[TowerTalk] Gain, S/N, and efficiency - anecdote

Fri Jul 30 11:00:12 EDT 2004

Here's a real-life anecdote.

VK0IR, Heard Island, was on the air.  He was on 20M where I had a 6-element 
monobander, 46' boom length (Telrex) at 120'.  A friend of mine near 
Philadelphia -- I'm in Maryland just east of Washington, D.C. -- had a small 
tribander at 30'.  My friend could not hear VK0IR, not a peep.  I had him S9 
plus about 20.  My friend couldn't believe it, called me on the telephone to 
have a listen.  I gathered, by the lack of callers, that many, many others 
of the DXers couldn't hear him either.

There are a number of variables there.  One, though you wouldn't think it'd 
make much difference on the path from Heard Island to PA and Heard Island to 
MD, these are two different QTHs on this end.  The "spot opening" effect is 
often noted on 10M and 6M.  Two, they were different antennas, different 
sizes of antennas -- e.g. different gain.  Three, different heights of the 
two antennas (which is probably the most important one).  Four, different 
feedline lengths and quality.  Actually, mine was a lot longer and probably 
more "iffy" performance wise too.

If I had to choose between a big 6-el 20M monobander to work DX like VK0IR 
and a small tribander -- even at the same height -- let alone a dipole at 
the same height -- I'd take the 6-el monobander every time, for receive as 
well as transmit.
I see that as "real world."

73 - Rich, KE3Q

----- Original Message ----- 
From: "RICHARD BOYD" <ke3q at msn.com>
To: "Tom Rauch" <w8ji at contesting.com>; "Jim Smith" <jimsmith at shaw.ca>
Cc: "towertalk reflector" <towertalk at contesting.com>
Sent: Friday, July 30, 2004 9:29 AM
Subject: Re: [TowerTalk] Funniest thing I've seen in weeks

> 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
> >
> >
> > _______________________________________________
> >
> > See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless 
> > Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
> > any questions and ask for Sherman, W2FLA.
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk at contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless 
> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> 