It depends...this is a complex (and interesting!) subject not covered in
too many places.
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.
If the vertical beamwidth halves, there's no S/N improvement. The
(unchanged) horizontal beamwidth "intercepts" as much isotropically
distributed noise as before, but presents it to the receiver 3dB stronger
than before (due to the gain increase). But the desired signal is also 3dB
stronger, for a net gain of 0dB S/N.
If the horizontal beamwidth halves, however, the antenna "intercepts" half
as much isotropically distributed noise as before (-3dB). The gain
increase washes this noise decrease out, -3dB+3dB=0dB change in noise power
presented to receiver. However, the signal strength did improve by 3dB, so
the net change in S/N is +3dB.
Achieving higher gain in a yagi antenna system using longer booms results
in (generally) antenna patterns with narrower vertical beamwidths.
Therefore, no net S/N improvement.
Achieving higher gain in a yagi antenna system using side-by-side arrays of
yagis will result in a net S/N improvement proportional to the decrease in
the horizontal beamwidth of the antenna system.
A practical example that some may be familiar with: which is "quieter" on
40m...a "deluxe" 1/4wave vertical or a shorty-forty yagi? Both are roughly
3dBd gain but they sure don't sound the same! The shorty-forty will give
you 5-6dB S/N improvement because of its F/S and F/B rejection over an
omnidirectional antenna.
Another practical example are so-called "wave" antennas like the beverage
or rhombic. The longer they get, the narrower their horizontal beamwidths,
with resultant improvement in S/N.
Yet another example is the ability to use small receive-only loops on 40m
and lower freqs. The argument cuts both ways...even if the antenna is
lossy, if it can receive atmospheric noise stronger than
internally-generated receiver noise, it's fully functional as a receive
antenna. And a small loop has enough horizontal directivity to make a
3-4dB improvement in S/N.
PS in typical temperate environments (like the US) the transition from
what's called "externally noise limited systems", an example of which is
the illustration above, to "internally noise limited systems", occurs
around roughly 20MHz. The transition frequency is higher in the tropics
and lower at the poles. The transition frequency also gets lower if the
receive antennas in use are inefficient (see the loop in the previous
paragraph).
Any flames or discussion welcome, although I note that I'm WAY off-topic
for [AMPS].
At 17:08 01.01.1999 +0000, Pete Smith wrote:
>
>I would argue, as well, thatit is fallacious (as K2UVG did) to count double
>for antenna dB, on the theory that they help both transmit and receive --
>my own belief is that below 6 meters, and maybe there as well,
>environmental noise is the limiting factor, not system gain. Hence the
>value of directive antennas on all bands, to improve the overall S/N ratio.
>
>As Tom says, the tradeoffs are mighty complex, and every time you change
>the baseline by making one improvement or another, they need to be redone.
>
------------
Scott Townley
nx7u@primenet.com
------------
Collector of:
Stoddard Aircraft EMI/RFI receivers and accessories
Big Parts for that Big Linear Amp
70's era RF test equipment HP/GR/Tek
Radio-related technical reference material 1940+
...anything else that will keep me off the streets at night
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|