I was asked to prepare a Radio Merit Badge program to be taught at the Camp Rainey Mountain Boy Scout Camp in Clayton, GA. One of the activities that we thought would be fun for the Scouts during the Friday class period, was a Fox Hunt. This would give the Scouts a chance for some hands on radio work, in a real life search and recovery operation.
I needed to come up with a fox transmitter that was durable enough to survive more than a single summer at camp. The other goal was to transmit in the 2m band, as the HT units we planned to use for receivers were single band 2m only.
Thanks to Eddie (W4JEM) for the inspiration and example design for this unit. He has been conducting fox hunts with home school and other youth groups for years.
This fox is based around a very simple simple transmitter. Eddie said “Oh it's really easy. Just hook up a 555 timer to an oscillator.”. But, of course the actual design and implementation was left to the student.
Based on Eddie's description, here is the circuit that I came up with.
The circuit consists of a 555 timer which turns a crystal oscillator on and off. Add a 9v battery, a voltage regulator, a few resistors and caps, and presto, you're done.
Obviously, there is a bit more to the design. The 555 timer is configured as a astable multivibrator, which generates a square wave with a 2 second period. This square wave is used to power an oscillator on and off. The oscillator frequency was chosen such that either a 3rd or 5th harmonic would fall into the 2m frequency band.
I built two different versions:
- A 48MHz osc which yields a 3rd harmonic at 144.000MHz
- A 29.491MHz osc which yields a 5th harmonic at 147.455MHz
Using a 7806 (6v regulator) to drive the 555 timer allows the output of the timer to reach a high of ~4.5v, which is enough voltage to turn on the 5v osc. You could use an adjustable regulator to dial up the timer output voltage to exactly 5v, but this seems to work well enough, and the osc frequency output does not need to be precise.
I was asked how this transmitter ID's itself.
Well, the short answer is that it does not ID.
However, the transmission is extremely low power. I'm guessing that the fundamental has an ERP of 15mW or less (3v signal into an ideal dipole). In the 2m band, we're using either the 3rd or 5th harmonic of the osc fundamental, so we're talking 30db or more below the 15mW.
When the transmitter is operating, it is received as an empty carrier (ie. like somebody is keying a mic but not talking). If you are not close by, and/or not using a directional antenna, you can't event see the signal above the background noise. However, when you are dialed in with the directional antenna, you can track the signal.
Here is the design of the pcb.
I cut a piece of plated 0.10 x 0.10 perf board to a size of ~ 1” x 2”. The component layout is shown above.
The pin connections are shown below.
Here is a picture of the finished pcb. Top view.
Note that the + ends of both 10uf caps are oriented toward the 7806.
And bottom view.
I like creating interconnects with bus wire, as this makes for a pretty rugged assembly. Also, the use of axial tantalum caps makes for a low-profile board, with no components sticking out to get broken off.
The pvc pipe housing for this fox is actually two pipe assemblies in one. The inner pipe assembly is used as a mount for the transmitter, battery, antenna wires, on/off switch, and transmit LED.
The diagram below shows the cut dimensions for the inner assembly. A Dremel with a cutoff wheel works very nicely for cutting out the slots which hold the battery and transmitter. This does kick up a bit of white dust, though. The inner pipe assembly is made from a length of 3/4” sch 40 PVC pipe with two end caps. The end caps do not need to be glued.
In this picture, the transmitter is attached and ready to slide into the slot in the pipe. The white and yellow wires are the antenna wires, cut at ~19” each, and are used to form a simple dipole. The battery clip is attached, and both the switch and led wires are attached.
See below for a closer look at how the transmitter is connected.
|White||Antenna - (19”)|
|Yellow||Antenna + (19”)|
|Brown||Connects switch to battery +|
|Brown-white||Connects other end of switch to Vin of the regulator|
This photo shows a view of the switch and LED. I drilled a couple of holes in one of the 3/4” pipe caps: one in the center to accept the switch, and a smaller off to the side for an LED.
A SPST pushbutton style switch is used. This needs to latch in the on position (no momentary contact). I picked a small low current LED for TX indication. The LED blinks on and off in sync with the transmission.
With the transmitter and battery tucked into their slots, wrap the antenna wires around either end of the pipe. Spacing really does not matter.
The outer pipe is made from 1-1/4” sch 40. Glued to either end are 1-1/4” pipe to 1-1/2” threaded adapters.
The larger 1-1/2” threaded couplers provided just enough room for our inner pipe assembly to clear when we insert it.
Note in the diagram above, a 1-1/2” to 1-1/4” slip nut washer is inserted into the pipe side of the adapter fitting prior to gluing it on. This washer seats at the base of the threads and provides a stop for the inner pipe assembly.
This picture shows the inner assembly slid all the way in, switch first, and resting on the stop washer.
This view shows the switch/LED end of the inner pipe resting on the stop washer. If you look closely, you can see the washer. Sorry, but it is a milky white plastic, so it is a bit hard to see.
Here is the finished PVC pipe RDF fox with 1-1/2” threaded end caps. Be sure to mark it clearly so that the FBI does not mistake your handy work for a pipe bomb.