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Fox hunting

This is an amateur radio activity that allows adults to play a sophisticated game of hide and seek. It is a team activity undertaken in vehicles to get close to the location of the hider - the 'fox' and then members of the team jump out of the vehicle and pursue the fox on foot.  The device that is hidden is a small amateur radio transmitter; usually around 1 watt output.  Once the fox has hidden the transmitter they call in the hounds and the hunt begins.

It is usual to have a team of at least four members - a driver whose job is just to drive the car, a combined beam swinger and equipment operator along with two navigators come runners. Some teams have done well with as little as two members, the introduction of small GPS  car navigation units has assisted these smaller teams greatly.

The beam swinger is an important member of the team as they decide which direction the vehicle should head. There are many theories about triangulation etc but the fastest way to get to the fox is drive straight towards the signal. Thus antennas used for foxhunting need to be unidirectional.

Foxhunting can be undertaken on a number of frequencies but the most popular is the two metre amateur band. In Melbourne foxhunting on two metres has been taking place on a monthly basis for more than 40 years. At other special events such as the South East Radio Group's convention in Mount Gambier hunting on other bands has been common. A Victorian championship is run each year where hunting on a number of frequencies is undertaken as well as pedestrian hunts.

Rules are fairly basic - fox is not permitted to be on private property and hounds should not enter private property to find the fox.  All road and traffic laws are to be observed at all times.

Fox hunting has attracted some media attention - in 2004 a short article appeared in "The Age".  There have been several TV segments - some in the news and others in lifestyle programs.

Typical  fox hunting vehicles set up for the hunt.   Two hound cars all set up
Mopunt Gambier hunt start vehicles in line Teams lining up for a start at Mount Gambier

Equipment

Receiver

The receiver needs to be reasonably sensitive and there should be enough attenuation for the signal level to be reduced to allow direction finding right up to another vehicle. Some of the more modern rigs are able to do this and a couple of teams have found that an IC706 MKII G is capable of being used by itself with no modifications.  A rig that received SSB is preferred as it allows for more modes to be  detected and has a more linear IF response.

Some teams use a receive converter - this converts 144MHz down to 28MHz with an attenuator in the lead from the antenna and another for close up work between the converter and the receiver. This can allow up to 120dB of attenuation and thus allows for close up direction finding on strong signals.

If using the wide range HF/VHF/UHF mobiles such as the IC706 MKIIG, IC7000,  FT817ND or FT857D. Apply all the attenuation possible within the rig first including turning off the pre-amp, switching on the attenuator and reducing the RF gain, then start applying external attenuation in the antenna lead.

 

Antenna

A unidirectional antenna is a must and one with limited unwanted lobes -  theory suggests that a three element yagi with 0.15 wavelength spacing of elements would be best and this has found to be so in practice. For a design of an antenna that has proved itself over the years see the VK3VT fox hunt beam.

The antenna needs to be mounted in such a way that it can be turned from inside the vehicle. In times past in Melbourne we have used a bracket on a roof rack holding a pipe as a bearing for the mast - which extends down beside the vehicle and allows the beam swinger to do just that. Changes to the road rules regarding overhanging loads meant that this arrangement was no longer within the rules so a number of teams have now constructed supports that have the antenna placed centrally on the roof with some form of linkage or bike chain to rotate the antenna.

 
Attenuator

These can be built or purchased - there are designs in the ARRL handbook using toggle switches which seem to work well provided that not too much attenuation is attempted per stage and that there is adequate screening between the stages. Note that you will need two attenuators if you have one between the converter and the receiver as well as the antenna and the converter.

 

Navigation tools

A map is essential - in Melbourne the best local street directory is the Melways and this is used almost exclusively by teams here. Most teams today also have a GPS and Laptop computer to display navigational information. Software in use varies and included home written code and packages such as Fugawi. An important feature of your navigator is that they can keep their food down while looking at the map. There have been some sad experiences here with several folk parting with their last meal!

 

Sniffers

These are hand held devices with a directional antenna and small portable receiver attached. They are used when the runners jump from the vehicle and look for the transmitter on foot. There have been a number of designs for these Ian VK3MZ had a design published in Amateur Radio Magazine and Bryan VK3YNG has developed several excellent units - he often has them for sale - contact him for details. See his website for details, including the manual.

Other designs for sniffers are around and these can be quite simple. One very simple unit can be built if you have a hand held that operates on the band you want to hunt on.  This is a mixer box that contains an low frequency oscillator and a simple mixer. The box is connected to the antenna and in turn to the radio. The radio is tuned to the required frequency plus or minus the oscillator frequency. 

With one of these, appropriate antennas and an  ICOM IC T81A Quad bander a foxhunter can  have a four band sniffer in a very compact unit. A unit such as this had been used successfully as  a two metre ARDF receiver as well as a sniffer on 6M, 70CM and 23CM

The mixer box was taken directly from the 1995 ARRL handbook and was built into a small die-cast box - using an AA battery. Full details are available -click here.  Once again a suitable antenna for two metres is the VK3VT beam -click here for details.

Page updated 13th Oct  2009  - VT


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