[TowerTalk] Measuring resonance of a yagi element

[jimlux]

On 4/22/20 8:00 AM, Grant Saviers wrote:
> Good point.  Modeling Yagis and optimizing with Auto-EZ is instructive 
> in how much one change changes a lot else.  Watching the automatic 
> optimizer try to find the several dimensions of target values shows 
> this.  Having designed a two band interlaced 30/17m I gained enormous 
> respect for those designers with proven performance multi-band designs. 
> A lot of hours at the PC.  Plus I wonder about how designers got it 
> right in the "old days".  My solution to the mount modeling problem is 
> all insulated elements.


In the "old days" people would make all sorts of claims, regardless of 
whether the antenna worked well or not.  If the sunspot number is >200, 
given 24 hours you can probably "work the world", and things like crummy 
F/B or gain that's 4 dB less than you think don't make a lot a difference.

That's why ARRL started requiring either range measurements or a model 
in ads that claim gain.

Beyond that, lots, and lots, of cut and try. If it's your *job*, e.g. at 
NBS, in pre-computer days, you've got some theory to work with, so 
you've got analytical equations for mutual impedance as a starting point 
(and tables for the Si() and Co() integrals you need).

Then, you make up a test matrix and systematically vary lengths and 
spacing and make lots of measurements. That's what a "good" antenna 
range has over a "not so good" - the ability to run the AUT up and down 
quickly.  If you can run an azimuth pattern in, say, 10-15 minutes, and 
then change lengths in 10 minutes with another 5 minutes to run up, and 
5 minutes to bring down, you can get a dozen tests done in a day.

A couple months later, you'll have the data you need to produce an 
"optimized" design.

You'll have previous knowledge of the range, so you know how high to put 
the AUT so that the "ground reflection" effects are known.  At JPL, the 
two longest ranges (1200 and 3000 ft) that are regularly used are over a 
canyon between the transmitter and receiver and both have adjustments of 
the height of both ends.  But still, there are anomalies when someone 
piles a bunch of steel light poles in a flat spot half way between the 
end points.
https://mesa.jpl.nasa.gov/1200_Foot_Range/index.html

We haven't done many HF antenna measurements on the JPL range, though. 
UHF (400 MHz) and up, for the most part.


One can see why modeling is popular.



> 
> Grant KZ1W
> 
> On 4/22/2020 06:38, Chuck Dietz wrote:
>> Hi Tom,
>> I don't think you are taking the right approach. I don't think you can 
>> just
>> "tune" one element of a Yagi antenna. You need to measure all the element
>> sections and placements including the element mounts and run them 
>> through a
>> computer design program. Changing any dimension will affect the other
>> dimensions to some degree. If it is a commercially sold Yagi, the file 
>> will
>> already be available and can be used to optimize the antenna.
>>
>> Chuck W5PR
>>
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