Re: [TowerTalk] Funniest thing I've seen in weeks

[Jim Smith <jimsmith@shaw.ca>]

Hi Tom and Jim L

First of all, thanks very much for your replies.

I devised this simple model in the belief that it would reveal to me why 
a yagi will hear stuff a dipole won't, even though noise level in the 
direction of the signal will obviously increase the same amount as the 
signal due to the gain of the yagi.  It seems to have done what I 
wanted.  From this, understanding of Tom's items 2.) and 3.) come quite 
easily.

Thinking that the better hearing on HF due to putting up a gain antenna 
is due to the gain is certainly an easy trap to fall into.  It was only 
after I did my little thought experiment that I was able to resolve the 
contradiction in my own mind.  It is now very obvious to me how this 
works. 

It has now also become obvious what a bad effect that big fat lobe 
pointing straight up on my low dipoles has on received SNR.

Yes, I have heard lots of folks claiming that a higher gain antenna will 
help both transmitting and receiving equally.  I used to be one of them.

The diversity combining info was interesting.  I have an R5 vertical 
mounted a foot or so above my old TH3 tri-bander.  The TH3 will often 
hear signals Q4-5 which are inaudible on the R5.  I have never bothered 
to look, but, under these conditions, the S meter should show noise from 
the vertical to be substantially more than that from the beam.

Hmmm.... maybe when I can hear a sig on both antennas I could listen to 
the beam on the main Rx in the MkV and the vertical on the sub Rx.  
Rats, MkV won't let me do that.  I could put the vertical on the old 
75A-4, though.  As all audio lines here go through a patch panel it's 
dead easy to patch the A4 audio through the mixer into one ear and the 
MkV into the other.

73 de Jim Smith   VE7FO



Tom Rauch wrote:

> > I'm told, by many people who know a zillion times more
> >    
> about this stuff
>  
>
> > than I do, that increasing antenna gain doesn't improve
> >    
> received SNR
>  
>
> > under the following conditions:
> >  The received noise is greater than the internal noise of
> >    
> the receiver
>  
>
> >  The noise has a uniform spatial distribution
> >    i.e. no stronger in one direction than any other
> >
> > This doesn't seem to agree with my experience so I
> >    
> conducted the
>  
>
> > following thought experiment.
> >    
>
> That explanation still misses a few things in cases where
> external noise sets the noise floor (the normal HF case).
>
> 1.) If the noise is EVENLY distributed, S/N improves by the
> directivity change.
>
> 2.) If the noise comes from one direction or a few
> directions, the ratio of response in those direction(s) to
> response in the signal direction and angle determines S/N
> ratio up to the limit where the noise in other directions
> finally dominates.
>
> 3.) If the noise all comes from the same direction and has
> the same polarization  as the signal, you can't do anything
> to improve S/N.
>
> In a city, for example, a VHF 1/4 wl GP can have about the
> same S/N as a collinear vertical because noise comes from
> the horizon, the same elevation and directions as desired
> signals! A omni-direction collinear increases gain in the
> direction of noise as much as it does signal level. We had
> that experience with VHF systems in urban areas.
>
> I have some stuff related to this on my web site. I
> initially put it there because people would claim changing a
> feedline over to lower loss line would help them hear DX on
> HF, and people would claim two Beverages close-spaced in a
> broadside configuration would improve signal to noise.  (If
> I add a second Beverage, gain goes up at least 3dB almost
> regardless of spacing. It is only at wide spacings (over 1/2
> wl) pattern changes and S/N can change significantly!)
>
> One thing for sure, without a pattern change there cannot be
> a S/N change. The exception would be if you are using
> something like a Knight Kit Star Roamer on ten meters.
>
> I wrote QST about this after they published an article where
> the author assigned a figure of merit to antenna gain for
> receiving based on gain, but they never really responded.
> IMO, the idea transmitting gain pays an equal dividend on
> receiving S/N is a very common myth that deserves to be
> corrected. Pay attention, and you will see that myth
> repeated over and over again.
>
> 73 Tom
>
>
>  

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