[TowerTalk] perspective

[K3BU@aol.com]

In a message dated 7/19/2000 8:18:23 PM Eastern Daylight Time, 
w8ji@contesting.com writes:

> Once the system is limited by external 
>  noise, gain is absolutely meaningless.
>  

Yep, neaming south and hearing atmospheric noise from around the equator. 
Then turn that antenna North and dig out those 10W JAs, and you will see if 
gain is meaningless.


>  An antenna with enough loss referenced to a dipole might give you 
>  problems, but there won't be any S/N difference between 0dBd and 
>  10 dBd gain when listening to a signal if the antennas have the 
>  same pattern. If the patterns are different, the antenna with the 
>  highest ratio of sensitivity in the desired direction to power received 
>  from noise sources will win.  

Your 0 dBd might not bring enough signal for RX to process. You might be too 
biased with low band situations where there is plenty of signal and high 
noise floor. On high bands all the way to moonbounce it is a different story. 
Gain means signal.

>  
>  > One more monkey wrench into philosophy that paths are reciprocal and can
>  > be nicely calculated (N6BT touches up on it in July QST p. 48): 1 or 3 dB
>  > can make a difference in getting the signal into the layer or duct and
>  > propagating it. Sometimes 1 dB can demonstrate itself as 5 or more dB at
>  > the other end. It ain't exactly linear. (I know about RX S-meters being
>  > all over.)
>  
>  If the ionosphere isn't linear, we are in for big time trouble. That 
>  means we have a mixer in the sky, mixing all the signals into sums 
>  and differences and harmonics there of.
>  
>  We all know that isn't true, so it stands to reason the ionosphere is 
>  amplitude linear. 

I don't mean in the sense of "amplifier" linearity, but in the sense of 
ducting, refracting or reflecting properties. If you exceed the treshhold of 
getting the signal into the media, it goes (plus some focusing, etc.) If you 
are "short" of it, no signal gets propagated. This is where dB is noticed 
most. 

>  
>  > Those with stacks try switching them and compare results on RX and TX. 
You
>  > might be surprised how much difference there is between RX and TX signal
>  > reports, I found more like 60% disagreement.
>   
>  I think you are saying the path isn't reciprocal. I'm sure that 
>  happens to a limited extent, in particular on low frequencies where 
>  the wave might be bent or rotated to slightly different paths. That 
>  isn't meaningful in justifying "free gain", because it would have to 
>  be a random event. In other words part of the time it would improve 
>  the reception more than transmission, and part of the time the 
>  other way around. Being random (and small), it would average out. 
>  

Playing with stacked Razors on 20 through 10 m I found more like about 60% 
disagreement between TX and RX path amounting to couple of S units 
difference. 

>  We'd never be able to define that effect unless we all had perfectly 
>  calibrated meters at each end, no QSB, or did some very fast back 
>  and forth data exchanges and recorded the levels.
>  
It takes simple test to see couple of S units difference, no need for 
calibrated gizmos.

Yuri, K3BU  

--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com