Simple direction finding

The fox hunt will be carried out on 2m probably 145.425MHz if clear channel. A simple direction antenna will normally have two orientations at 180 to each other at which maximum signal is received, (You could use a multi-element narrow beam Yagi or similar so one lobe was much more sensitive than the others, but this would be rather cumbersome to lug around). Having orientated the antenna for a maxima at your current position, you draw a line through your current position on the map in the compass direction of the antenna's maximum. This will pass through the fox, but at this stage you do not know which direction along the line the signal originates. Move position by a few miles and repeat. You now have two lines and where they cross is roughly where the signal is originating. Look at the map and decide where to go for two new positions nearer to the signal where you can produce two new lines that are likely to cross at about 90deg at the rf source ( this gives maximum positional accuracy), lastly move again to add a third position and you will have the triangle of probability formed by the crossings of all three lines. (Three are used to eliminate any spurious reflections with either of the first two). Navigate to the centre of the triangle formed on your map. Using the directional antenna at a max walk slowly towards the source using the s meter to fine tune your pacing.) Shoot the fox. (Metaphorically of course).

 

eg We move to Casswell's Bridge and the first antenna maxima gives us line A see fig. 1. The signal could be coming from the West or East of us.

 

fig1

 

We relocate to Middle Farm and get direction line B see fig. 2 this means the signal is from the East of us line A and B diverge to the West but cross to the East.

fig 2

 

We now move to a position that will let the third line cross A or B at 90 deg (ish) see fig 3, The three lines form a triangle and the source of the signal is most likely to be at the green dot X in the middle of the triangle. We move to X and use our s-meter and eyes. Walking past a terrace of houses, with no antenna on a hand held, will usually give a maxima outside the house from which a signal originates, however, on our excercise the signal will be outdoors and on legally accessible land.

fig 3.

 

Manufacturing a suitable antenna

You will need - sorry image compatability problem will be resolved just as soon as possible - est mon pm