Once you get into QRP, you'll find yourself thinking more and more about antennas, and that's what happened to me. I like to operate my QRP rigs when I go camping, or maybe just in the local forest preserve. It was a natural thought to wonder about what kind of antenna would be lightweight, takes up little space, is easy to set up, and works great. That's a tall order for any antenna.
Then I read Rich Wadsworth's (KF6QKI) article about the "Twin-Lead Dipole". It didn't dawn on me at first, but after a while I began to think this might just be the design I was looking for. The details of the antenna are described in Rich's article which appears in Feb-2002 QST, p 36-37, so I won't duplicate that. Instead, I'll just give a brief description of the antenna and how I use it.
Physically, the antenna consists of a piece of 300 ohm twinlead which is split into the two legs of the dipole at one end, and fed at the other end directly by the rig. The secret to this antenna is that the twinlead feedline is exactly a half-wavelength at the frequency of operation (14.060MHz for my application). This results in the feedpoint impedance of the dipole legs being reflected to the QRP rig. So if you can achieve a 72 ohm resistive impedance at resonance for the dipole, you will see that same impedance at the rig end of the feedline. That's about a 1.4:1 SWR.
I actually use my dipole in an inverted vee mode. This has a couple of advantages. First, I only need one tall support for the center of the antenna, and I can tie the two ends to stakes in the ground using 1/8" nylon rope (see sketch below). This makes it very easy to setup. Second, since the legs of the dipole are at about 90 degrees, the impedance drops to close to 50 ohms, which makes for an even better match with the rig.
This has become my "go-to" antenna for 20m portable operation; I always take it with me. When there is no tree branch at the right height, in the right place, I fall back on my homebrew PAC-12 vertical.
Hope this has been of some help, and if you have any questions, please drop me a line.
73,
ed - k9ew