[TowerTalk] If you had a choice

[David Robbins]

near field measurements are full of problems also.  besides the problems of projecting the near field pattern to something that is useful in 
comparing in the far field there are all the little distortions caused by coupling to nearby objects, feedlines, trees, etc...  A fun thing might 
be to use one of those little quad-copter things to fly an orbit around an antenna at various altitudes while making measurements.  Doing 
that you could possibly get an idea of how the antenna works in-situ.  of course to get back to the comparison to a predicted pattern/gain you 
still have to remove or compensate for nearby ground effects.


Oct 17, 2012 09:02:49 AM, jimlux at earthlink.net wrote:

On 10/17/12 2:38 AM, Stan Stockton wrote:
> Perhaps a much more practical and objective evaluation would be to
> have someone who has the proper equipment and knowledge of its use to
> spend the time to determine the exact properties of any loading
> components involved (if any) and then create accurate models.

yes.. but there are a significant number of hams who do not believe that 
modeling works (particularly for loading components), and, 
realistically, a well done test also addresses things like construction 
and assembly tolerances.

When I bought my 6BTVs, I wanted to build a model that would match their 
measured (impedance) performance, particularly with respect to mutual 
coupling in an array, so I started out trying to figure out how to model 
the traps, that led to a challenge of trying to *measure* those traps. I 
never did find a good solution.

One could, if one knew the details of construction, probably build a 
full wave very fine scale model in something like HFSS, but that gets 
back to the construction tolerances issue.

For that matter, as much as we'd like manufacturers to provide validated 
NEC4 models for their products, I'll note that when we buy antennas 
commercially, they don't generally come with a model, they come with 
pattern data.

That said, I've been wondering if there is a way to do some sort of near 
field measurement for an HF antenna using a probe/source suspended on 
(non-conductive) cables.
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