TopBand: Radial current
Wed, 4 Jun 1997 11:17:00 -0700 (PDT)
Gee - I had been feeling pretty good about my antenna system...
That was - until W0UN sent me a copy of a recent article he saw.
It seems likely that my 20 elevated radials aren't all equally sharing the
current. This article (sorry - I left it at home and forget who
to credit as writing it) simply shows that if any one of the radials
is near zero impedance (electrically a quarter wave) that the lion's
share of the current will go into that radial.
Compare the current into a zero ohm resistor and a 2 ohm resistor
and it becomes clear.
As you change frequency, the distribution of current in the
radials will change as a different one approaches the right length.
The article also provides some solutions. The simple approach appears
to be to either shorten or lengthen all of the radials by a quarter
wavelength (or something close to that). Now, you will have impedances
that still may vary by a few ohms, but none of them are near zero
Ohms. Then use some loading to tune the radial system for the desired
frequency (using an inductor if they are short, or a capacitor in series
if they are long).
This seems to make a lot of sense - and all from using nothing more
complicated than Ohm's law.
The advantages are a better pattern because of more uniform current
flow. The other advantage is elimination of a possible horizontal
polarized component because of unequal distribution. I didn't think
of that before, but having symetric current flow around the base
of the antenna is important in this regard. It would also follow
that I need to be careful about the physical distribution of the
radials as well. Currently, I have a higher concentration in
some directions (i.e., Europe and the east coast).
So - I guess I will cut my radials down in length (perhaps to 3/16 wave)
and put an inductor in series with them and adjust it for minimum
impedance on 1825 kHz. Perhaps I will make some current measurements
before and after to verify the results. I could tune each radial
individually to make them all the same impedance at 1825 before
connecting them in parallel.
Of course, this will remove all the magic from the system and it
will never work the same...
Been trying a few CQs around 0500Z looking for a ZS or something,
but no luck yet. Received a QSL from HP3FL yesterday.
73 Tree N6TR
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