Topband: Non-resonant receive antennas

[Tom W8JI]

> When talking about a low dipole, the question comes up
> as to why it must be low to work.  Actually we don't
> know that it must be low to work.  Very few of us are
> in a position to put up a "high" dipole, so the question
> is basically moot.  However, in an attempt to gauge the
> influence of height, I A/B'ed two full size dipoles at
> 30 and 60 foot heights over a period of 6 months.  The
> one not in use was floating to avoid interaction with
> the active one.
>
> I never noticed any difference in receiving performance.
> What seems to happen is that the signals are a few dB
> higher on the 60 foot wire, but the noise is commensurately
> higher.

Low is always in wavelengths, feet are too general. :)

Your results are expected, because anything below 1/8th wave has little 
difference in pattern (except for loss of efficiency at low heights). On 
receive, only the pattern (and polarization) matters until you get into 
system internal noise floor limitations.

The idea a really low dipole is quieter or has better high angle signal is a 
big problem with NVIS antennas and emergency services. People park 80 meter 
dipoles at a few feet and they are no quieter except for loss of efficiency 
(and in some poor installation cases less feedline radiation at low height).

I had dipoles at ~300, maybe around 130-150, and 60 feet on 160, as well as 
non-resonant low dipoles. Once below ~150 ft, they all pretty much work the 
same except for gain reduction with reduced height. That gain reduction can 
be somewhat mitigated with a ground screen of some type, until the antenna 
gets too close to the screen.

During the rare times the any dipoles were working very well, they all 
pretty much "worked". The high dipole was closer to the vertical than the 
low dipoles at other times.

Of course inside a few hundred miles was different. The high dipole, like 
the verticals, were dogs. Any dipole below ~150 ft was pretty much the same 
in close on sky wave.