Hi Scott,
> The prevailing theory (from a CCIR bulletin whose nomenclature doesn't
come
> to me right now) is that "static" on HF is primarily caused by
> thunderstorms in the tropics, propagated ionospherically just like
desired
> signals. The CCIR bulletin makes comment that although in some localized
> instances one can avoid noise spots using directivity (i.e., use an
antenna
> of sufficient *azimuthal* directivity to reject azimuths of high
> noise/thunderstorm concentration and therefore improve S/N), experiments
> seem to indicate that tropical noise appears isotropic...coming from all
> directions.
>
> So say you have a 6dBd yagi, with some S/N for a given signal. Now you
> (magically?) swap it out for a 9dBd yagi for a 3dB increase. For a 3dB
> increase in gain, one of the half-power beamwidths (horizontal/azimuthal
or
> vertical) has to halve.
At HF or lower, directivity of the antenna (which does NOT consider
efficiency, just the sinX/X patterns) and NULL depth improve receiving
capabilities. Gain has nothing to do with receiving performance, unless the
receiver or feedline sets the noise floor. At HF, in normal situations, the
receive improvement is just the ratio of response in the desired direction
to the response in ALL directions from which noise arrives, gain being a
meaningless by-product of the desired parameter.... directivity.
Only the absolute power radiated at the desired wave angle(s) and polarity
affects transmitting performance, and that can be FAR less than the actual
maximum measured gain. The transmit advantage is usually not more than the
gain, but it can be if there is destructive multi-path propagation.
For example, with an antenna with 30 dB maximum null depth compared to its
main response and 6 dBi gain the advantage on transmit is usually some
number below the maximum gain. It might be -20 dBd or 6 dBi, but never more
than 6 dBi unless multipath is occurring. If multipath is occurring,
anything can happen!
On receive, the same antenna's improvement can be as much as 30 dB (or more
if multipath is a problem) if noise primarily arrives from one direction in
the null and the signal arrives in the main lobe. If noise arrives from ALL
directions and the signal is perfectly aligned with the main lobe, the
improvement is equal to the directivity or 6 dBi.
If the signal arrives from the direction and with the same polarity as the
noise, the antenna won't help a bit on receiving although it will help on
TX.
This issue is so complex, an FOM is impossible. The FOM will vary from day
to day and moment to moment.
FOM's often sound good on the surface, but are often totally useless. At
HF, it is amazingly difficult to do a statistically accurate FOM on an
antenna system. Actually, it is impossible in most cases.
Only one thing occurs with certainty. Less feedline loss or more
transmitter power ALWAYS improves things with a given antenna. Everything
else "just depends" on what day it is and what way things are blowing in
the wind.
73 Tom
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