[TowerTalk] [Bulk] Re: antenna choices for K4XS

[Jim Lux]

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.