[TowerTalk] If you had a choice

[Joe Subich, W4TV]

> 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...

Ground ranges have been used for years with good accuracy compared to
free space.  With horizontal antennas but the direct wave and the
first lobe have good correlation with the free space pattern.  Tests
like those conducted by Steve and Ward use a consistent range and
single reference so any range distortions fall out when making
comparative measurements.

> 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.

I doubt that those devices have sufficient operating height to make
practical far field elevation plots - even at one mile which is still
not completely in the "far field" on some bands.

73,

    ... Joe, W4TV


On 10/17/2012 9:11 AM, David Robbins wrote:
> 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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