Fox Hunting

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Q&A on Fox Hunting and the Offset Attenuator



Fox Hunt V7 Kits and Assembled units

Fox hunt V5 kits

Are you an expert fox hunter?

We’ll get this one out of the way with a definite NO.  I just like to do a little hunting during the summer months.  But over the decades I have tracked down many things from foxes, jammers, interference, and in the shack RFI issues.

Why a beam antenna?

I have played and seen multiple fox hunting antenna types over the decades including phased verticals and doppler types such as the dopplescan ($$$) and double ducky direction finder.  In all the tape measure beam with an offset attenuator seems to be the biggest bang for the buck.

What kind of antenna do you use?

The tape measure beam by wb2hol.  An update adding a choke is here. These can be made for almost nothing using some PVC pipe and T’s, an old tape measure (Think Harbor Freight free coupon!), and a few other odds and ends.  Don’t have hose clamps?  No problem, substitute cable ties, etc.

One change for those to try…… There is a hairpin at the feed point of the driven element.  Some some strange reason (maybe just me) but I find the antenna to work better without it.

What does the offset attenuator do?

The offset attenuator is nothing more than an oscillator, 4MHz for example, and a mixer diode.  The input signal from the antenna is mixed producing lower level signals at plus and minus the receiving frequency.  So for example you are hunting a jammer on 146.52MHz.  Just tune your radio to 142.52MHz or 150.52MHz and start swinging the beam.  The attenuator has a potentiometer to raise or lower the amount of drive going from the oscillator into the diode.  This in turn raises or lowers the offset frequency signal level, in cases to almost zero!

In other cases the signal can still be too strong, especially when your within feet of the fox box.  In that case keep tuning up or down in multiples of 4MHz (138.52, 134.52, 130.52 or 154.52, 158.52, 162.52….. in the above examples).  Each 4MHz harmonic mix will be even more signal reduction.

Any other hunting tips?

There is a treasure trove of information with more tips, making fox boxes, and other events at


Find a local fox hunt:


Videos to explain fox hunting, antennas, and how a fox hunt offset attenuator works:

The biggest advice I can give you is to know your antenna!  I use the the local Nation Weather Service radio station to learn where my beam has it’s peaks and nulls.  This gives you a nice fixed location and transmission.  Speaking of nulls…… you may find on some beams that the backward or side NULL makes a better detector than the forward peak.  It all gets back to taking some time to lean how your particular antenna works.

If the nearest NWS radio is too far away and if your radio has broadcast FM built in you can use that as well although the 2M antenna will not be as efficient.  The trick is to find a local FM broadcast station that has a clear unused channel 4MHz away.  For example if you have 101.5FM then try the offsets of 105.5 or 97.5 if there is no active station there.  Hard to find in the bigger cities!  You might find a station in the 88-89MHz range and tune 4MHz lower if your radio will go below 88MHz.

Need a good cheap radio for fox hunting?  Check the garage sales and thrift stores for hand held programmable scanners.  You can pick them up CHEAP – around $1-$10!  Other advantages are they have a REAL squelch knob, non-ham/kid proof (no PTT to transmit or destroy the attenuator – perfect for scouts, etc.), and pre-programming the fox frequency in 4MHz increments to quickly change attenuation levels.  Did I mention dirt cheap?  I have yet to pay more than $5 for them.


STL and FreeCad Files for alternate V7 bracket mounts including 3/4″PVC (1″OD) and Arrow antenna booms:


Where can I get an enclosure for the V5 offset attenuator?

My first enclosure was an old hand me down Bud box re-purposed.

The second one was hand built using aluminum flashing and a bench vise for a hand brake.  It looks pretty bad, like it took a fall out of the car on I-75.  So we won’t show a picture of that one!

The latest V3 – V5 boards is a slightly more rectangular size and fits in a 1/2″ PVC junction box available from the big box hardware stores for about $2.50.  Search for UPC 0-34481-20749-8.  The only downfall is there is no room for the 9V battery.  but bigger boxes are available.

I prefer a good metal RF tight enclosure but this seems to work pretty good.  Of course when the RF is so high it’s bypassing the antenna then it’s time to switch over to the coffee can probe.

