On 4/22/20 7:28 AM, john@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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