Rich,
It's an issue that's not usually seen in the broadcast engineering world
where one frequency is transmitted for broadcast. Typically a transmitter
will fold-back delivered power when its output Z is fixed (e.g., 50 or
70-ohm) and SWR exceeds some predetermined amount set by the manufacturer.
This is typical of broadbanded solid-state amplifiers with a fixed output Z
that use no output matching network. These transmitters are designed to
work into just one line Z and hence a strict maximum SWR at the
transmitter's output terminals.
It's possible to recover and re-direct the reflected wave (created by the
line to load mismatch) back to the load if a network is used to tune the
line for reactance cancellation and match the output Z of the transmitter.
The usefulness of this is highly dependent on line loss. In the ham's world
where low-loss open and balanced feeders are often used, it's quite common
to have a 20:1 line SWR, yet attain 90% or better transmission efficiency.
Of course, as line loss increases, the ability to take advantage of this
approach becomes less effective. By using a network directly after a solid
state, fixed Z transmitter, the line Z can vary wildly, but the network
just installed will manage the tuning and matching function to ensure the
transmitter is matched into the network.
Back to the broadcast world: early AM stations would use multi-conductor
open feeders with no ATU at the vertical tower's base -- nor in cases of
station's still using flat-top Marconi "T"s. For normal AM broadcast (but
not C-QUAM stereo or IBOC), the entire tuning function is quite capably done
at the transmitter even hundreds of feet away from the vertical. Line SWR
is high, but it matters little as line loss is extremely low at MW. Of
course with high SWR, voltage handling between conductors and insulator
spacing needs to be well-managed.
As directional systems became necessary along with increased use of coaxial
lines, it meant that each tower needed its own ATU at the tower base to
perform an exact line to feed Z match -- with all phasing conducted in a
common phasor cabinet located near the transmitter -- a universal set-up
we've seen used probably since the late '30s, maybe even earlier.
Paul, W9AC
-----Original Message-----
From: Richard Fry
Sent: Friday, December 05, 2014 6:54 PM
To: topband@contesting.com
Subject: Re: Topband: EZNEC 5.0 +
We can have 67% reflected power and still have nearly 100% of transmitter
power getting into the antenna and being radiated.
Then could someone please explain why the manufacturers of ham, broadcast
AM/FM/TV, and other transmitters specify the maximum SWR (e.g., minimum
return loss) for the loads they may drive at full, rated output power (no
foldback)?
If "nearly 100%" of the r-f power output of such transmitters was radiated
by the antenna system regardless its VSWR/return loss, what would be the
need for such OEMs to specify a maximum load SWR?
R. Fry
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