--- Tom Rauch <w8ji@contesting.com> wrote:
>
> Unless we have well defined noise arriving at zero
> degrees
> elevation 90 degrees azimuth from the peak response,
> some
> amount of spacing beyond 1/2 wave is always better.
> Overall,
> the more energy removed from the sides the better
> S/N
> becomes whether we are in a city or country
> location.
I disagree.
I have looked at the EZNEC plots and I see the same
thing and arrive at different conclusions.
I see the nice nulls at 30 degrees elevation changing
to peaks at 2 degree elevation. I see more nulls,
true, but they are narrower than the nulls at narrow
spacing. It's a matter of area under the curve and I'm
not prepared to do the math to prove it.
I did do extensive field testing however. My AZ QTH
was bad for noise. I measured the noise at the house
at some 35 dB over the high noon noise in the desert
50 miles from town and 10 miles from the nearest power
line.
Using my rotatable flag as the "gold standard", I
could rotate it 360 degrees and not see more than 2 dB
variation in the noise. With a measured 30 dB null off
the back, it's unlikely there was a single strong
noise source in the neighborhood to upset the
measurements.
With a pair of flags spaced at 315 feet I could see NO
improvement in S/N over the single flag. I took data
over a month long period on DX, while I was trying to
figure out why the pair didn't work as advertised.
Looked at transformers, phasing, common mode, and
everything else I could think of.
The day I moved the pair closer together, I saw an
immediate 3 dB inprovement in S/N over the single
flag. This improvement was constant over the entire
remainder of the season that I used this pair.
I will agree that EXACT 1/2 wave spacing is not as
desireable as SLIGHTLY wider spacing. In my case the
spacing was about 275 feet as opposed to 268 feet half
wave. I will also agree that one or more strong noise
sources that either fall on a null or a peak in the
response will alter the measured results in a
practical field installation. In those instances, best
S/N might be had by "tuning" the rearward response by
changing the spacing if you can't turn the whole
array.
To my knowledge no one has ever made a QSO using EZNEC
or any other CAD tool for an antenna. At some point
you have to drag bits of metal out into the yard and
assemble them. Once that happens all bets are off. No
one can model your backyard. Local conditions will
vary dramatically for everybody and no "one size fits
all" cookbook design can be guaranteed to work as well
for you as it did for your neighbor. Look at all the
possibilities and make changes to see what happens.
You might get lucky, who knows? Above all remember
this is a hobby, have fun!
73,
Larry - W7IUV
DN07dg
http://w7iuv.com
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