Fw: Re: [TowerTalk] DB's, ad nauseam

[Tom Rauch]

Hi Roy,


<italic><color><param>0000,0000,0000</param>>   I think you're wrong. 


</italic></color>It won't be the first time for that. All it takes to convince me I'm 
wrong are facts that don't fly in the face of wave mechanics or 
physics. Wive's tales and pathological science don't go far with me.


<italic>> International short wave broadcasters have, in</italic>

<italic><color><param>0000,0000,0000</param>> some instances, used ionspheric focusing as a means of enhancing their

> signals into an area, taking advantage of this phenomonon. 


</italic></color>I've been involved with SWBC antenna design from time to time. 
While focusing antennas on a target area is always desirable, the 
change in signal level is absolutely linear with the gain change in 
the direction and angle of the target.


There are two exceptions:


1.) The signal has multipath.


In this case there can be destructive interference between two 
paths. The main effect of multipath is fading increases, along with 
distortion of the signal's modulation. You can hear this using SSB 
on amateur bands when people change antennas and their audio 
seems to change, or on AM as the signal has "selective fading" 
and the carrier or sidebands are nulled randomly making the audio 
distorted with fades. This is a fading problem, not a level problem.

     

2.) The transmitting system is so powerful it actually changes the 
ionosphere. That requires many millions of watts ERP.



<italic><color><param>0000,0000,0000</param>Last month, in

> QST, Tom Schiller(hope i spelt that correctly) wrote an article which

> recognizes this phenomonon.


</italic></color>Because something appears in print, that does not guarantee it 
correct or factual. That's especially true if the goal is selling 
something instead of a technical article.


<italic><color><param>0000,0000,0000</param> I have personally experienced it. Basically

> its this: An antenna with well known and predictable characteristics,

> compared to another lesser antenna(also with known and predictable

> characteristics), producing results via ionspheric propagation

> disproportionate to and far in excess of any known  differences between

> the two. Do you have your own stories to tell about this 
phenomonon Tom?


</italic></color>I don't doubt people "think" they have seen that effect, given the 
fading on the bands...poor calibration of S meters....a tendency to 
exaggerate changes...etc.


For example, I did a series of A-B tests on 75 meters changing 
from a G5RV to a dipole. In virtually every case, the receiving 
station reported the dipole was better. The amazing thing was I 
never changed antennas! The test was really to see how much 
difference I could get with only an antenna "name" change.


Sorry, but this stuff about non-linear amplitude response in the 
ionosphere is pure unsliced bolognia.


<nofill>
73, Tom W8JI
w8ji@contesting.com

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