[TowerTalk] Measuring resonance of a yagi element

[jimlux]

On 4/22/20 7:28 AM, john at kk9a.com wrote:
> Actually checking the single element resonance is a good way to check it 
> against a model, especially if it has non-insulated boom clamps that are 
> difficult to model. If the element has linear loading this would be a 
> good way to adjust it to ensure that the Yagi will match the design.  Of 
> course the element has to be measured in close "free space" conditions 
> for the data to be useful.
> 


And it's that "free space" that's tough - easy for a 2 meter or 6 meter 
Yagi, somewhat of an ordeal for a 40m antenna.

One could run a model with the antenna at various heights above ground 
and see where the influence of the ground starts to fade away.

or, as a "build check" then you're comparing the resonance you measure 
at, say, 6 feet off the ground, with what the manufacturer measured at 6 
feet off the ground (with some "allowance" for varying soil properties)





For what it's worth NBS Circular 688 on Yagi design recommends a length 
tolerance of 0.003* wavelength, which is about 2 1/2 inches on a 20m 
Yagi. However, the author has no backup for that tolerance value, so it 
might be more like "that's what we could make them in the NBS shop for 
our 400 MHz test frequency" (it's about a tenth of an inch for the 75cm 
wavelength)
https://tf.nist.gov/general/pdf/451.pdf


> John KK9A
> 
> Chuck Dietz W5PR 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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