Hi gang:
Ed's math analysis of the loading section of the SDRV is beautiful! I wish I
could do that kind of stuff!
My actual approach was quite a bit simpler. Since the shorted twinlead is
roughly equivalent to a lump inductance, I stuck a 2" dia piece of coil
stock at the base of the radiator, tapped down to where I was close to
resonance, counted the turns, used HamCalc pgm to figure the actual inductance, then another of HamCalc's pgms to figure out out much shorted twinlead yielded that inductance, cut the twinlead about 6' longer, and trimmed down to resonance. The end result was pretty close to the calculated length --
as Ed points out, dielectric etc affects that length, but more importantly
the conductive coating on the pole affects the calculated results.
As I explained in the article, Denny's piece of twinlead cut to 15'5" yielded
exactly the same base impedance measurements on 40m. What I like about
our cooperative work on the SDRV-234 was the duplicability of the results,
using identical dimensions, but not exactly the same twinlead --mine was
brandnew lowloss from R.S., and he had an older piece. I tried some old
junk high-loss stuff, and it was about 2' diff. on 40m, and some new
cheapie hardware store stuff, and it too was about 2" shorter. So, it seems
safe to say that the dimensions will be close enough regardless of the piece
of twinlead as far as lenght is concerned.
One important point I forgot to mention in my article regards the "critical" area as far as impact on resonant frequency. It is right at the base where the radiator
connects to the output side of the twinlead loading section. Highest current
point. Adding 2" between the output of the loading section and the radiator
throws the freq a 100kHz or so!
It was a joy to use the SDRV back in PA after Dayton last year on 30m. Worked
30m EU DX at will with good sig reports and good QSO's. So, as it stands now,
when I retire, I have a difficult choice -- either head to AZ for the sun and
fun and almost NO EU DX, to FL for fun and sun and beach and scads of
DX, or to PA for great EU DX butvery little sun and fun. Ironically, this hellhole
of -55 windchill is better to EU than from AZ! Life choices...gads!
As I noted in the article, you can't put 15-10m radiators on the pole. The mutual
coupling kills that possibility -- and incidentally, the same is true with the
W6MMA vertical because of the conductive coating on the pole. So, I though
I'd whip up a dipole to hang from the top for those bands.
The FOX (KF4AR) was very nice in SD tonight and the pack was sheer
bedlam. I love hearing all those powerfuil QRP sigs!
72, Ade
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