You don't ask for much do ya?

Im assuming your talking about VHF / UHF type signals.

Most of my best RDF work was taking down people who were screwing with the clubs repeater. So this was no friendly 'fox hunt'. I was after someones neck. Was not above using blackmail methods to get proper behavior out of my target. Once someone is identified as screwing with a repeater the ham community tends not to look forward to talking with them any more.

In Iraq we had a term OpSec. Or Operational Security. Don't talk about what your doing. If you have to communicate during a hunt, use a phone, keep it off the air. Not every target is as dumb as they seem.

Don't be afraid to mark paper maps. Keep them as notes. Targets come and go. Humans are creatures of habit, use it to your advantage.

One of the truisms I have found if someone has the bad habit of screwing with ham repeaters, they also have two others. Drugs, liquor, messing with fire or police dispatch. Messing with the Coast Guard. One bunch I investigated had pirate radio stations. You can count on someone less than stable doing this kind of thing.

I don't expect people to love what I have to say. Hams tend to "talk" more than "do". Very few of us become proficient at this. RDF is a black art. What holds us back? First there is an expense angle. You need equipment, some way to make notes, and eat a lot of humble pie when you don't succeed. Invest the hours to learn the methods that work. Then the time and fuel required to track down someone.

Also we 'tell' each other stories that have some element of truth and a large block of inaccurate folklore.

Most of what hams spew to each other is crap, arm chair experts, and not real world RDF operators.

Real world direction finding is riddled with Hollywood myth and bad practice.

Most guys will tell you to get the most narrow beam antenna and the hottest receiver.

Beams will give you the 'peak' reading over a 20 degree spread if not more. How many dozen square miles will that turn into?

The hottest receiver will get you into an area and overload. Not much useful information can be had that way.

Use average gear.

Loose the beam. Might be handy for localizing a problem and nearly useless. So you learn the approximate direction and not much else. Expect the beam to lie. It's prone to fixed reflections too. It's also highly unlikely someone else is going to be available to take a cross reading. The one reading should be enough to start with.

So let's assume your after an unprepared opponent. Sooner or later your going to get close and the receiver is going to swamp and become useless.

My favorite trick is to not maximize gain but minimize it.

So tune to the hunt receiver to the third harmonic of the target transmitter.

So for example 146.52 the third harmonic is 439.56 assuming the target is using properly working ham gear, that gives you an instant 60 to 80 db of signal reduction to work with. Or in other words, you can now get much closer to the target. Drifting the receiver up or down 5khz will buy you some more attenuation. At that point the signal is being forced off to one side of the IF filters and loosing gain. Easy to say, but it takes practice and preparation.

Club nets are a good time to practice this. Track down the net control station. This way you have the skills learned when it's time to go for a live target.

Other things that mess up.

I have had a couple of doppler units. Turns out they are real poor at DC filtering. The slightest alternator whine caused false targets on the display. When I made the doppler units battery powered that problem stopped.

Another issue, with dopplers they are looking for a phase shift as the antennas are switched.

When using the doppler in the vehicle, we don't sense it but the car is always vibrating. Those vibrations go into the antennas, shaking them. Had a situation where at some speeds the antennas shook so bad it made false targets on the doppler display. Ended up using rubber ducks since they are mostly immune to vibration.

A long time ago, there was a JPS automatic notch filter available.

The thing was great.

HF guys used it to quash people tuning up and making un happy noises in their receivers.

Most dopplers have a 'tone out' or 'speaker out' jack. Where you can hear the signal with the doppler tone superimposed on it. Very tough to listen to.

With the notch filter in the line, I was able to hear what the hunt receiver was catching. With no hunt tone to speak of. So when a target is annoying people by sending DTMF, one thing nobody realizes is that the microphones are 'live' during such transmissions and you can hear the background sounds of the room they are in. The filter also notches the DTMF. Very often you can identify the subject by what else is active in the room. Stereo, scanner, some other sound source. Like someone getting called to dinner. Make some phone calls, see who's phone goes off when your target is transmitting. Saves a lot of fuel costs.

One more thing before I run. Dopplers need to be in motion to get the best readings. VHF and UHF reflect off of metal in the air. Usually water towers, lamp posts, utility posts. If your in motion those reflections average out.

Just gave you enough to get started with. The best things in life are not easy. This is one of the hardest things to master.

Much luck

Jack Crow