The VDA or Vertical Dipole Array - a simple & awesome wire antenna
The VDA is a mono band antenna, often used by DXpeditions, for a good reason.
It consists of 2 elements, the driven element and a parasitic one, the reflector. This endfed halfwave gives a very low take off angle and is very suited for chasing DX.
The design can be optimized for either front to back attenuation or fwd. gain. My version gives approx. 4dB fwd. gain and max amplitude at 19 degrees.
The red element is the driven one, and you can see the impedance matcher in the white enclosure are the bottom right. The VDA is in fact a 1/2 wave radiator, so the feed impedance is very high, ~2500 ohm for my version.
This could seem like a stupid idea compared to the 36 ohm on a GP ant, but it has one crucial advantage. A very minimalistic ground plane need.
Due to the high impedance, next to no current will run to ground so in most scenarios you coax cable on the ground will suffice. I tend to use a single radial layed out in the signal direction.
20 meter version.
You will need 2 pieces of wire, one 9,4 meters for the driven element and one 10,4 meters for the reflector. The crossbar in the middle is 4 meters in total, and the 2 small ones at top and bottom are 50cm long.
I've build mine on a 10 meter heavy duty fibreglass pole.
Then all you have to do is build a matching device. Theres lots of info on the net, but the easiest one is the 1:49 toroid balun. This will divide the 2500 ohm feed impedance to match your 50 ohm TRX.
Twist the wires for the first 2 turns tightly, this will give better performance. The capacitor can be omitted, it will however give a better match on 10m.
