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Q&A on Fox Hunting and the Offset Attenuator

Hunting

 

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 http://www.homingin.com/

 

Find a local fox hunt: https://foxhuntlist.com/

 

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.

Fox-Hunt-Scanners

STL and FreeCad Files for alternate V7 bracket mounts including 3/4″PVC (1″OD) and Arrow antenna booms: FoxhuntV7-Holders.zip

 

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.

FOAD-BEAMs FOAD-Front
Business end of the tape beam.Yup – the battery is just taped on. Top View
FOAD-Case FOAD-SO-239
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.

Can you make me a turn-key fox hunting solution?

I get asked to build transmitters and complete receiver systems w/ antennas.  Unfortunately there are only so many hours in a day and time is not available for the task.  I have been working on my own transmitter idea for well over 2 years now (both VHF and UHF) and it’s still just a pile of parts waiting to be assembled.  So below is how to get started the easiest way.  I also have not personally tried some of the equipment below so caveat emptor!

  1. Transmitters
  2. Receivers
    • A good brand name (Kenwood/Icom/Yaesu…..) handi-talkie is recommended.  A newer one is preferred if you are going to use an offset attenuator as older radios will not receive out of band.  If the fox transmitter needs to be remotely turned on (which saves transmitter battery life!) a radio with a touch tone pad is needed.  Using a lower quality radio may get your poor receiver performance (overload) or unwanted birdies and spurious signals.
    • Old handie-talkie sized radio scanners work great and also have to option to not accidentally transmit.  Great for non-hams and kids.  With many public services moving to 800MHz digital these can be found for a couple dollars at a thrift store or yard sale.
    • When all else isn’t available a Baofeng will work (I have an UV-82 in my arsenal).
  3. Receiving Antennas  – I recently purchased an Arrow antenna.  They have one for 2M fox hunting (3 or 4 element) but I ended up getting a split boom LEO model to also work with VHF/UHF satellites.  I find the antenna to be high quality and the split boom makes for compact travel.  https://www.arrowantennas.com/arrowii/146-4ii.html
  4. Receiver Attenuator Pad – When starting a hunt far away you might be able to rotate your antenna and listen for a peak/null signal.  But as you get closer you will need to attenuate the signal.  The 3rd harmonic technique (Fox on 146.000MHz but listen on 146.000×3=438.000Mhz) does work but there are point where it will still be too weak (not heard) or too strong.  In this case a “step attenunator” can be used to reduce the strong signal.  You can mostly find them on Ebay or AliExpress.  I have one from AliExpress but found the push switches to be a little flimsy and would recommend one with good toggle switches.  Make sure to get one in a metal enclosure to keep RF from bypassing the unit.
  5. Receiver Offset Attenuator – As a replacement or compliment to the attenuator above an offset attenuator removes the problem when getting closer to the transmitter.  You will find at some point, depending on your radio, the RF signal will bypass the antenna and go straight into the radio.  The offset attenuator removes this problem by listening to a frequency offset from the fox transmitter (typically a value like 1, 2, or 4MHz).  You can find more information on offset attenuators earlier in this page or also on the Foxhunt V7 Offset Attenuator page (An Arrow mount is available):  https://kc9on.com/product/fox-hunt-v7/
  6. Receiver Cables – You will need some cables to run from the antenna to the radio, as as well as an attenuator in between.  For that you will need some 3-6ft cables with the correct connectors or adapters.  You connectors will of course be depending on the above equipment.  In find Amazon to have a good selection of RG-174 or RG-316 type cables with the proper ends needed (usually BNC or SMA).  Here is an example:

What is the difference between the different offset attenuator versions?

Version:

V5 (Bare Bones)

V7 (Complete)

Oscillator:

4MHz Crystal Sine

4MHz Crystal Sine

Component Style:

Thru-Hole parts

75% SMD parts

Available as a Kit:

Yes

Yes – SMD parts pre-soldered on and tested

Available as Assembled:

No

Yes

Switch:

SPST Power only

DPDT Power and Bypass

Connectors:

None

Choice of BNC or SMA

Enclosure:

3D files available

Includes enclosure

Battery:

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:

-20dB

-10dB

Attenuation ON at Fox Frequency:

-8 to -18dB

-6dB

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

N/A

Notes:

  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.

What about the mysterious JP1, should I use it?

There are some that insist the radio/antenna ground must be connected to the oscillator ground.  However that is not the case with very little difference between using it or not (your mileage may vary).  I’m not a formal electrical engineer but lets dive a little deeper into that area of the schematic and see why it’s not needed.

RV1 starts by adjusting the amplitude of the oscillator signal.  The oscillator signal (red path) is through R4, the diode D2, and R5.  Resistors R4 & R5 provide isolation between the radio/antenna side RF (we don’t want the outside RF going into the oscillator) and also to control the biasing of the diode so the adjustment is as much in the diode linear region as possible.  We now have D2 turning on and off at 4MHz and acting as a very crude lossy mixer with varying amplitude.  From the radio side the path (blue – the fox signal) is from the antenna, through capacitor C3, the diode (which mixing now becomes purple containing fox&offset signals), back through capacitor C4 and out to the radio.  The 2 capacitors act as DC blocks to keep the radio/antenna from interfering with the biasing but still allow the RF to pass through.  The resulting mixing effect will have the following frequencies going to the radio (FOX=frequency the fox transmitter is on, OSC in this case is 4MHz):

OSC          FOX (-6 to -10dB down)          FOX+/-OSC (-30 to -60dB down)          FOX+/-2xOSC(-35 to -70dB down)          FOX+/-3xOSC(>>80dB down)………

So what happens when we tie the 2 grounds together?  Not much.  If you jumper JP1 that will connect the radio/antenna ground to the oscillator ground.  In these cases R5 and R4/RV1 are far greater than 50 ohms so there is no RF path to speak of.  However, farther up in the oscillator section there can be a (really) small impact on C2/C5/C6 and you might see a little difference in the frequency or amplitude of the oscillator depending on the antenna/radio connected, usually not enough for a noticeable change in the normal operation.

 

 

 

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