> At 03:44 AM 6/26/2004, Peter Sundberg wrote:
> >So f/b on groundwave is important and so far, from what I
have seen the
> >results are pretty close to what the software model tells
you. >
>
> My recent experience leads me to agree, Peter.
> I understand that anything near the antenna can and will
affect groundwave
> field strength, but since the only variables in this test
were the pointing
> of the antenna and the frequency, this result seems to be
a pretty good
> indication that the reflector is tuned too high.
Something either is a good method, or it isn't. Not being
good doesn't mean every case won't work nor does it mean
most cases won't work, this method simply has the potential
for anything up to a very large error and you have no way to
know if it working or not without using another more
reliable method to crosscheck.
If you are fortunate enough to have an uncluttered area, NOT
just in the path but in all other directions around the
antenna, and if the antennas at both ends have very low
response to vertically polarized signals then you probably
can use longer distances to indicate zero angle gain and F/B
ratio. But we have to remember what we are measuring:
1.) We would be measuring the pattern at zero degrees,
something we don't often care about.
2.) We would be measuring through a "filter" that often has
significantly more attenuation for horizontally polarized
signals than vertically polarized signals.
3.) The most reliable way to measure gain or F/B of a
horizontally polarized HF antenna is with spacing in the
order of hundreds of feet, not thousands or more. The
"measure at a mile" stuff came right out of the thin air,
and became a myth. People actually think it is better when
it is very much worse!
The error probably is modest (a few dB) in most cases when
sites are reasonable clear of sources of re-radiation at
both ends (in *every* direction from the antennas) and the
antennas have minimal vertically polarized zero degree
radiation. The main point is the data is unreliable. There
CAN be a significant error, like the example I cited where a
person became totally convinced he improved antenna
efficiency dramatically by changing only one thing, his
matching device, from a tuner to a stub. There is at least
one commercial Yagi and one small Quad being sold that use
totally sloppy methods to claim more gain than exists in
theory. You can bet the gain is 2-3dB less than claimed, and
is actually in the same ballpark as other antennas the same
size.
I've even seen one person claim, with a straight face, his
special design quagi beam had five or ten dB gain over a
Hygain 204BA! Pure nonsense, but you'll never convince the
person who has managed to get the results he wants through a
flawed test that he might be mistaken.
A lot of nonsense comes from using unreliable measurement
techniques and not cross-checking the results. A measurement
is either reliable, or it isn't.
73 Tom
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