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Whip AntennaA traditional 80m ARDF antenna is 8m of wire, suspended from a tree branch, with one or more 8m radials & feed from an ATU tuning unit.Potential problems with this solution.
A 6m fiberglass roach pole is used to support a vertical wire with a loading coil wound directly on a lower section of the roach pole The coil being approximately 1m from the bottom of the pole, with 68-70 turns of 24-26 swg wire, determined using the MFJ antenna analyzer. No radials are used! A standard 10mm copper covered steel earthing rod will penetrate the hardest ground, provide a reasonable ground connection and support a roach pole, which is sleeved over the rod. This arrangement gave an SWR better than 3:1 and a reliable, good workable signal. Proven on a number of events in 2006 with a 3watt ON7YD/ATX-80 transmitter, and using no additional tuning device. It should be noted however, that this design has been depreciated for active use in favour of the tuned short wire devices presnted below. 80m antenna erection at the Butterwood ARDF event April 2006
Short Wire AntennaThe obvious disadvantage of the roach pole is deployment of the relatively bulky and heavy components also, variations in the environment can easily detune the antenna from its preset optimum.Experiments with foxOring micro transmitters using antenna 2m long or less, showed that a very short tuned, resonant antenna could be usefully detected over far greater distance than expected. As a result, various options to match a simple 5m wire antenna were investigated. At 80m, any antenna using a wire length of 8m or less , has a high input reactance with a very low radiation resistance. A suitable matching device for this configuration, is the parallel tuned ATU, similar to that used in the ON7YD 80m micro transmitter and the W3EDP balanced tuner. Ken M0AET, was the proactive developer of this system shown below.  A 5m wire and counterpoise is used with an out-board ATU built into a small plastic box, 3"x2"x1".   (note: this will only work in a high impedance network) Using the parallel tuned circuit described with a MFJ current meter, an RF current of 120-150mA was measured. This antenna and ATU system was successfully used throughout 2009, including the British championships An alternative is to use the 'L' match network as suggested by AA3SJ. Effectively, a cut-down version of the popular 'T' match.   The 'L' match was built into a 80m transmitter as shown. It shows how a wider box (AB23) can accommodate an integrated ATU.  Using the 'L' match, with a MFJ current meter, an RF current of 150-180mA was measured. The LED tuning indicator is from a design by Peter Zenker DL2FI. The 10pf capacitor was removed to make the voltage peak more obvious. However, of the two LED voltage indicators described above , the M0AET design gives a more pronounced peak. Wire Antenna deploymentA 6m roach pole is used to 'lift' the 5m antenna wire to a suitable tree support, with a specially designed hook bent from a wire coat hanger or similar guage wire. The hook slots in the highest hollow section of a roach pole. (The top section is solid, so first needs to be removed).A firm tug on the wire will un-spring the hook from its support ( watch-out for the falling hook!! )  references
last updated 01 Jan 2010. | ||||
