"Reg Edwards" wrote in message
...

"Dave VanHorn" wrote -
Reg wrote -
But to be on the safe side, allow for a dissipation of 1 watt per foot
length of dipole wire which will then get warm in the centre, somewhat
above
ambient, when in a slight breeze.

But the current distribution isn't linear, and the loss is I^2R, so I
would
think you'd have to know the max current, and work it that way.

============================
Yes Dave, I know. But I did say the wire gets warm at the centre. Sorry
I
forgot to say it remains cold at the ends except for the small conduction
of
heat along the wire. And it does not get quite so warm when it rains or
snows.

Actually, the current distribution along a 1/2-wave dipole is exactly a
sinewaveform. This is arithmetically very convenient because the linearly
distributed wire resistance then behaves as though it is lumped at the
dipole centre with a value exactly half of the overall distributed value.

Now the end-to-end resistance of a 1/2-wave, 160-meter dipole, made with
26-gauge copper-plated wire, is about 22 ohms. Therefore the feedpoint
resistance of the antenna is the radiation resistance of 73 plus 22/2 ohms
=
84 ohms.

Therefore, with Tx power equal to 1000 watts and a current at the centre
equal to 3.45 amps, the overall loss of power dissipated in the antenna
wire
is equal to 3.45 squared times half of the end-to-end conductor resistance
of 11 ohms. Which (as I said in my previous message) equals 131 watts.

Which raises the wire temperature a harmless handfull of a few degrees
Celsius. Actual temperature, of course, depends on the open-air ambient
temperature at noon, in June, in the New Mexico desert.

Antenna radiating efficiency = 73/(73+11) = 84 percent, or a loss of 0.76
decibels, or 1/8th of an S-unit. Which, by no stretch of the imagination,
is likely to influence the results of a contest.

Isn't it remarkable what a little bit of ohms-law and arithmetic can do?

Mathematics - don't make me laugh! smiley

Mathematics is what English infants school teachers refer to when whining
for a pay rise. To such a level are what the western-world's standards of
education have been reduced.
----
Reg, G4FGQ


Reg
I went to public schools in Texas in the 50's and 60's.
There were no standards then. School was a pathetic joke.
If I hadn't had ham radio, cars and flying as teenage passions, I never
would have learned math or physics.
73
H., NQ5H