Simple QRP Doublet Antenna

Here is the antenna and feed line I made this weekend. It is designed to be used with an ATU for portable QRP use on 20, 30 and 40 meters. DO NOT USE THIS ANTENNA NEAR OVERHEAD POWER LINES. The doublet and feed line are made from two pieces of wire, each 75 feet long. 25 feet of each wire forms the radiator, and 50 feet of each wire forms the feed line.

There are no mechanical or soldered connections between the feed line and antenna since each side is one continuous piece of wire. At the center of the doublet, I used a couple wraps of plastic electrical tape to hold the insulated wires together.

The parallel feed line is simply the insulated wire taped together at 2'-6" intervals and runs down the mast. There is 50 feet of feed line. If I want to make the doublet longer, all I need to do is un tape the center of it and re-tape it at the selected length. For example, I could make the doublet 100 feet long and the remaining parallel feed line would then be 25 feet long. No cutting or soldering of the wire.

The wire is the 20 gage underground dog fence wire. The insulation is UV resistant HDPE. Since the PFR-3 has a tuner, I figured it would load on all three bands, which it did. I got a good match (dark SWR LED) on all bands and made a couple good contacts, on 40 and 30m.

No, the feed line conductors do not use spacers and there is not an intentional air gap between them, and so it likely does have some slightly greater attenuation than a traditional open-wire feed line due to the dielectric properties of the HDPE insulation and closely spaced conductors. But as far as I could tell from reading, the maximum attenuation would be no more than say 4db per 100 feet on 14 MHz, and less on 10 or 7MHz. Since the feed line is 50 feet long, the maximum signal penalty would be about -2db (maybe -1db on 40m) and so a useable signal to noise ratio can easily be maintained at the receiver/transmitter.

The first contact on 40 meters using this antenna was with NW3DC, Washington, D.C. (610 miles). My first contact using this antenna on 30 meters was W4JFR/Jim in Troy, Alabama, a distance of 690 miles from my statiion.

The impedance of the feed line might be on the order of 100 to 150 ohms, but with the tuner being able to match the line, apparently most of the RF is going to the antenna. I was going to make the feed line twisted, but then realized I would have more flexibility with the incrementally taped feed line, and I couldn't think of a reason that local/random "imperfections" of the line would make any practical difference.

The support is a piece of PVC with a short metal extension at the top and is about 18 feet tall. The ends of the antenna droop down to tree branches within reach from ground. With 50 feet of feed line, I had to route all the excess on the handrail of the deck and would trim it to length for a permanent installation, but in the spirit of experimentation I will keep it this size for now so I can add and subtract length to the doublet for fun.