Foxhunt gear – offset attenuators
I attended a great fox hunting presentation at Dayton this year, hence a few posts on the topic.
I had some posts in the past about tape measure beam antennas. Really neat antennas and they have multiple purpose use (they are GREAT for hitting distant repeaters when you orient them vertically!). Much like the picture on the right (courtesy of Joe Moell K0OV) they are more useful for fox hunting when you add the active attenuator to your setup. FYI, Joe is the co-author of the great book “TRANSMITTER HUNTING, Radio Direction Finding Simplified” available where most ham books are sold. His website has more information on the book at http://www.homingin.com/THRDFSinfo.html and he contributes to CQ and CQ VHF.
So….. just what is an offset attenuator? Joe explains it on his “Homing in” site as:
An RF attenuator is a device that goes between antenna and receiver to reduce the signal strength down to within the range that the receiver S-meter can handle. Without one, you may think you’re close to the fox when you’re still far away. You won’t be able to get close enough to a camouflaged hidden T to identify it. The amount of attenuation should be adjustable so that you can add just a little when your S-meter first pins, up to a lot as you get within a few feet. Special ARDF receivers used by champion foxhunters have electronic attenuation built in, but ordinary handi-talkies don’t. Adding it would require major micro-surgery in the HT.
His attenuator page is:
I recommend his site in general, many great projects:
On his attenuator page, he has full schematics to make an offset attenuator.
But wait…… there’s more!
Further on his page, you see one made in a sweet Pomona box. I like this box and thought it was a bit pricey at first, until I did the math and figured out the cost/time to do it myself. These boxes are shielded with the connector of your choosing (BNC/SMA/259, etc).
They generally cost around $25 or so and are shielded! Great to have. When you add the cost of connectors and such, it isn’t really so expensive after all.
I really advise using such a case or a metal case in general, makes things work out much smoother in the end. More information on this box at: http://www.pomonaelectronics.com/index.php?i=prodsub&parent=BOX&cat=BONCONN&getDetails=
But wait….. there’s even more!
Marvin Johnston KE6HTS is now offering a “semi-kit” for this attenuator on his website. I’ve seen this kit when I was at Dayton this year and encouraged a friend to pick it up and build. I may end up running a buildathon here in CT on these attenuators.
The price is really not bad at $22.00. You can purchase them built for a few dollars more.
Information on the kits and pre-built models are at:
And yep……. there’s even more (again!).
If you would like to “roll your own” from parts you may have on your bench, but don’t want to make a circuit board, you can get one from……. you guessed it…… Far circuits. I picked one up and am going this route myself.
There are a few boards/projects on the Far circuits website at:
There are a ton of great resources out there on the web, these will really get you going right from the start. Fox hunting is a really fun and useful part of our hobby and one that doesn’t cost a ton of money to get started in. If there are no active fox hunts in your area – start ‘em! There are plenty of options as far as transmitters and such and really doesn’t cost a club much money to get started.
I’ve recently seen an old article showing how to build or assemble a passive attenuator from a panel mount BNC, the grounding shell of a PL-259 and a Female BNC to SO-239 addapter. Can’t find the link to it now, but it may represent a simple assembly at a cheaper price for an in line attenuator. Anyone know the link?