[TowerTalk] antenna impedance and amplifier matching

[Jim Lux]

I think the issue is more about design of the amplifiers and what the 
expectations are.

The comparison is being made between a narrow band tuned tube amplifier 
and a broadband untuned SS amp. There's going to be differences.  And in 
the ham world, you're talking about a cost sensitive application; you're 
not going to see automatic matching networks or isolators or designing 
so you're not on the peak performance Zout.

It's more of a system engineering thing.  Hams have been happy for years 
using antennas with 2:1 VSWR and an amplifier that has a tunable 
matching network to drive it.  The run of the mill 100W SS output PA 
these days is designed to take a fair amount of mismatch without having 
severe problems, and they have protection circuits so that the output 
power is reduced.

you CAN build a high power SS amp that will drive a 2:1 mismatch, or a 
3:1 mismatch or whatever.  But it will be expensive (go check out the 
products from Amplifier Research, for instance).

Or you can use a fast autotuner (which you could put at the feed, so the 
feedline loss is less).  That lets you build an antenna that has a 
feedpoint Z that is less wonderful, but might have better F/B or gain 
over the band.

 From a functional standpoint with respect to amplifier/antenna 
matching, there's not a lot of difference between a SteppIR, a tube amp 
with an adjustable output network, or a SS Amp with an autotuner.  It 
just moves where the "adjustable" part is.  Some of this is driven, 
also, by the odd way in which amateur station output power is regulated 
(e.g. there's a limit on transmitter output power)  That means there is 
a bias to "matching inside the transmitter", because any losses don't 
count against the power limit.

If you want the ultimate in no adjustment.. a 3 dB pad in series between 
the amp and the antenna will present no worse than 3:1 VSWR at any 
frequency and any load impedance.  6 dB pad (12 dB return loss) is 
1.67:1 VSWR.  Build yourself a 15kW SSPA, put a 10dB pad inside the box, 
and you've got a legal limit amp, ready to go, that will tolerate pretty 
much any load you care to hook up to it.



With respect to how this sort of started, and log periodics.. you can 
make a LPDA have any arbitrary feedpoint impedance, and any arbitrary 
smoothness of VSWR.  It's all about how many elements you want to put in 
it.  So manufacturers pick some *reasonable* value for VSWR ripple and 
performance and size/weight, and that's what they build. Given the large 
non-ham market for HF LPDAs, I suspect that the impedance of the 
antenna, with a couple hundred feet of some common transmission line, is 
perfectly compatible with some off the shelf HF transmitter from Harris, 
Thales, etc., intended for use on the roof of an embassy or similar.