Dan Zimmerman N3OX wrote:
>>> Do you have any issues with interaction or funky pattern distortion that
>>> close to your flag?
>>>
>>> Scott
>>> -----Original Message-----
>> Just an academic question.. how would one know if the pattern is distorted?
>> I've given quite a lot of thought and experimentation to figuring a good way
>> to measure HF antenna patterns as installed (for hams.. I know how to do it
>> if I had hundreds of thousands of dollars.. RELEDOP, for one)
>
> How would I know if it's distorted? Easy in azimuth with a rotatable
> antenna. 1843.2 kHz TTL can oscillator driving a six foot whip
> against a short ground spike stuck out along a riverbank about a
> wavelength from my antenna. G4HFQ's "polar plot" software and my
> radio's AGC turned off, gain adjusted so it doesn't distort.
>
> Patterns here:
>
> http://n3ox.net/projects/flag/160pattern_lg.jpg
very cool..
But that's only one bearing relative to the antenna site.
If you had, for instance, a huge mile high metal wall on one side of the
property (opposite the direction to your pinger), your polar plot
wouldn't show this "site specific" effect.
The reason I've been thinking about this off and on for the last several
years is that site effects and interactions are more and more an issue,
as you make antennas smaller and you're trying to squeeze them into a
suburban lot.
If one's talking about a yagi up on a tower, well clear of the
surroundings, the pattern in use is probably pretty close to the free
space pattern, modified by the terrain, which you can model with
something like HFTA. But, when it comes to verticals, and things down
among the houses, trees, swingsets, etc. it gets a bit dicier. And,
modeling is nighmarish (sure.. what are the dielectric properties of
your house? )
It would be nice if there was a way to objectively evaluate this, as
opposed to the "worked 100 DX on 5watts" sort of evaluation.
I've thought about pingers and GPS on balloons, kites, and RC model
airplanes, for instance. (I even started building such a thing.. a poor
man's reledop). As you've noticed, you could put together a very small
cheap test source pretty easily. Maybe one could attach them to small
helium balloons, launch them, and track them photogrammetrically. I'm
willing to throw away $5 test payloads, but not $100 GPSes. And, even
with the $5 throwaway, it's still going to take a lot to get good az
coverage (hence the RC model airplane idea.. you can fly it around in a
circle)
Another strategy is to set up a small interferometer (small, so it's not
affected by its own design) to measure the incident field from signals
of opportunity. You could then compare the response of your "reference
standard" to the antenna under test. But that would still be subject to
some of the site effects (although the "interaction" effects would be
smaller).
Jim
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|
