[Moon] dish question

[Joe Martin K5SO]

Hi Sven,

No, the blockage due to the ring isn't 3.42 dB.

The signal power intercepted by a dish is directly proportional to  
its aperture (cross-sectional area).  If the aperture of your dish  
without the ring is 38013 cm2 and the active area of the dish with  
the ring is 36628 cm2 then the loss in signal power due to blockage  
by the ring is:  dB = 10 log (36628/38013) = -0.16 dB.

73,  Joe K5SO

On Jan 8, 2008, at 2:37 PM, SM5LE 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 = 36628/2¹ =  
> 36628/6.28 =5833 and Ã5833=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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