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.
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