On Sat, 2 Jan 1999 05:52:34 -0000 Michael Tope <W4EF@pacbell.net> writes:
>(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.)
>
> Scott is correct only if the noise is coming from the same
> elevation angle as the desired signal. In this case, he would
> be correct. In reality though, more often than not, I suspect
> that there will be noise contributions distributed over a
>range
> of elevation angles.
Elevation and azimuth. A tropical storm noise source may be propagating
at the same angle as a desired station on another heading. Pattern
discrimination may render that noise meaningless.
In this case, decreasing the vertical
>beamwidth
> of the antenna would tend to improve the SNR of the received
>signal
Absolutely. H plane compression can be via boom length, stacking or both.
> as long as the desired signal remains in the boresight of the
> narrower antenna.
Stacking with switching capability can be used very succesfully, even if
it decreases the desired signal at a lesser rate than the noise. It is
the ratio that counts.
Noise comes in many flavors, including other hams. When contesting
actively I had 4 high stacks on 10, 15 and 20M with extensive switching
capability. We could often reduce Western EU almost into the noise and
run the East EU and other deep multipliers, etc. Alternate switching
allowed almost constant runs as long as the propagation held out. It was
fun to listen to and we could keep both groups calling as long as we
switched patterns every minute or so. Rotating antennas in different
directions also had benefits.
Also, as you pointed out, as boom length
>increases,
> both the E and H plane beamwidths will narrow. At VHF
>frequencies
> where the receiver noise temperature may be a limiting factor,
>the
> extra gain helps just as much in either E or H plane as the
>noise
> is coming from inside the receiver.
I designed and built my 8el 6M yagis with maximum F/R and minimum side
lobes in both planes to eliminate as much noise is possible.
For a continent wide opening into EU I use a 16el Collinear that is 30'
tall and fixed to the side of the tower. It covers from North EU to North
AF and is great for rapidly changing condx but is very noisy. Switching
to the yagis at times gives me the best possible SNR at the expense of
constant rotator turning.
On 2M external noise is still the limiting factor for terrestial work.
For most EME it is the RX.
>
>>>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.
>
>You are right, Scott's thesis assumes that all of the offending
>noise was concentrated in the 3db beamwidth of the original antenna.
>Increasing the forward gain 3 dB can cause the rejection in an
>"off boresight" direction to increase by a much greater factor.
>If the noise happens to be coming from that direction, the SNR
>improvement could be much more dramatic than 3dB.
Again, absolutely correct. The YO yagi program is a great place to learn
how antennas really perform as compared to outdated traditional thinking.
>>>
>>>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
>>>he 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.
>
>
> Good point!
>>>
>>>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.
>
>If that is the case then how come electrically small
>ferrite rod antennas can effectively null a sky wave
>BC signal?
I havent played with a ferrite rod in ages but I suspect they perform
different than a true loop.
For 160/80 I often have to use a 2 turn balanced loop to null local
noise. It is 4' in diameter and made of 1" CATV line and remotely tuned
from the shack. It sits 6' off the ground and is turned by a TV rotator.
There is often no directivity noted on the desired DX signal and
ionosphere propagated noise but local noise goes down 20-30dB or more.
I have tried just about every type of active and passive RX antenna in
order to increase low band country totals. Several do appear to have a
slight edge at one particular moment but also add to confusion. I have
decided that 6 Beverages plus the loop do the job 99% of the time. Being
limited to only 1200W QRP on those bands is often the deciding factor.
>
>>
>>>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.
>
> How do you make a distinction between between absolute
> and perceived trasition frequency. Is there a defined
> internal noise temperature for HF equipment? Is it 290K,
> 29K, 2.9K? The fact of the matter is, the transition point
> depends upon the prevailing ionspheric conditions, antenna
> noise temperature (component due to thermal noise), feedline
> loss, and receiver noise temperature (post detection).
You obviously stated it much better than I Mike. I have been trying to
keep it brief since we are still quite off-topic.
But in any case it has been pretty well accepted by the true VHF experts
( not I ) that external noise is the limiting factor up thru 2M. This
assumes state of the art equipment, feedlines, antennas, etc. And yes
there are those rare moments where a 1dB NFpreamp can overcome some of
the 10-12dB NF in the average ricebox xcvr on 10M. But 20MHz as a
transition, no way IMO.
>>>
>>>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.
>
>Carl, I didn't know you were on the amps reflector editorial board.
Anyone with a keyboard is an instant author, critic and associate editor.
As far as I know only Paul has the voice of God. (- :
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
>
>73 de Mike, W4EF
>
>Not very many countries on any band
>ERP prabably < 1 watt on 160
>
>
>
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