> From: W8AV@aol.com
To: <topband@contesting.com>
> Date: Fri, 1 Aug 97 03:01:38 +0000
> In a message dated 97-07-31 02:21:53 EDT, you write:
>
> << Someone (I've long-since forgotten who) once told me that to find
> the correct termination value, I should apply a small amount of
> RF to the antenna and walk down the wire with a field strength meter.
> The best value of termination resistance would be when the
> sine-wave fluctuations observed while I "walked" the
> wire were least, i.e., "flat". >>
Correct termination would produce a smooth uniform taper in
voltage or current, with minimal fluctuations. I use a clamp on RF
current meter.
Properly terminated, my 500 ft Beverage has 50% current loss over
it's length. So setting the current for EQUAL current at both
ends does not work at all.
An abbreviated method is to measure the current (or voltage) at two
points exactly 1/4 wl apart, and adjust them to equal values. There
is some error in this, because it does not allow for power loss along
the wire. But it gets you close.
> Hmmm.....interesting idea. When I matched my beverages which are about 600'
> long I used an operating impedance bridge that I borrowed from work and
> measured the impedance of the beverage against ground and terminated the
> beverage in the resistance value of the beverage.
Hi Goose,
I've hear that before, and it is on some web pages, but there is a
flaw in that method. The impedance is measured with standing waves on
the antenna. Depending on the exact wavelength of the antenna (odd or
even 1/4 wave), the impedance will be higher or lower than the actual
surge impedance.
Another similar method uses a "wild guess" terminated antenna. But
again if the antenna termination isn't "guessed" correctly, the
length will change the feedpoint impedance.
A proper method is to sweep the antenna over a wide frequency
range, and adjust the termination input impedance for minimum
input impedance change at the ANTENNA terminals (but not down any
length of feedline). Virtually any SWR meter at the Beverage
feedpoint could be used for this, a battery powered antenna analyzer
is ideal.
The goal is to adjust the termination for the most constant SWR as
frequency is varied, and NOT the lowest SWR.
Another method would be to sweep the antenna input and find the
lowest feedpoint impedance frequency and highest input impedance
frequency nearest the operating frequency range. By taking the
geometric mean of these two impedances, you will come very close to
the ideal terminating value. This method works with the antenna
terminated incorrectly, not terminated, or of course with it
terminated properly. Unfortunately it requires using an impedance
bridge at the antenna feedpoint.
A final method is to "ping" the antenna with a TDR at the feedpoint,
and adjust the termination for minimum return pulse amplitude.
Because Beverages have such high loss, they tend to self-terminate.
Even if you mis-terminate them, they still have noticeable F/B ratio.
But closer is better.
=====
Earl,
You asked about copper wire. I buy #16 tinned soft-drawn copper buss
wire new on 5000 ft rolls for just over $100 per roll. If you want
the source let me know.
Tinned copper lasts forever, and solders easily. I silver solder
mine.
73, Tom W8JI
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