Business end of the tape beam.Yup – the battery is just taped on. Top View
Exploded view SO-239 to Radio – It’s a 1/4″ of PVC threaded onto a chassis connector and stuffed in the hole.

3D printed enclosure files are available!

A huge thanks to Kevin N7RXE for the design and files for the initial enclosure! Foxhunt-V5-3D-Enclosure


The KC9ON Version of the Enclosure with chassis mount BNC areas:  Fox5Box1

And another box using an SO-239 chassis connector from K4MID on Thingiverse:  FoxhuntV5C Case


In those pictures are a Baofeng radio….. how does it work?

First off, I like my Baofeng.  It’s a cheap radio for the price but it has it’s downfalls. The receiver is a bit wide and easily picks up interference.  If it ever gets trashed or dunked in the lake I won’t feel as bad as dunking a $500 HT!    The big secret is turning the squelch all the way OFF (menu #0 and SQL to 0).  With the squelch on the  response is way too slow to trip.  Besides you WANT to hear between signal and noise.

Does it work on other frequencies too?

Yes!  We have tried this out in the 440MHz (70cm)  amateur band as well as the FRS (!467MHz) service area.  Close range attenuation offsets up to +/-24MHz work just as good in the 2M band.  The initial in-line attenuation is ~-10db as opposed to the -20db seen on 2M.  Below you can see the relative attenuation on the 446MHz and 442MHz offsets using a 450MHz signal.

Experiments have also been carried out on frequencies as low as the short wave and AM broadcast band!  For example 700KHz WLW is a strong station here in the winter nights, putting the offset attenuator in line found WLW at 4.700MHz, 8.700MHz, and 12.700MHz offsets.  Why would we want to offset attenuate an AM radio station?  Well, we do have long cold winter nights here 🙂

I’m real close to the fox, can’t attenuate any more and just can’t find it…..What next???

Here is a neat trick I discovered 2 weeks ago when getting ready for a fox hunt.  One of the fox hunting setups is a Uniden handheld scanner that’s used so kids can hunt and not accidentally transmit.  I had the antenna-less scanner sitting on the boom of the antenna right over the reflector.  The offset attenuator was turned on but the cable going to the radio was not connected and hanging on the floor.  When the nearby fox box was turned on I was surprised when the scanner squelch broke.  There is enough mixed RF coming out the antenna end to work with the scanner.  Upon experimenting it looks like this configuration with the unhooked scanner on the reflector works pretty good from about a dozen feet in!

Another trick is to use a coffee can probe.  Take a 3lb METAL coffee can, drill a hole for a BNC connector about 1″ from the bottom end.  Add a 1-2″ piece of wire to the connector and bolt down to the can.  The shielding of the can makes a nice close-in sniffer.

What is the difference between the different versions?


V5 (Bare Bones)

V7 (Complete)


4MHz Crystal Sine

4MHz Crystal Sine

Component Style:

Thru-Hole parts

75% SMD parts

Available as a Kit:


Yes – SMD parts pre-soldered on and tested

Available as Assembled:




SPST Power only

DPDT Power and Bypass



Choice of BNC or SMA


3D files available

Includes enclosure


9V not included

9V Included

Frequency Range:

<500KHz to >500MHz

<500KHz to > 500MHz

Typical Attenuation Levels (all models):

2M (146.00MHz)

70cM (450.00MHz)

Attenuation OFF at Fox Frequency:



Attenuation ON at Fox Frequency:

-8 to -18dB


Attenuation at +/- 4MHz offset:

-30 to -60dB

-35 to -70dB

Attenuation at +/- 8MHz offset:

-35 to -70dB

> -80dB

Attenuation at +/- 12MHz offset:

-60dB to -80dB



  1. Unit can run as low as 3V (2xAAA) but minimum attenuation levels will be higher than above
  2. It is recommended to not transmit more than 5W briefly in bypass mode with the V7 otherwise damage may occur.
  3. NEVER transmit in offset mode – It WILL fry the mixer diode.
  4. Unit can be modified to a 2MHz offset using a 2MHz crystal.
  5. Performance under 100MHz has not be tested but will operate down to 500KHz
  6. Performance has not been tested above 500MHz but slowly degrades to about 700MHz maximum usable frequency.
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