Thursday, February 11, 2016

Fox Box - Can it be this easy?

For quite some time both Arlene and I have looked at article after article on something called "fox hunting" also known as direction finding.  This is an amateur radio "sport" of locating a hidden transmitter by using various methods such as a radio with a handheld Yagi antenna.

We have some of basics we need to go searching but no one was doing fox hunting in our area.  At least we had never heard of anyone doing it.  Therefore, we decided to do a hunt for ourselves. But wait there was one thing, one very important thing, missing.


I decided to look into building a fox box, which is typically a weather proof box which contains a low powered transmitter and a controller of some sort that fed the transmitter some information to, well, transmit.   I looked through the internet resources and found some great little gadgets that work on one or maybe two frequencies at very low power usually in the milliwatts range.  This was fine, but what if you wanted to use a little more power and be able to try fox hunting on 2 meters or 70 centimeters and vary the frequency from hunt to hunt let's say.  I really found nothing.

This of course this led me to building one for myself.  I sat and thought about it, assembling a parts list and drawing possible designs and then suddenly it hit me.  What do you really need for a fox box?
First you need a transmitter.  Well, as you probably already know there are several handheld amateur radios that are so cheap, everyone considers them as disposal radios.  Could I use one of these?  Let's say an old Wouxun or Baofeng or one of the other no name manufacturers.  Seemed reasonable.

Now the hard part was the controller.  What does a controller do?  On a periodic basis it sends information to the transmitter so that people can hear it, to basically find it.  I looked at Raspberry Pi, Audrino and specialized units, but being someone who is frugal to the point of cheapness, these were all too expensive.

Again I pondered what did I have in the house already that would do the same thing.  It had to give audio to the handheld's microphone port.  That port on a Wouxun is an 3.5 mm jack.   Then the light went on.  I have an MP3 player, which I brought for $30 AND a Wouxun radio.  Is that all I needed?
Could it be that easy?

I recorded a 3 minute MP3 audio file which has in it the following:
"Here is the K7RFH fox box operating on a frequency of 145.65 FM, K7RFH fox box"
The 3 minutes included a period of silence to make up the total of 3 minutes including the message. Now, this can be any amount of time, so I made several version of varying times.  The shortest being 30 seconds and the longest my original 3 minute version.

I then setup the Wouxun to accept VOX input.  Using a  3.5mm to 3.5mm plug cable, I plugged one end into the MP3 player and the other into the Wouxun microphone input jack.  Set  the VOX level to 3.  Turned on the radio.  Turned on the MP3 in "repeat a single track" mode. Now, every 3 minutes the radio announces its call sign and a little more.

Since I have never done this before, I am not quite sure if this is a good way to do it.  I think I need to stay in control of the radio, so that means I will need to know were it is at all times and be able to get to it quickly in case something goes awry.  This is because the fox box operates in the ham band and runs 2 to 5 watts.  Although given the antenna, it may be quite a bit less power.  The control part  should not be a problem. The high power may be an issue for the hunters.  With the higher power, it means that the signal will be stronger and may ricochet giving false directions.

Finally, I needed a weather proof case.  Fortunately, I had purchased an ammo can set at Sam's Club. The larger can was used to make a radio in a box. You can see that one here.  However, the smaller one was just one of those sizes that might have worked to carry a small 7 AH battery and maybe some extra parts.   However, now I have a handheld which is 124.5mm × 61.49mm × 33.88mm plus an MP3 player which is 0.6 x 1.6 x 2.6 inches.  These together would be nothing inside the ammo can.  So, perfect!

Laying the ammo can on its side, this was clearly a good spot for an NMO antenna mount.  I had one just laying around.  Drilled a 3/4 inch hole and poof, the antenna mount was ready.

Final issue is power.  The Wouxun should be able to transmit for about 6 hours and the MP3 player is good for about 18 hours, all on their own power.  If  I wanted to make sure  I had more battery power I could put a 7AH battery or a USB power brick in the case.  Both would fit.  So now with just a few more parts the entire box could probably run for 24 or more hours.   Of course, fox hunts are usually over in just a few hours.  Everyone would get too bored if it took much longer than that.  So each devices' on-board power will be just fine to start out.

Personally, I was very surprised how easy it was to create this very simple fox.  Total cost for parts I did not already have is... well nothing.  You may not have all the share parts that I keep around so here is the estimated breakdown of costs:

  • Handheld radio = $30
  • MP3 Player = $30
  • Ammo can = $10 (part of set)
  • NMO mount and connectors = $12
  • Antenna = $12
  • Miscellaneous wiring = $2
  • Total is about $96.  
  • You can probably do better than this on e-Bay!
Now on to building some hunting devices!

Days have passed and I now have two options for a fox box.  In addition to the original MP3 version, I decide to spend some money and buy a unit that could be controlled via radio signals.  The new option came from

The first option as described above is the MP3 player/Handheld transceiver. This provides voice transmissions and timing is controlled by how long the voice portion plus a period of silence is recorded.  That is it.  Nothing special. No control of unit.

The second option replaces the MP3 player with Byonics PicCon unit.  The PicCon unit costs $42 as a kit or assembled for $66.  What the PicCon provides is pretty amazing.  When connected to a transceiver via the microphone in and speaker out, the PicCon can be remotely programmed and controlled.  Connecting the PicCon and radio required either building or buying a compatible cable.  I selected to buy the cable from Byonics for $19.  I figured the ease of assembling the fox box accounted for some time well spent.  The PicCon's capabilities and user manual can be found at

 Below are photos of the two configurations.

This is what the Fox Box looks like from the outside.  Photo 1 is the "carry" look.  

Photo 2 shows the NMO cap removed.  

Photo 3 shows the 2 meter antenna attached.

This is the MP3 version. Note the headphone jack on the MP3 player is connected directly to the microphone input on the HT.  The HT is setup with VOX control on the microphone.

This is the PicCon version with 12V battery pack to power the PicCon "Hidden Transmiter Controller". The PicCon is controlled by DTMF tones from the HT or via tones heard over the air.

1 comment:

Andrew Lamoureux said...

Nice job, I like how it's a bunch of easy to get off the shelf devices. I have to look more into this piccon device, especially the programmable over the air feature.