In article , John S
wrote:

If you ran the analysis with the loop closed on those two bands, would
you mind doing it over with the loop open?

7.05MHz 117.3 - J 0.2996 ohms closed loop
3.525MHz 9.365 + J 111.4 ohms open loop
10.125MHz 253.9 + J 60.16 ohms open loop


Those are free-space values. I forgot to put the ground in. Here they
are revised for the bottom to be about 1.3m above ground...

7.05MHz 123.6 - J 0.02019 ohms closed loop
3.525MHz 13.57 + J 105.7 ohms open loop
10.125MHz 261.7 + J 14.89 ohms open loop

And for completeness:

14.05MHz 265.2 + J 78.03 ohms closed loop
21.15MHz 181.4 + J 286.6 ohms closed loop
28.05MHz 322.6 + J 397.5 ohms closed loop


Many thanks. That confirms what I thought, except that I had expected
the antenna to be on the capacitive side of resonance on 30 metres. All
the figures indicate that my 142.7 ft loop is just a bit too long. The
math I did when I worked for a living was pure math, but I'm all in
favour of being experimental when doing things like this. Now that I've
got a license I'll soon be buying a rig (likely an Elecraft KX3) and
before I take the thing backpacking I'll go to a nearby park and play
with antenna lengths, running a few SWR vs. frequency curves, and come
back home and do a little calculating to see how much I want to shorten
things. It looks from your figures as if it's pretty close to resonance
on 30 metres, so I'll probably shorten it a bit so as to get it too
short on 30 and still a bit too long on 20, and equally bad SWR-wise.
That probably won't change it very much on 40, and will improve it
(slightly) on 80, where the SWR won't matter so much anyway. I'll have
to see whether it improves it very much on 15 and 10, and if not, maybe
shorten it a bit more to make them better if I can do that without
making the lower frequency bands a lot worse. I don't want to fix up 15
and 10 at the expense of making the lower frequency bands, which I'll
undoubtedly use a lot more, really bad. Maybe the best thing to do is to
build a separate antenna for 15 and 10 (and perhaps 6). Anyway, it looks
as if it's time to do some experiments.

I still wish I could do something like what EZNEC does, without going to
the trouble of either buying a PC or getting something like Parallels
(which lets one install Windows on a Mac) and then buying some version
of Windows which I wouldn't use for anything else besides EZNEC. I had
hoped that CrossOver would do the job, which it did very nicely for the
Windows exam preparation software I downloaded from the Industry Canada
and from the Radio Amateurs of Canada websites. Does anyone know of
antenna modelling software for the Mac?

David, VE7EZM

--
David Ryeburn

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