One of the tenets of Amateur Radio is that we as hams are “self policing” – hams that violate the FCC rules are largely notified and corrected by other hams (usually the ham doesn’t realize that something is wrong and is grateful for the notice). Only willful and egregious violators are turned over the the FCC for further action. So if there is another ham that has a stuck transmitter or some other problem that you can’t notify him on the air about, how do you find out where he is? Through radio direction finding!
The task of finding and identifying an unknown transmitter is known formally as Radiolocation (using the radio waves themselves to determine the location of the source) but it’s informally known as Foxhunting. The “fox” is the transmitter, and we “hunt” the fox until we know where it is.
We can use all the tricks that we as hams know about signals to find the location of most any fox/transmitter. In some cases, it’s just for fun and practice, but in other cases it is really needed.
The Basics of Foxhunting
What do you need to know to find the location of a transmitter? You need to know the direction (relative to your current location) that the transmitter is, and you need to know just how far away it is from you. These two pieces of information are all you need if the transmitter is a block away or it’s half way across the country. The difference is just how you find out those bits of information!
Finding the Direction
In most cases, you will need to use some sort of directional antenna to get an idea of which direction the fox is (relative to your current location). The problem is that directional antennas are not perfect – they cannot identify to the sub-degree just where the signal is. So you will need to take multiple readings and do some head calculations to find your best guess.
Finding the Distance
Thanks to physics, we know that signals that are closer to us are stronger, so we can use signal strength to determine whether we are getting closer to the fox. As with direction, no measurement is perfect and you will need to take multiple measurements and find your best guess.
Work in Teams
As with most things, teamwork usually makes the job easier, and working in teams can make foxhunting easier, especially if the team is distributed and can take readings from different locations at the same time, and then coordinate and discuss the findings. Teamwork is almost a necessity for finding real transmitters, as a bunch of the “rules” for fun foxhunts do not apply (such as geographic boundaries and restricted placements). For fun foxhunts, working in teams close together (in the same car for example) allows you to strategize and discuss your thinking.
The End Game
Once you get close to the fox, a lot of the tools you used to get there will no longer work. Directional antennas for example lose their directivity when close to the source (near-field signal). So you will need an additional set of tools to “finish the hunt” – these tools vary based on the type of signal that you are hunting and the equipment that you are using, but this is why practice hunting is both fun and necessary. With sufficient practice, you will develop a set of “end game” tools that you can use in any situation.
How Do You Get To Carnegie Hall?
The age old joke is still true. Practice! Here in Richardson, the RWK usually has at least one foxhunt every weekend so you can develop your skills and also have some fun. We want you to learn how to hunt and there are others that will be glad to teach you!
You can read more about our weekly foxhunts here and be sure to check the club calendar.
Foxhunting Equipment
Advanced Techniques
Next: T700 – Intro to Amateur Radio Satellites
RWK University original content is Copyright 2022 by the Chip Coker KD4C and the Richardson Wireless Klub. Reproduction, extraction or use of this material by any other entity is with explicit permission only.