[Moon-net] sending dish question and answer ones more de SM5LE

[SM5LE]

Hi
Again : moonnet and moonbounce is fantastic !

I can forget about gaining 3.4 dB - hi (otherwise the offset dish should be VERY popular).
Here are some answers :


... 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. 

... area of 2.2m dish is 3.8m2  and area of ring is 0.14 m2 thus effective radiating area is 2.2 - 0.14 = 2.06m2
10 log 2.2/2.06 = 0.28dB, however with an offset you would get less ground noise.

... the total blocking is 3.4 per cent = 0.16 dB

... offset dishes require giant feeds (in wavelength)
... you have to consider the feed's illumination profile over the dish, which is not linear. 
Shadowing in the center is not equal to shadowing at about half radius.


TNX to all   /Sven  SM5LE


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 = 36628/2Pi = 36628/6.28 =5833 and V5833=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