Wednesday, June 01, 2011

20/11/10m.monoband end fed halfwave antenna

Working portable demands an easy setup of your antenna. The antenna also has to give enough punch at a low radiation angle without the need of radials.
The vertical end fed half wave antenna is my choice for it’s easy to employ and it's a perfect DX antenna . This antenna is grounded thus giving les static noise to your receiver.
A 20m. version is about 10 m. long and has aprox. 3000 Ohms at the feedpoint. So we need a simple tuner to reduce this to 50 Ohm .
Put 20 turns ( covers half of the toroid ) of 1 - 1.2 mm enamel wire on a T130-2 toroid with a tap at 3 turns from the ground .The first picture shows the first test setup with a 2.5 turn tap. You also need about 25 cm of thin Teflon coax (RG 178 b/u) that will act after tuning as a high voltage capacitor of about 22 pF.
The pictures explain the simple construction. The cabinet is a PVC type G302 made by Velleman.




Tuning : The trick to tune this transformer is to find the working frequency first by tuning the capacitor. Then we look for the the 50 Ohm point on the coil .  Connect a 3300 Ohm resistor from the top of the coil to the ground of the antenna plug.
Adjust the length of the open coax line until you reach the lowest VSWR in the centre of the band with an analyser. The coax will be about 23 cm in total resulting in  22pF .
Now remove the resistor and connect the antenna wire of aprox. 10 m. PVC coated copper wire. Tune the antenna for best VSWR by reducing it's length carefully. My antenna is 9.9m. long and is mounted on a 12.5 m. fishing pole. A near 1:1 VSWR  on 14.200 MHz can always be achieved ! Before tuning , fix the toroid with some hot glue and wait fore it to cool off. Beware of the high voltage at the antenna feedpoint , this can rise up to almost 500V at 100 Watts.

10/11m version
For 10/11m band  use 16 turns 1-1.2mm enamelled wire on a T130-6 toroid with a tap at 2-3turns.
A 15 cm piece of coax forms the capacitor. Antennawire length is aprox. 5.5m .

My first DX contact with a vertical half wave end fed antenna was with VK6MV in SSB using the long path,  with a 5-5 report.
 I was also able to work  Japan,Taiwan, Thailand, India, Indonesia, South-Africa, The Falklands ands. in SSB with good reports. Every end fed / vertical needs some kind of counterpoise to push against and in this case that's the coax , so don't forget a line choke near to your transceiver. 
Good DX !

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