[amsat-bb] Dishes v Yagis at 1269MHz

[Charles Suckling <charles.suckling@xxxxxxxxxxxx>]

Someone on the BB recently posted:

"I have to ask, why use a dish on L-band? A Yagi, loop Yagi, or helical 
would give you a much better F/B ratio with a much smaller and more 
manageable antenna, and would give you substantially better gain than even 
a 2-3 meter dish (2 meter dishes aren't really even big enough for C-band, 
let alone L-band) in a much more portable package. I might start thinking 
in terms of dishes for 2.4+ GHz, but 1.2 is more Yagi or helical territory."

The reason for wanting to use a dish on L-Band is simply that the above is 
not true!

Long Yagis at 1.2/1.3GHz certainly work, but in my experience often suffer 
from quite large sidelobes, and do not match the gain claims often made for 
them  I have had, via AO-40, the opportunity to compare two  antennas, a 46 
el Loop Yagi (made by the designer himself) and a 5ft dish.   The dish is 
fed with a 2.25 turn helical feed scaled from the G3RUH S-Band design and 
positioned in the dish for maximum sun noise.  The loop yagi is linearly 
polarised.

The result of extensive AO-40 comparisons is that the dish outperforms the 
loop yagi by at least 6dB. Some of this is due to the linear polarisation 
of the loop Yagi losing 3dB, but the intrinsic  gain of the dish is 
certainly higher.  (Trying to compare the antennas using sun noise was 
difficult - too much pick up of ground noise by the sidelobes of the yagi).

Results on tropo have been similar - the 5ft dish wins every time.  But the 
loop Yagi is the one on the tropo tower, windage and visual considerations 
taking precedence!

I would guess that the point where dishes have similar gain to long 
yagis  is somewhere about 3 - 3.5ft dia.  I know that this is rather less 
than the textbook 10 wavelengths!

Charlie G3WDG

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