In preparation for the pedestrian fox-hunt that happened a couple of weeks ago, I decided to take a look around for some antenna designs that I could build at home. I came across a blog post which showed off a Time Difference of Arrival (TDOA) antenna that someone had designed and built.

A simple and effective fox-hunting antenna

How it works

The antenna relies on a timer, in my case a 555 timer, to generate an audible signal. I have mine calibrated to a frequency of 1kHz. This signal is level-shifted so that it goes between -4.5V and 4.5V. The use of a set of diodes causes the circuit to quickly switch between the two dipole antennas mounted to the unit (tuned to 145mHz). When the signal received by the two dipoles is out of phase, the receiver emits a tone. When the RF source is equidistant from both antennas (when you are pointing at the “fox”) the signals are in phase, causing the tone to disappear.

This design is fully functional regardless of how close you are to the source, providing an advantage over other antenna designs. The audible tone also provides a clear indicator of direction. However, the main issue is the fact that you get two readings, at 180° to each other.

Parts List

The list of components required is quite simple, so I had most of them laying around.

• 555 Timer
• 4.7K  Resistor
• 100K Resistor
• 2x 470Ω Resistor
• 2x 0.01uF, 50V Capacitor
• 10uF, 25V Capacitor
• 0.001uF, 50V Capacitor
• 4x 1N4007 RF Pin Diode
• 2x 10uH Inductor Coil
• RF Choke, 8 turns (salvaged from a motherboard)
• SPST Switch
• 9V Battery
• 9V Battery Clip
• Antenna wire (single core) or 4x Telescopic Antennas
• 1.5M of RG-58 Coax

Build

When building the antenna, I used two blog posts for reference. You can find them here(pdf) and here. For some reason I ended up combining both of the designs, using the Bryonics.com design for the dipole circuits, and the 146970.com design for the rest of the circuit. This probably isn’t advisable, but it worked for me. Here is the circuit I used:

You can test the 555 timer circuit by hooking the output pin up to a speaker. You should hear a tone when power is supplied to the circuit. I started by breadboarding the timer circuit, and then moved it to veroboard for the final build. Once the circuit was complete, I zip tied the PTT button down on my PMR446 Walkie-Talkie, plugged the antenna into my Wouxun KG-UVD1P reciever, and tuned into the 446mHz signal. The antenna worked perfectly first time, providing a clear tone that faded out when I pointed the antenna at the radio.

For the physical construction of the antenna, I used some thick, single core, cable to make the dipole antennas, and mounted the whole thing on a 60cm long piece of wood. Lots of glue gun was used as I couldn’t be bothered to find some screws. I also drilled holes in the ends of the wood to poke the coax through. The dipole elements were attached using screws, and can be bent between a folded and deployed position.

Evaluation

I took the antenna out for the pedestrian fox-hunt that happened a few weeks ago. The antenna picked up the first signal very well, and the audio tone allowed me to find the direction of the transmission to quite a high accuracy. Unfortunately for me, I headed off into the wrong direction, as the antenna will give you two readings at 180° to each other. I had it running all evening, without the battery flattening. The RF choke fell off quite quickly, as I had not done a very good job with the soldering. I was also worried that the connection between the RX input coax and the board would break, so I would consider using a small connector in the future (such as an SMA socket). Overall the antenna worked very well, albeit sending us in the wrong direction! In terms of improvements, I would also like to mount the circuit board properly, and put it in an enclosure.

If anyone has any questions, or wants me to bring mine with me on a club night, just let me know.

-Peter Barnes