Topband: Receive antennas

[Tom Rauch]

> I have looked at the EZNEC plots and I see the same
> thing and arrive at different conclusions.

Take the average gain in the 3d plot in EZnec+ . Find the
difference between average gain and maximum gain in the
direction of desired response to average gain. The result is
the directivity, and with evenly distributed noise
directivity sets the S/N ratio. Not gain, not how nice
pattern looks.

> 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.

With a single dominant noise direction it all comes down to
the null in the noise direction being some safe amount
deeper than the average noise arriving from other
directions.  It's a coin toss of where nulls fall and how
deep they are. If the noise is around the sides then you
only need a null deep enough to knock that noise down to a
level less than accumulated noise from other directions. The
random optimum condition will almost always be some spacing
wider than 1/2 wl because:

1.)  Nulls deeper than those required to make the single
direction dominant noise source fall below distributed noise
are wasting null depth.

2.) It's statistically more likely the noise is forward or
rearward of the sides rather than straight off the sides.

> 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 wouldn't know why that would be, unless you really did
have a nearfield noise source. It's very unlikely for any
location to have bothersome farfield groundwave noise
sources that are evenly distributed within 2dB.
How could that happen? I can't imagine the luck involved in
that. There would have to be dozens of noise sources in all
compass directions with each one having just the right path
loss and initial strength to be nearly equal.

> To my knowledge no one has ever made a QSO using EZNEC
> or any other CAD tool for an antenna.

I agree....but some basic idea of how it works helps. It's
significantly easier to work our way through a list of most
likely good choices than to start with unlikely choices.

Another big problem with models is the feed system. It's
easy to forget that the feedline can easily be a large part
of the antenna when feeding small ground independent
antennas, and it is very difficult to remove it from the
equation.

What works for each of us is what works for each of us,
especially when the antenna is in a cluttered environment,
but 1/2 wl is generally too close. It would be a very
special case where 1/2 wl spacing would be optimum.

At my location models prove to be very reliable. Better
directivity antennas work better. Directivity is calculated
by finding the difference between average gain and peak
gain.

73 Tom