the mike seem to work acceptably.
Now the tracking aircrew DFs the signal as they would an ELT or EPIRB. Its the same principle, really. Continue to DF until you've located the target aircraft or you get indications of "station passage," as described in manuals for an ELT search. If you achieve station passage, LOOK for the other aircraft, turn around, and DF some more. Although the target aircraft is in motion, this shouldn't be apparent to the tracking aircraft until at a close range. It is critical during this practice that both aircraft remain at their prebriefed sanctuary altitudes. While both aircrews should be clearing visually as much as possible, the hard altitudes briefed will ensure deconfliction and prevent a midair.
Have fun!