This is a copy of an article I wrote for the Radio Officer’s Association. 

The Horsham Amateur Radio Club hold four “Fox Hunts” every year, two evening ones in the 
summer and two Sunday morning ones in the winter. These days we use the 144MHz (2m band), although in the past 1.8MHz has been used, which has more similarity to the direction-finding systems we were used to at sea. 

The equipment needed for a 144MHz Fox Hunt is, a 2m receiver/transceiver, a directional aerial, usually a Yagi or HB9CV and most importantly an attenuator, as most handheld transceivers are very sensitive and have no RF gain control and are poorly screened.  Ordinance Survey maps of the area, a rule and pencil to plot the bearings and a compass to ascertain which direction your antenna is pointing when taking bearings. Signals are transmitted in FM and the strength can be determined by the bar graph on the transceiver or by listening to background noise when using the attenuator.

The way the system works is as follows: one person is nominated as the Fox, and he finds a suitable location up to 25 miles distant for the daytime fox hunts, and no more than 15 miles distant for the evening fox hunts from the designated start point. The Fox makes sure by reference to propagation software or by checking himself, that his signal can be received at the start point. The transmitter is usually located in a car with a vertically polarised aerial, but this may not necessarily be the case, especially during the evening transmission where the Fox may be closer to the start point using a handheld. 

The participants assemble at the start point and are handed a sealed envelope with details of the location of the Fox, also the location of the pub to meet afterwards.  This envelope is handed to the Fox still sealed when he is found and the arrival time noted. 
  
Transmissions from the Fox are of two minutes duration and are repeated every ten minutes for two hours on the daytime hunts and ninety minutes on the evening ones.  All participants take a bearing at the start, mark the direction on the map and set off in that direction in their car.  The next thing to consider is where you can stop just before the next transmission to take another bearing and check on the accuracy of the first bearing. 

Accuracy of bearings and signal levels on VHF are very variable and may be affected by elevation, objects in the path, polarity and reflections of the transmitted signal from various things like buildings, overhead cables and even hills, and it pays to rotate the antenna between vertical and horizontal to obtain the best bearing.  In recent years sat-navs have become very useful as you can locate your own position at any point when taking a bearing. 

In our Club the member that completes the Fox Hunt first must be the Fox next time round so this stops the same person constantly winning. On the Sunday morning hunts the Fox also must find a pub near his lair that can be booked for Sunday lunch for those that require it. I reckon coming second is the best place to be!

There is another method of direction finding which uses a Doppler shift system (a change in frequency caused by relative motion) to determine the direction of a radio signal source. 

A typical doppler DF system consists of: 

• A set of four ¼ wave antennas (one in each corner of the car roof) that are sequentially switched by a control module to simulate motion by electronically switching or rotating the 
  signal reception. 
• As the antennas "move" around, the received signal experiences a cyclic Doppler shift - higher frequency when approaching the source, lower when receding. 
• This shift creates a sinusoidal pattern in the received signal. 
• The phase of the sinusoid is read by the control module and a display (a ring of LEDs) indicates the bearing (direction) of the signal source with reference the direction the car is 
  moving. 

There are a small number of amateur equipment/kits available on the internet, but we have no experience of them. 

Many years ago, I took part in a Fox Hunt with John G4HUK who then worked for a certain government agency and he ‘borrowed’ one of their Datong Doppler systems for the weekend. The 
unit comprised of an electronic aerial switching box connected to a control/display unit. All the user supplied were the antennas and the FM 2m radio. The four antennas were connected in order to the four switching ports on antenna switching box and the fifth port was connected to the radio antenna input. An audio lead was then connected from the radio to the control unit.

Once this was all placed in the car, calibration of the system was required, this involved standing directly in front with a hand held and setting it to transmit on the wanted frequency and checking that the ring of LEDs indicated in the forward direction, you then walked around the car in a circle to ensure that the LEDs followed you round, if they didn’t you knew that the antennas on the roof of the car were in the wrong order.

How does it work (The Doppler Principle), a VHF Doppler Direction Finding (DF) system accurately finds the direction (bearing) of radio signals using the doppler effect. A system uses four fixed 
antennas and electronically rotates or sweeps the incoming signal around the four antennas at a fixed frequency, 300 times per second (300Hz). As the car moves towards the incoming radio wave, the frequency appears higher (compressed waves); as it moves away, the frequency appears lower (stretched waves). The receiver picks up these frequency shifts, producing a distinct audio tone at the same frequency as the antenna's rotation (the "sweep frequency"). In the control box the phase of this audio tone, compared to a reference signal, directly indicates the bearing (direction) of the radio source on the ring of LEDs. 

So, all you need to do is drive in the direction of the signal by keeping the LED indicator facing forwards. The system worked very well, allowing us to find the fox in 12 minutes! This was all very 
good, but to be honest, took all the ‘fun’ out of DF hunting, so we took the decision to ban the use of doppler DF systems from HARC Fox Hunts, as such doppler systems took away ‘skill’ required to locate a hidden station, besides what can you do for a couple of hours waiting for the pub to open! 

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Adrian – G4LRP