On Fri, 01 Jan 1999 15:52:23 Scott Townley <nx7u@primenet.com> writes:
>
>It depends...this is a complex (and interesting!) subject not covered
>in
>too many places.
Sure is, try reading various contest pubs...they take it to real depths.
>
>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.
Thats because many antennas have poor elevation angle rejection curves.
There is no free lunch in antenna performance.
>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.
That makes no sense whatsoever. Anytime you reduce the E or H plane of a
Yagi pattern you will tend to improve SNR. The unknown is the arrival
angle of the noise vs the desired signal but in general the longer the
boom the more compressed the 3dB points ( yes H and E planes use 3dB
points as references; in loooong VHF yagis the H plane response is often
more important.)
>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.
That is again a totally simplistic approach. Reality requires a
multitude of factors to be considered.
>
>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.
Again way wrong. Narrower beamwidths= lower SNR and the ultimate way is
to stack antennas using a PC program to optimize. You might try YO and
see the results.
>
>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.
Yep, and worn out rotators. I know of no serious contester or DXer using
horizontal stacking, even those with unlimited budgets. ( yes there will
always be exceptions until a storm blows thru).
>
>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!
Nor perform the same if you fail to specify height parameters for the
yagi. A 2,3, or 4el vertical array will often blow the socks off a modest
height yagi on 40M. I mean DX, not continental/ local chit-chat. It can
and will when installed correctly be much quieter on RX in the rejected
direction.
The shorty-forty will
>give
>you 5-6dB S/N improvement because of its F/S and F/B rejection over
>an
>omnidirectional antenna.
Wrong still again. Even a CC 2el 40 will give solid 15-20 dB rejection
off the sides over a very wide range of elevation. The rear rejection is
typically 10-15dB over the 180 degree area.
Work it out and you may see why a yagi has a much higher "apparent" gain
when compared to a dipole, its all in the angle of the dangle as an old
friend used to say.
>
>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.
Up to a point only. They also compress the H plane.
>
>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.
A small loop has no real horizontal directivity on a desired signal, you
better go back to the books. Its primary function is to null out very
locally generated noise that is strictly vertically polarized. It does a
great job in that respect.
>
>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.
Agreed. That is why I often find a 1dB NF preamp very usefull on 15-10M.
The transition frequency also gets lower if
>the
>receive antennas in use are inefficient (see the loop in the previous
>paragraph).
Wrong again. The PERCEIVED transition may appear to change only.
>
>Any flames or discussion welcome, although I note that I'm WAY
>off-topic
>for [AMPS].
No flames but I do wish you put a bit more research effort into the post.
73 Carl KM1H
272 DXCC on 160
309 on 80
Too many on all the rest.
Getting bored, thus spending more time here! And on VHF
Only off-topic when Paul roars.....
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