> ....someone was mentioning using wirewound potentiometers for
> terminating receiving antennas...not circuit board pots but
> bigguns...oldies like you see in the flea markets at hamfests...are
> these easier to obtain than honest to goodness carbon resistors...and,
> can you actually see the performance peek as you sweep the pot...of
> course I guess you need to get a buddy who is willing to stand in the
> middle of a dark field late at night whilst you listen on the RX for
> things to peak up...definitely NOT single op stuff!
A few cautions:
Be careful with inductive resistors, like wire wound pots, at radio
frequencies. DC resistance can have little to do with RF resistance.
If you have a MFJ antenna analyzer, you can sweep the analyzer
frequency at feedpoint and watch SWR change with frequency.
Properly terminated antennas have little SWR ripple over a wide
frequency range.
Note it does NOT matter what the actual absolute SWR value is,
what you want is minimum SWR **change** with frequency.
If we measure input Zo of the antenna, proper termination comes
from sweeping frequency and finding the highest R and lowest R
values at frequencies where the antenna is an odd1/4 wl and even
1/4 wl long. Multiply R highest times R lowest, and then find the
square root of that.
Say we find R highest at 2.5Mhz with a value of 800, and R lowest
of 200 at 2 MHz.
We know a lot from that! We know the antenna Zo is sqrt of
200*800, or 400 ohms. Termination would be 400 ohms against a
perfect ground system.
We know the antenna is 492/ 2* (FhiFlow) long in electrical feet, or
492 feet long electrically. We can measure through a transformer (if
we keep leads short) if we know the transformer ratio.
I can find the termination resistance of ALL my Beverages without
help, using a standard MFJ259B. I can not only tell what the
antenna impedance is, I can tell how long the antenna is
electrically.
Because of the velocity factor, all Beverages are noticeably longer
electrically than they are physically. It helps to know the electrical
length of thee antenna you are actually using when making arrays
of Beverages, or looking for a certain pattern.
73, Tom W8JI
W8JI@contesting.com
________________________________________________________________________
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