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Re: Formula for antenna polarization loss



Chris,
You can deduce the formulae required for the various theoretical combinations 
from the descriptions on 9-6 and Table 9.1 on page 9-7.  The only combination 
not provided is LP to LP, opposite sense, which is identical to Type 5 Link, 
with infinite attenuation.  Effectively, the formula is a linear function 
with a slope of 1 and an intercept of 0, where the zero point (0,0) is 
infinite attenuation from opposite polarity (either LP-LP or CP-CP) and the 
100 % point (call it 100,100) is matched polarity (either LP-LP or CP-CP).  
For example, when comparing the signal level of a CP wave on a LP antenna 
(any orientation), the value of the signal (integration of the area under the 
curve) is halfway between 0 and 100, or 50:  -3 dB from the maximum "matched" 
combination.  This would be exactly the same scenario if you took a perfectly 
vertical signal and received it on an antenna at 45 degrees.  Take that same 
antenna and move it down to 22.5 degrees and you will have half again as much 
signal (-6 dB).  Move it to 11.25 degrees and you will be -9 dB.  Move it to 
9 degrees (10 % of the "matched" orientation) and you will be -10 dB.  If you 
can define the percentage of polarization offset (LP) or ellipticity (CP), 
you can simply apply it as a linear ratio to the 100 % value and then take 
the log conversion for dB.  OK?

Again, these are theoretical concepts and really have little in common with 
everyday satellite groundstation operation.  I have never observed any 
satellite signal that is constant in polarity or orientation for very long.  
As real-world observations, high-gain LP and fixed-CP antennas "see" enough 
of the ellipticity in satellite downlinks to hear them even when greatly 
mismatched in polarity or orientation.  Changing CP is most valuable when 
doing weak-signal work, such as AO-10, or when the antenna is very small and 
at the margin for effective communications.  A perfect example of this is the 
fading common in omnidirectional CP antennas, like eggbeaters and QHA's.  
When the satellite CP favors them, they hear very well, but when it changes 
to the opposite polarity (typically several times a pass for the LEO's), they 
lose the signal because they do not have enough gain to get an acceptable S/N 
ratio from the ellipticity error in the satellite downlink.  Theory says the 
attenuation is infinite, but practical observation says it is more like -20 
to -30 dB. 

Sorry for so long ... hope this helps,
73, 
Jerry, K5OE


> Thanks for the reply Jerry,
>  
>  > K2UBC's "The Radio Amateur's Satellite Handbook," ARRL, 1998, 9-4 - 9-13.
>  
>  Unfortunately, I have this manual and the above pages do not give any
>  formula.
>  
>  > "The ARRL Antenna Book," ARRL, 1991, 19-19 - 19-10.
>  
>  Does this manual have the formula? If so I will go out and purchase it
>  pronto!
>  
>  Thanks
>  
>  Christopher Cox
>  
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