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[TowerTalk] wire antennas

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Subject: [TowerTalk] wire antennas
From: (Jim Reid)
Date: Thu, 03 Jul 1997 08:53:25 -1000
At 01:41 PM 7/3/97 +0200, Jurgen Geldhof wrote:
>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 antenna-feed 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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