by Peter Parker VK3YE - first appeared in Amateur
Radio, August 1998
Foxhunting, amateur radio direction finding
(ARDF) or hidden transmitter hunting is a fun activity where people compete to
be the first to find a hidden radio transmitter. They do this by using
receivers with directional antennas to hone in on the transmitted signal.
Apart from the excitement of the hunt
itself, those who like making small receivers and directional antennas will
enjoy it for the challenge of building equipment that is rugged, reliable and
does not give false readings. Nevertheless, constructional ability is not
required to enjoy the activity - many beginners use hand-held VHF transceivers
or scanners to receive signals from the 'fox' transmitter.
Regular foxhunts are held by local radio
clubs or foxhunting groups. Participants may go on their own or be part of a
team. Hunts are conducted either on foot or in vehicles. An amateur
transmitting licence is not necessary to participate.
A variant of foxhunting is Amateur Radio
Direction Finding (ARDF). This is a rapidly growing international sport and
calls for a degree of physical fitness not possessed by many amateurs. Details
on ARDF are presented elsewhere in this article.
Most foxhunts use the two metre (144 MHz)
band. The national ARDF frequency is 145.300 MHz, though some groups still use
other frequencies. There is also some ARDF activity on 3.5 MHz. Low power
transmitting and receiving equipment for this band is very easy to build.
Compact directional receiving antennas for 3.5 MHz are also interesting
projects. Because most amateurs already own a portable VHF scanner or
transceiver, this article concentrates on foxhunting on the two metre band.
Except for ARDF, which is an international
sport (see end of article), local foxhunt groups set their own rules. These may
include things like driving carefully and requiring that the transmitter be
within a certain distance of the starting point. Other rules are fairly
informal.
The person setting the fox goes off and
hides the transmitter. Meanwhile, participants ('hounds') gather at the
starting point. They may monitor a local repeater for liaison purposes. When
the transmitter has been hidden, the fox setter switches it on and announces
that the fox is transmitting and that the hunt has started.
Hounds first need to know which direction to
travel. They madly swing their beams around until they can get a bearing on the
fox's signal. They may then consult a map and start heading in the direction of
the signal.
The first individual or team that finds the
fox is the winner. Those who have seen the fox transmitter walk away from it to
avoid giving clues to following teams. The transmitter is turned off when the
last hound finds the fox or announces on the liaison frequency that they have
given up. The winning station or team is then entitled to set the next fox.
Either another hunt is run or participants may socialise at a participant's
house or cafe.
The transmitter used in the foxhunt must be
compact and rugged. Its frequency should be stable (crystal control is ideal)
and be able to run for several hours off a nicad or sealed lead acid battery.
RF output powers as low as 20 milliwatts are satisfactory for pedestrian-based
events of a few hundred metres. Higher powers (1 to 5 watts) are better for
longer hunts. A fox transmitter with two or three RF output power settings is
desirable to save power and/or fool the competitors.
Because many people will be using FM
receivers, it is desirable that the fox's signal is frequency modulated with an
audio tone. This tone can be keyed to transmit a Morse ID. Programmable ICs are
often used to send the Morse. However, 20 second digital message recorders are
so cheap nowadays that these are the logical choice for those wishing to build
a Morse or voice ID for a transmitter.
Antennas for fox transmitters can be almost
anything. A quarter wavelength piece of wire is recommended for beginners.
However more experienced groups have used fences, bridges or sheds as antennas.
The use of directional beam antennas can also be worthwhile. This is because
they can fool competitors into thinking that they are very close to the hidden
transmitter. Also competitors can be given misleading bearings by orienting the
beam so that it bounces the signal off a large building or hill some distance
away. Other interesting effects can be had by experimenting with the antenna's
polarisation. Effort should be made to camouflage the antenna and feedline to
make finding the fox harder. For example, a tree branch and fencing wire can be
made into a yagi antenna that is almost invisible when concealed in a tree.
Similarly, a wire antenna could be dunked in a lake or river.
This is a matter for the individual
competitor. The equipment used depends on whether the hunt is vehicle-based or
pedestrian-based.
Competitors in vehicle-based hunts typically
have some sort of steerable antenna mounted on the car. Some keen hunters have
bored a hole through the roof of their vehicle to allow for a rotatable pole
for the antenna. Others use an antenna on the roof rack or a vertical piece of
dowelling protruding through a passenger window. This last suggestion is
preferred for those without beam heading indicators installed for reasons
explained later.
A two or three element quad or yagi is the
most common choice for competitors. This should be optimised for maximum
front-to-back ratio rather than forward gain. A sharp null off the back or side
can be very useful in direction finding. A suitable antenna was described in Novice Notes February
1997.
It is important to know the direction that
the car-mounted beam is heading. Some people use remote-control motors and
indicators. However, this is not necessary for the beginner. A simple approach
that works well is to have a nail knocked in to the side of the antenna support
dowel that faces the direction at which the antenna is aimed. This method can
course only be used where the antenna support dowel protrudes through the
passenger window.
