Date: Mon, 12 Dec 2016 13:32:16 +0000 (UTC)
From: Catherine James <catherine.james@att.net>
To: Amps group <amps@contesting.com>, Bill Turner <dezrat@outlook.com>
Subject: Re: [Amps] MOSFET amp filtering - was: auto-tune
I hear a lot of recommendations here for antennas that are flat across the band
and very close to SWR of 1:1 to keep solid-state amps happy. That is
completely unrealistic on the low bands.
I have to re-tune my 160 meter dipole for even small excursions of a few tens
of kHz. A cage dipole would be broader-banded, but that is difficult to build
and install, especially for a 270 foot long antenna that hangs in the trees. I
certainly couldn't put one up in the treetops where my current wire dipole
lives, as it would get all tangled in the branches.
So much of discussion and recommendation around antennas seems to unconsciously
assume that we are talking about the high bands. I've lost count of the number
of discussions where someone asked for a reco on an HF amp, and was told over
and over, "don't start with an amp, improve your antenna system, put up a beam,
etc."
At this point in the cycle, I am spending more and more time on 160, less on 20
and 40, and essentially none at all on 10 and 15. Few hams can put up a beam on
the bands below 20. The longer the wavelength, the wider a given band will be
as a fraction of that wavelength, and the less broad-banded the antenna will be
without tuning. Tuning is a fact of life, and amps are more important on bands
where the ham cannot have a rotatable directional antenna.
160 has been amazing lately. 10 doesn't appear to have opened at all for the
contest last weekend, at least not here in New England. And 160 is where you
really, really want an amp!
73,
Cathy
N5WVR
## actually its pretty straight forward to shift a 160m dipole up or down
in resonant freq. A small DPST relay is used to shunt out either a small coil
in each leg , right at the feedpoint, or shunt out a small loop on either side
of the feedpoint.
This assumes a full sized 160m dipole, that is coax fed. 1 relay will
provide for 2 x segments. 2 relays will provide for 4 x segments. 3 relays
will provide for 8 x segments. 4 relays will provide for 16 segments.
The voltage for the relays can be fed up the coax, so no control cable
required.
You can also achieve 7 x segments by using 2 SPST relays in one leg and just
one SPST relay in the other leg.
Relays + coils is the easiest method. Since the relays are only at a 50 ohm
point, the peak V across their contacts is very low.
Relay contacts will easily handle RF current on 160M.
Same trick can be used on 80m. On 40m, swr is not usually an issue with full
size dipoles. Use 6-8-10 gauge wire for max BW, then no cage required.
6-8-10 gauge works good on 160 + 80m too, albeit heavier..and more $$.
A fan dipole will also work. 2-4 dipoles with a common feedpoint. But All
stagger tuned. Then no relays etc. But u end up with more weight and $$.
Jim VE7RF
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