Thanks Fred. I'm familiar with measuring broadcast fields for both
directional and non directional systems. The variances over the seasons
with varying moisture levels in the ground and the difference with and
without vegetation is clearly measurable and predictable. But still the
efficiency of the radiator was not clearly defined.
As to hams, I suppose we individually evaluate a given antenna under the
conditions we have available. From that we can say that given antenna
ZZ is more or less efficient than antenna XX. What ever that XX antenna
happens to be. While others may say that their XYZ is the best antenna
they have ever had, this may be true, that is until one may find
another antenna to be better. What ever "better" is defined. And
again, each of us will have objectives in terms of what our antenna and
station must attain. As Rob Sherwood said when asked "what is the
best receiver", his answer; "what ever satisfies your needs and you feel
as comfortable to operate and can afford". I suppose antennas are
much in the same vein of characterization.
Yes, at VHF and UHF there are means and facilities to accurately measure
antenna efficiency. Usually we find those to be in the 60% to 80%
range. Unfortunately some of the applied power is converted to heat,
the result of IR loss, and thus is lost in terms of electromagnetic
radiation. Again the means and the equipment required, as Jim K9YC
stated, generally is well above and beyond the means of most hams.
Some years ago I was fortunate to have supervised access to the antenna
test range at the Motorola facility in Florida and also at the anechoic
chamber owned by IBM in S. FL. These supported my graduate studies.
No further answers required on my part. I've launched into a "reading"
project to further educate myself on the topic.
73
Bob, K4TAX
On 9/9/2018 5:48 PM, Fred Jensen wrote:
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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