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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