[TowerTalk] Funniest thing I've seen in weeks

[RICHARD BOYD]

What's an FOM?

And, for us non-engineers, bottom line, can we still safely say more 
efficient antennas with more gain are going to tend to be better, in 
general, than less efficient antennas with less gain?

Realizing that, though it seems intuitive that more efficient is ALWAYS 
better than less efficient, that there are exceptions, such as low noise 
receiving antennas that may be less efficient.

Realizing that in some circumstances omnidirectional is better than 
directional, etc.

Resulting in the conclusion we often hear, "You can't have too many 
antennas," meaning it's nice to be able to switch to different antennas to 
find the best one in the circumstance.

?

73 - Rich, KE3Q


----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "Jim Smith" <jimsmith at shaw.ca>
Cc: "towertalk reflector" <towertalk at contesting.com>
Sent: Friday, July 30, 2004 8:23 AM
Subject: Re: [TowerTalk] Funniest thing I've seen in weeks


> > I'm trying to understand your statement below.
> >
> > If the received noise is uniformly distributed in azimuth
> I would think
> > that additional gain, presuming that it comes from a
> narrower azimuthal
> > pattern, would increase the SNR of the desired signal.
>
> Presuming it does, you are correct. Any additional
> directivity (not gain) increase in the dorection of signal
> would result in the same improved S/N when the noise is
> uniformly distruted.
>
> > If there is a lot of noise from one direction and the
> increased
> > directivity is such that there is now a null in the
> direction of the big
> > noise then the SNR of the desired signal would increase a
> lot.
> > Am I on the right track here or totally out to lunch?
>
> No, that's how it works. Directivity and the direction of
> signal and noise determine the change in S/N ratio.
>
> Gain is directivity times efficiency, so we have to sort out
> if a gain change moved response away from noise and towards
> signal (better S/N) or gain increased without greater
> attenuation of noise and/or focusing on signal.
>
> Let me give you an example:
>
> I have a receiver, like most non-microwave receivers, that
> limits on propagated noise. I change the feedline and remove
> 10dB  of loss. I'm 10dB louder in Europe. Do I hear more DX?
>
> Not at all. I hear exactly the same.
>
> I have a two-meter repeater antenna in a clear field and the
> primary noise comes from the horizon at zero elevation, from
> all the junk in the distance around the antenna. I change
> the 1/4 wl groundplane to a 6dBd collinear, do I get better
> S/N?  Maybe a tiny bit, but nowhere near 6dB worth. The new
> antenna still primarily focuses on  the noise. To obtain
> gain, the collinear removes response at angles other than
> the horizon where there is very little noise. Since the
> noise mostly comes from 0 degree angle, S/N barely changes.
>
> It is a common myth that is repeated over and over again
> that any improvement in antenna gain pays a double payback
> as an improvement in receiving. A recent QST article
> assigned a FOM to improvements, and the author incorrectly
> always assigned a receiver improvement to antenna system
> gain improvements in his FOM. The incorrect idea that more
> gain=better receiving is so woven into our minds that no one
> caught or corrected the error.
>
> An increase in gain can change S/N any amount from a
> decrease to a huge improvement.
>
> 73 Tom
>
>
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