Agree with all of that. I realized after I wrote it that I should have
stressed that zeroing out the reactance at the antenna from the shack
end of the feedline does NOT match it at the the feedpoint and that
you'd still have SWR losses in the feedline and also in the matching
network. I was referring only to the reactance and should have made
that more clear.
Also agree about the output impedance of the transmitter, but I was
trying to simplify the discussion and didn't want to make it more
obscure than it had already become.
73,
Dave AB7E
On 2/20/2018 11:51 PM, Jim Brown wrote:
On 2/20/2018 9:35 PM, David Gilbert wrote:
"Antennas are tuned with wire cutters or a hacksaw so the feed
impedance is the desired value, ideally 50 ohm resistive or at least
a match for the feeder concerned."
I think we're getting a bit off track by lumping "tuning" and
"matching" together as if they are the same effects. They are not.
Yep. And many antennas are better matched to other than 50 ohm
feedlines. My high dipoles are in the range of 85-90 ohms, so I use a
good RG11 to feed them. And those dipoles are for 80 and 40M, so their
lengths are tweaked so that, with stub matching in the shack, the Z
presented to the 87A (tube) power amps is within the range they are
happy feeding.
At least in my world, tuning refers to compensating for reactance at
the feedpoint by some means that nets it out to zero. In that
respect, doing so with a network at the other end of the feedline
accomplishes the exact same thing as using wire cutters or a hacksaw
on the antenna itself.
Don't agree with that -- the match between the antenna and the
feedline determines the loss in the feedline. The tuning network
between the transmitter and the feedline does NOT change the loss in
the feedline, it simply gives the transmitter a load that it's happy
driving, and to which it can deliver maximum power. AND -- we are NOT
matching the antenna Z to the TX output Z -- for most real
transmitters, their output Z is LESS than the load they are designed
to drive, and that output Z changes dynamically with signal level!
Back in the olden days, we young EEs learned about how do determine
this with "load lines."
They are not different. As you say, the current and voltage
distributions are not the same as if you cut the antenna to length,
but the lack of feedpoint reactance is.
But with most antennas, zero feedpoint reactance can be achieved at
only one frequency -- it takes something like a SteppIR to move that
zero point around the band. I may resonate my 80M dipole at 3625, but
I almost never operate there. Rather, I'm mostly between 3500 and
3600, and between 3650 and 3850.
And then there are directional antennas like Yagis. :)
73, Jim K9YC
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