Hints and Tips

Carrying all the stuff!

Map, compass, receiver, ruler and pencil

So what exactly do you have to take with you? Here is a list for a 144 MHz competition:

  • Receiver
  • Aerial
  • Compass
  • Map fixed to some form of semi rigid lightweight board
  • Something with which to mark the map
  • Control Card or electronic registering device
  • Whistle

The whistle is easily hung round your neck on a piece of string. When a control card is used, it is best waterproofed (if it is not already printed on Tyvek) and then pinned to your outer clothing with safety pins. For electronic ‘punching’ at the hidden transmitter, the SI ‘dibber’ is provided with an elastic strap to carry it on the forefinger.

The receiver, aerial and compass are best made up into a single unit if possible. Home constructors have a field day building a receiver into the boom of the antenna.?The map needs to be waterproofed on the start line. If you hope to make marks on the outside of the waterproofing, the map must not move about inside its covering.

Marking the map

Unless you are blessed with incredible powers of memory you must mark some of the information about the bearings and strengths of the Foxes, onto the map. If the weather is fine and dry, you can carry the map unprotected but in the wet this is a real problem. Pulling the map out of a plastic bag, marking it with a pencil and then returning it to the bag is one option, but hopelessly time consuming.? A better idea is to mark on the outside of the plastic cover, providing the map does not move about inside the cover. Spirit based felt-tips will do this (the fine pointed ones are best) but will not work if the map is already wet. However, marks made on a dry map will not run.

There are pens that will mark on wet plastic. Those with pressurised ink tanks are good. An example is the Mitsubishi Power Tank.

Finally, chinagraph pencils will mark wet plastic and are available in a wide variety of colours.

How to decide the order in which to take the Foxes?

This is the hardest part really. Focus on the known location of the finish; try to make some sort of judgement about signal strength and make your decision. What you want to avoid is going to a Fox near the finish and then have to return, in the direction of the start, to find another Fox that you need.

Choice of Receiver

The ideal radio is a small, light AM receiver with accurate frequency setting. It should possess a very wide range gain control. In reality you will get as close as you can to this ideal. Here are some of the possibilities:

  • Use a hand held rig designed for the amateur radio market. For vhf this will almost inevitably have an FM receiver with a built in limiter. This feature makes DFing harder but far from impossible as evidenced by the hundreds of Club DFers up and down the country. Not used by any serious competitor.
  • Use a portable amateur rig suitable for the reception of AM signals. The antenna would have to be implemented separately and there is no guarantee that the gain control range or the screening of the radio will be sufficient.
  • Build one of the kits available. See the home page of this site for details of UK sourced kits and ready built receivers.
  • Roll your own, probably to a published design. In this case the receiver can be integrated into the boom of the antenna.

Multi-path on 144 MHz

VHF Multi-path propagation mapIn well contoured areas, multi path propagation at vhf has to be overcome by a successful DFer. Here is an account by David Williams of how he overcame the multi path at Transmitter #3 at Eymore Wood.

David came from the East (right of the map) and took his first bearing (nearly due north) from the top of a NW facing slope. The top of this slope ran NE to SW and TX #3 was located near the top of the slope but away to the SW. Instead of getting a good bearing to the transmitter, because the direct path through trees and around irregularities in the face of the slope, a much better signal was reflected from the south facing slope on the far bank of the stream running W to E.

Moving down the path gave a bearing skewed a bit more NW via a similar mechanism. Plunging down to the stream (not a good place to take bearings on 144MHz) the reflections now appeared to be coming from well down the stream. David charged off through the grot only to find the bearing rotated through 90 degrees when the TX next fired up. By this time the direct signal was quite good, only disturbed by the screening in the immediate vicinity of the stream.

Moving south westwards David reached the path for the next transmission only to find the TX was to his left, behind and up. He was now close enough to the transmitter that even an attenuated direct signal was stronger than the reflections and he moved up the slope and into the TX.

The moral of the tale – be aware of potential reflectors and be prepared to react to ‘illogical’ changes of direction.