[Moon-net] dish question

[Rastislav Galuscak]

Hello Sven,

   dish antenna gain is calculated from formula G = 10log([(D^2 - d^2)pi^2/lambda^2]*efficiency)

Where D is dish diameter, d is feed diameter, lambda is wavelenght (0.231m at 1296MHz)

So for your dish 2.2m without feed is directivity 29.5192 dB
Dish equipped with 0.42 m has directivity 29.3579 dB
Penalty for dish blockade is only 0.1613 dB
Consider efficiency 50%
Your expected antenna gain on 23 cm is 26.35 dB

  73, Rasto, OM6AA

SM5LE <sm5le at telia.com> wrote: Hi all
 
If I use a small dish like 2.2 m; how much do I gain to go to offset dish (with zero blocking) ?
I am using a 42 cm ring and did this calculation. 
Can I compare square cm:s when calculating the blocking from ring ?
 
My choke-ring blockage. (In theory) :

2.2 m dish is 38013 cm2 (my dish).
Area for ring (42cm diameter) is 1385 cm2.
Fictive area (dish with ring): 38013-1385 =36628 cm2
Fictive dish diameter (dish with ring); radius is: r2 = 36628/2π = 36628/6.28 =5833 and √5833=76cm. Gives diameter = 1.52 m
Gain for 2.2m 26.73dBi (VK3UM software) 
Gain for 1.5 m 23.31dBi (VK3UM software) 
and finally; ring blockage: 26.73-23.31= 3.42dB
 
Is it realy that much , 3.4 dB ... 
 
Tnx for some answer
/Sven SM5LE


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