Inside the vehicle is a switchable RF
attenuator. This is used when the signal from the fox is very strong but you
still need to get a bearing. Descriptions of suitable attenuators appear in the
standard handbooks. Good quality construction is important to reduce signal
leakage.
Lastly there is the receiver. This should
have an s-meter so that it is possible to get an indication of the strength of
the received signal. SSB receive capability may also be desirable. A reasonably
small multimode two metre transceiver (eg Yaesu FT290R) is ideal for this
application. Alternatively, a home-made receiver with a variable tone output to
indicate received signal strength could be used instead.
The equipment mentioned above is of course
the ideal. However, do not be put off if all you have is an FM handheld
transceiver - foxhunts have been won by stations using these as the receiver.
Tuning off frequency is sometimes a useful technique to effectively attenuate
the received signal.
In many cases, vehicles cannot be parked
close to the fox's hiding spot. Alternatively, signals may be so strong as to
render the vehicle-mounted direction-finding system ineffective. The solution
is to use a hand-held 'snoop loop'. This consists of a hand-carried two or
three element yagi, attenuator and simple receiver. This may either be a
handheld transceiver, portable multimode transceiver, or home-made receiver.
Especially important is effective shielding to prevent leakage into the
receiver other than through the antenna connection.
Pedestrian hunters are limited by the size
and weight of equipment that can be carried, especially if the walk will be
several kilometres. A compact multi-mode transceiver or homebrew receiver,
attenuator, two-element yagi and map are all desirable for the pedestrian
hunter. If the attenuator is built properly and the transceiver is
well-shielded, such equipment can be used to locate the transmitter to within a
metre.
Often the last hundred metres of a fox hunt
can take much longer than travelling the several kilometres required to reach
the general vicinity of the transmitter. This is particularly so if the
transmitter, feedline and antenna are well hidden and signals are strong.
Effective triangulation of the location of the fox (including searching up and
down, using horizontal and vertical polarisation and careful observation) is
important here. It is quite possible for a team to be the first to reach the
general area but squander this advantage to later arrivals by having poor
equipment and/or poor powers of observation. Always remember that the signal
radiates from the antenna and not the fox transmitter. Thus all bearings will
be towards the antenna. For this reason, the antenna is often the first part of
the transmitting equipment located and you will need to follow the feedline
along to find the transmitter itself.
To many, finding novel and unusual hiding
spots is the best part of foxhunting. There is a peculiar pleasure in hiding a
transmitter that takes other people several hours to find. If you join a
foxhunting group or team, you will hear many anecdotes about past hunts where
transmitters were hidden in strange places. The following are a few ideas for
those whose turn it is to hide the transmitter.
Where possible foxhunts should be held where
there are concentrations of people. Examples include main streets, shopping and
restaurant areas. The reason for this is to increase the visibility of amateur
radio and foxhunting in the general community. Pedestrian foxhunters are
normally in a better position to answer questions from the public than those in
vehicles.
Amateur Radio Direction Finding or ARDF is a
form of orienteering which was started in 1933 by the Swiss Army. Since then it
has become very popular throughout the world. International competitions are
held every year, mostly in Europe, and a World Championship every two years.
International competitions are held over a 4
to 7 km course. A total of five transmitters are to be found within a set time
period of about 120 - 140 minutes. The competitor with the fastest recorded
time is declared the winner, provided all transmitters are located.
The only assistance given is a detailed map
of the area with the start and finish only marked on the map. A compass is a
necessary piece of equipment. For a team event the times of the members of the
team are added together and once again the lowest time would be the winner.
ARDF requires competitors to have reliable
equipment, be physically fit, be able to interpret beam headings and read maps.
It combines electronic, map reading and physical skills in the one activity.
All the transmitters are on the same
frequency but do not transmit all the time. Instead they are switched
sequentially so that only one transmitter is on air at any time. Each
transmitter comes on for one minute every five minutes. Transmitters send a
simple Morse code signal so that competitors can identify each one.
At each transmitter there is a punch which
is used to mark a card the competitor carries to show that the transmitters
have been found. Transmitters can be found in any order.
There are several categories for the
competitors. These are:
Every five minutes a group of competitors
start one from each of the categories above. Each category is required to find
a different number of transmitters, so that following someone is not
necessarily a good idea!
Two Amateur bands, 3.5 MHz and 144 MHz, are
used. Receiving and transmitting equipment is readily available, however the
transmitters must be controlled by a licensed amateur radio operator.
Information regarding simple receiver and
transmitter kits can be obtained from Ron Graham VK4BRG, PO Box 323, Sarina,
Qld, 4737, telephone (07) 4956 1155.
Abridged from material supplied by Wally
Watkins VK4DO
I would like to thank Wally Watkins VK4DO,
Ron Graham VK4BRG and Neil Pickford VK1KNP for their assistance in the
preparation of this article.
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This page was produced by Peter Parker VK3YE parkerp@NOSPAMalphalink.com.au. Material may be copied for personal or non-profit use only.