80m Short Wire Antenna
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.
The ‘L’ match network used is similar to that published by AA3SJ. It is a cut-down version of the popular ‘T’ match. By operating with a single frequency & fixed ‘short’ antenna less than 8m long, we therefore only require a transformation from a low impedance output (Tx) to the high impedance antenna.
The ‘T match’ circuit can be simplified, using a coil with a fixed number of turns and a single series capacitor. The variable capacitor can be a low cost poly type (about 200pf).
A high voltage can develop across the variable capacitor. The two small trimmers on the back of the variable capacitor are prone to arching over, so it is recommended that the rotating vanes in the trimmers , are removed before assembling the ATU.
Using the ‘L’ match with a standard ATX80 transmitter, an RF antenna current of 150-180mA was measured with a MFJ-853 meter.
The LED tuning indicator is from a design by Peter Zenker DL2FI.
The 10pf capacitor can removed or reduced to make the voltage peak more obvious.
above: The ‘L’ match incorporated into a ATX 80m transmitter using a wider box (AB23).
above: Separate ATU in a small abs enclosure.
Wire Antenna deployment
A 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 gauge 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!! )
nb. Substituting a number of shorter lengths of wire for the counterpoise, (eg. 3x2m instead of a single 5m length) is just as effective, can be easier to deploy and less of a trip hazard.
References and further reading.
ShortWhip Antenna Tuner: http://www.qsl.net/aa3sj/Pages/Whip.html
Lambda/2 no Counterpoise ATU: http://www.qrpproject.de/UK/multifuchs.htm
One knob ATU by G3RJV: PracticalWireless Feb 200