I cut my top loaded "T" (85' at top, 10' high feed, elevated 8x 125'
radials) for 1825. Then 3 equal value series capacitors are shorted by
PCB relays (10a, 5Kv coil insulation) to switch in C, C/2, and C/3
(C=4000pf for this antenna). That yields 4 by 40KHz spaced segments
with > 50KHz 2:1 bandwidth for full band coverage (I don't cover above
1980). The feed is 25 ohms with a TLT with a 1.1:1 resonance swr, and
the caps are on the low Z side (higher current, lower voltage). I
paralleled silver micas to get an adequate current rating. EZNEC shows
the C, currents and voltages needed, all reasonable with the correct
micas. The relays never hot switch and are mounted in a plastic water
tight box at the feedpoint. So far so good at QRO and 3 years of casual
operations. I used a 4 position rotary switch and some diodes to select
the relays. My amp has a tuner that is ok < 2:1 swr, but the idea (not
mine) can be morphed to whatever frequencies desired if a tuner isn't used.
Grant KZ1W
On 2/16/2016 6:50 AM, Tim Shoppa wrote:
Jim, I mentioned relay cut-in of L's and/or C's at the base of an antenna
to help cover all of 160M. On my tuner web page I mentioned does not show
this recent update I have made at my shack, but I have a relay that shunts
out turns of my 160M coil to help me QSY to top end of 160M phone in
contests.
Another choice that I left out, if you want to QSY all the way between 1800
and 2000 kc with a limited-bandwidth antenna, is relay-cut-in-and-out of
extra length at the top of the antenna. There have been QST articles, as
well as some write-ups on this mailing list, about doing this. The
electrical voltages involved, as well as complications of remotely
controlling relays at the top of an antenna, have made this a non-choice at
my station. I'm very self-satisfied about my relays at the base of my
antenna but the complications of high-voltage relays at the top of an
antenna, is not something I've done myself.
Tim N3QE
On Tue, Feb 16, 2016 at 9:05 AM, Tim Shoppa <tshoppa@gmail.com> wrote:
...snip..
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