I responded privately to the OP earlier in the day with the following. Since
this thread has taken on new meaning. I'm posting it here:
Frank,
Maybe a little thought experiment would help.
Consider that a K3/100 (I assume that is your worry) puts out at best 100 W and
with most modes and operating styles, the average is much much less.
If you were running a full-carrier mode like FM or RTTY, the duty cycle is 100%
while transmitting, but assuming a 50-50 split between transmitting and
listening, it's 50%. So in the worst case, let's say the average output is 50
W. On CW or SSB it will be even lower.
Now this is at the output connector. With almost any run of cable, there will
be some loss, but to be conservative, assume that it is zero. So at the
antenna feedpoint the average power is 50 W. Hopefully, most of this is
radiated and escapes the confines of your attic. In fact, a dipole is very
nearly 100% efficient, but assume for a moment that it isn't and 50% of the
available power is converted to heat.
This means that 25 W is dissipated as heat, and the other 25 is radiated. Most
of the heat will be in the neighborhood of the feedpoint because that is the
area of higher current. However, it isn't a point loss but spread over some
length of the wire.
But let's pretend that all of the heat is generated in a very small spot on the
wire and contemplate this.
I like to sometimes turn these things into other questions, so I would ask
myself, "Self, if I was trying to heat up that wire to solder the transmission
line to the wire, what is the likelihood of doing the job with a 25-W soldering
iron?
Pretty unlikely isn't it? And that is the worst case scenario. With a small
amount of power distributed over any appreciable length of copper there simply
isn't going to be any significant temperature rise.
End Quote.
Now to address David's remarks that follow:
A lot of this is conjecture at this point since the size and composition of the
attic, the electrical length of the dipole and the frequency are all unknowns
at this point.
However, using my "backwards" logic from above, consider actually trying to
absorb the transmitter power in the building without it leaking to the
outside. Having done measurements in anechoic chambers and free-space antenna
ranges, I can tell you that it's difficult.
Or try to imagine purposely attempting to heat the attic with a 50 W heating
element. If it's a cold climate, it's not going to happen and if it's like
where I live where last week the air temperature was 102F and the solar
insolation on the roof was 1 KW/m^2, what effect would another 50 W make?
Answer: none. This is a non-issue.
Wes Stewart N7WS
> Frank MacDonell wrote:
> > I am using a center fed dipole in the attic for a K3.
> Does the antenna
> > generate any measurable amount of heat during TX?
> Thanks.
> >
>
> Yes. Subject to using suitable measuring
> instruments.
>
> If the antenna is a reasonable length, a lot more heat will
> be generated
> in the building structure (conceivably more than 50% of the
> power) than
> in the antenna wire. If the antenna is, in
> particular, electrically
> very short, a lot of the power could go to directly heating
> it.
>
> I will also depend on the size and construction of the
> wire.
>
> Why? If you are considering the total thermal load, I
> think the
> building structure dissipation will be the most important
> factor.
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