[Elecraft] ARRL book on receiving antennas

[Bob McGraw K4TAX]

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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