[Moon-net] dish question answer : only -0.16 dB !

[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 3.8 - 0.14 = 3.66 m2
10 log 3.8/3.66 = 0.16dB,

... 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.

... You can get high efficiencies with offset dishes but they are harder  to construct


TNX to all   /Sven  SM5LE