At 01:41 PM 7/3/97 +0200, Jurgen Geldhof wrote:
>Hi,
>Here's my question :
>
>Is there a formula to calculate the necessary thickness (or diameter) of an
>antenna wire to support a certain amount of power at a certain frequency ?
>
>I know, the thicker the better, but I prefer it as light as possible.
Hi Jurgen, Seems to me it will depend upon the effective radiation
resistance of your antenna. A rhombic wire antenna has a pretty
high value, so the voltage on the line would be high, but
the current will be low, for a given power level, compared to
say, a simple half wave dipole wire up a half wave or so where the
effective resistance would be around 75 ohms or so.
Consider some imaginary situations: transmission lines in which
it has been attempted to match to the rhombic antenna, but we
still have a 3:1 VSWR in the antennafeed system. Using the
following feed lines, we would measure (I think):
50 ohm coax will have a voltage of 474 rms, and carry a max
current of 9.5 amps.
72 ohm transmitting twin lead, voltage, 570 volts rms, current, max, 8 amps.
450 ohm twin line, rms voltage, 1423; max current, 3.16 amps.
The losses which I assume concern you, would be related to the
current losses of a particular wire size. So the answer you seek
depends upon a lot of factors of the situation, and it is going
to be impacted by the height of your wires, there length at a
particular frequency, etc.
I just measure the forward voltage on my 50 ohm coax going to my
tribander. Find a vswr of around 1.2 or 1.3:1 (tough to measure
exactly when so low). With 1500 watts or so peak showing on SSB
voice peaks, read a max voltage forward of about 300. Forward
current, max, then is the voltage divided by the line impedance,
or 300, by 50, or about 6 amps max. Did this on 20 at 14154, no
one else around there right now, so didn't QRM anybody that I
could here from out here anyway.
Anyway, just some thoughts about your question; am sure someone
else will be able to give you better, and perhaps even a correct
answer. My answer may be way too simple minded!!
73, Jim, KH7M

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