Saturday, February 21, 2015

Tips for 80 meters

I've had this 40/80 meter trap dipole for a couple of years:
Shown coiled-up for shipping.  In use those four loops are uncoiled and straightened out.

Works great, especially on 40 meters, with an SWR < 1.5 across the entire band.  The big advantage is that it can also work on part of the 80 meter band while remaining yard-sized.  A full-on 80m antenna is about 135 feet long, but by using some LC resonant voudou (that would be those funky PVC-core coils in the above picture) this thing is a good 50 feet shorter.  Back to the qualifying phrase "can also work on a part of the band," the downside to this contraption is that it only works on about a fifth of the 80m band, maybe a third of the band when persuaded by my transceiver's little 3:1 tuner.  The sensible thing to do in this case is to set it and forget it in the sub-band you mostly want to work, which I've done up in the General-class phone end of things, 3.8-4.0 MHz.  That's where the action is.

But I occasionally want to use PSK31 on 80m too, and that's centered down on 3.580 MHz.  I could go through an hours' exercise in adjusting the outer ends each time I want to change sub-bands, or just:

clip on an extra length of wire to each end of the dipole to lower the resonant frequency.  In practice, it came out to about 12" for these tip wires, but if you're tuning from scratch start with about 20" and trim it down a couple of inches at a time.  The black clip on the left attaches to the end of the antenna; the red clip on the right holds things stretched out with paracord and a little ceramic insulator.  Once everything's tuned up, it takes about 2 minutes to step into the back yard and attach/detach the tips to switch sub-bands.  It even pulls in the bottom-end of the Extra-class phone band, a nice bonus.

It took an hour or so to trim everything just right, but it's worth it to get PSK31 coverage on 80m.  Because PSK31:

It's kind of like The Matrix, only much, much nerdier.

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