On 4/13/15 9:12 AM, Bry Carling AF4K wrote:
Very good point. I do not know whether these deep scientific studies were done
at NEAR
FIELD to the antennas or at distances of hundreds of miles away.
For HF antennas you don't have to be hundreds of miles away.. The rule
of thumb for "field is sufficiently plane that the gain is correct
within a few tenths of a dB" is that the field be flat to within 1/10th
wavelength over the aperture of the antenna.
For a yagi, the "aperture" is probably the length of the boom, rather
than the length of the elements. Or maybe the diagonal (e.g. twice the
turning radius).
THis is the origin of the 2*D^2/lambda guideline which really only works
for fairly narrow beamwidth antennas and dishes..
If you're a few 1000 meters away, I think you can get fairly accurate
gain numbers, if you account for ground reflections. There's two ways
to do that: move the antenna (or the probe) up and down several
wavelengths and take data at multiple points. From that you can
calculate the ground bounce (even if the antennas are only a few hundred
meters apart)
The other way is to use a comparison antenna with known gain (e.g. a
dipole) mounted at the same location as the AUT. There could be a freak
specular reflection that causes weird results, but I think you'd see
that as you did your azimuth pattern.
This is what the report alluded to earlier in the thread did. I think
their methodology is basically sound. It's a heck of a lot of work to
collect this kind of data: you have to beg borrow or buy the antenna,
then assemble it, then hoist it up, then take it down, dismantle it, and
give it back to where you got it.
The actual measurement part is easy.
If someone had a stack of 10 antennas already assembled that they wanted
to test, an 80 foot boom bucket truck with a suitable rotator in the
bucket would be the easy way.
The articles / books claim that these yagoi beams have virtually no gain at
all. I wonder
whhow NEC would stack up against their measurements? It's in a boob, or article
so it MUST
be true. I am just left pondering the same as you - trying to decide where ALL
that extra
power is disappearing to...
Hmm, I don't think the articles claim no gain, at least if they're
showing actual measured data. In fact time after time, yagi antennas
come in reasonably close to their modeled gain (within a dB or two), and
an antenna that doesn't show that kind of gain in a test has something
suspicious about the design.
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