Yes, very straightforward theory. Just gather all the watts actually
radiated by the antenna and divide it by the watts you put into Rr.
Unfortunately, I did not really address Bob's question ... "How do you
sweep up all those watts?" :-) That is a nearly intractable problem at
HF unless you'll tolerate significant inaccuracies and assumptions.
It's much easier at UHF and uWaves.
An alternative is to measure/compute the losses. Did something similar
on a 10 KW FM broadcast TX, calculating the power it took to heat the
exhaust air on the premise that the rest went up the coax to the antenna
and I knew what the PA input power was.
KFBK in Sacramento CA [1530 KHz] eliminated a lot of the unmeasurable
variables by employing a Franklin antenna [center-fed half-wave
vertical] over the rice fields of the southern Sacramento Valley [nearly
always standing water, and always wet]. The center-fed vertical exhibits
far less ground losses than bottom-fed monopoles ... at 50 KW, it's
colloquially known as the "Flame Thrower of Sacramento." It may be the
only Franklin left in NA. KFBK is also famous as the birthplace of the
RCA Ampliphase transmitters and the radio birthplace of Rush Limbaugh.
NEC models coupled with terrain models can be used to establish upper
and lower bounds on antenna efficiency with pretty good fidelity to
reality. But Bob still posed a good question.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
On 9/9/2018 2:01 PM, Don Wilhelm wrote:
Skip,
That is a great formula for theory - I vaguely remember it from my
electromagnetic fields course.
But how you measure it?
With practical measurement equipment, it is difficult to isolate to a
single plane.
That may be do-able with fully characterized equipment in a controlled
antenna field space or in an EMC lab, but it certainly is not
practical in a typical ham antenna installation - and even the
radiation resistance is not easily measured.
Antenna modeling done properly will provide a much more easily
produced result. Comparative results between different antennas can
be obtained from a reference pickup antenna, but that can only show
the relative performance, we still have to guess at the efficiency.
73,
Don W3FPR
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