Eric is an excellent antenna guy, and I'd like to add a little to
what he says.
The usual procedure is to get the terminating impedance right (for
maximum F/B, and then adjust the input transformer. Remember some
transformer can be so crummy (especially ones with needlessly high
low frequency Ui cores), you will get a low SWR with nothing hooked
to the transformer! Because of that, feedline SWR tells us nothing
about antenna performance.
Eric wrote:
> If the value is correct, the response from
> the back will be minimized. The value required for this is
> dependent on nothing but the effective surge impedance of the
> antenna when operated as a transmission line (it varies depending
> on the exact circumstances of each antenna).
If the termination resistance is not exact, the antenna doesn't go
"dead". You will not lose all the signal-to-noise advantages. You
will lose f/b ratio, and if there is strong noise coming from the
direction of the "null", s/n ratio will decrease by the amount
of the null loss.
What that means is under most operating conditions you won't
see much difference, but under some you can see many dB of change
with a small error is termination resistance.
Assuming a lossless antenna, the f/b ratios for different
terminations are:
Termination SWR f/b
3.6 5 dB
1.9 10 dB
1.4 15 dB
1.2 20 dB
1.12 25 dB
If we "guess" the correct value is 450 ohms, and it is really 500,
we can have 25 dB f/b ratio.
In a normal single direction Beverage, the termination resistance can
be "guessed at" with good results, just as Carl said. The problem
occurs when you build a reversible Beverage, because now the
termination resistance is presented through transformers and
transmission lines. All the small SWR errors in the termination,
antenna, and transformers can add up, and you can wind up with a
"crappy" system that took a lot of work.
A little more effort usually isn't that much trouble. The
antenna can provide years of (listening) pleasure, or be a nag
at times because you don't pay enough attention to it from the start.
There have been several post on how to determine the surge impedance
of the antenna. There is also some bad information out there
floating about on how to measure the antenna termination impedance.
The only correct ways are:
1.) Excite the antenna with a small signal, and measure the current
along the antenna with a clamp-on meter (simple and cheap to build)
or an RF voltmeter / FS meter (not as reliable).
Adjust the termination until the common mode voltage or current has a
uniformly smooth taper towards the far end. (Current will never be
equal all along the antenna, if it is....watch out).
2.) Sweep the antenna over a wide frequency range, and adjust the
termination for a constant SWR measured right at the antenna. (Not
lowest SWR, just the most steady SWR. After you are done you adjust
the transformer ratio for lowest SWR.)
3.) Measure the lowest feedpoint end impedance of the wire (as
frequency is changed to find those points), and the highest feedpoint
end impedance of the wire with an RF bridge. Multiply those numbers
together and the square root of the result is the antenna impedance.
4..) Install a remote signal source in the null area and adjust the
termination for a null.
You can just put it up and guess, or measure it. Unless it's
a reversible antenna or unless you like to spend an extra hour to
do a careful installations, a guess will often suffice. Especially if
you are in a good location.
But remember when you "guess", you also don't check the system for
any errors or flaws.
73, Tom W8JI
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