[Amps] MOSFET amp filtering - was: auto-tune

[Jim Thomson]

Date: Mon, 12 Dec 2016 13:32:16 +0000 (UTC)
From: Catherine James <catherine.james at att.net>
To: Amps group <amps at contesting.com>, Bill Turner <dezrat at 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