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Re: IC-910 / 3731

True, I did skip a step.
This is all from direct integration of the aperture distribution, 
neglecting edge diffraction and an 11.5dB edge taper:
The 4/24 case (4" dia feed blockage on a 24" dia dish) shows only a 0.084dB 
loss in efficiency.  Interestingly enough this is very close to the simple 
geometrical method I alluded to (10*log(1-0.03)=-0.132dB).  I don't claim 
the geometrical method to be accurate for larger cases!
Where did you get the 0.7-0.8dB figure?
One oft-neglected additional effect of central blockage is a raising of the 
sidelobe levels for the overall dish pattern.  In this case, the first 
sidelobe level goes from -25dB in the unblocked case to -20dB in the 4/24 
blocked case.  That of course adds a bit to your antenna noise temperature.
An 18" center-fed for the same feed size has a somewhat surprising 
result.  The directivity only loses 0.8dB but the first sidelobe level goes 
up to -11dB,  worse than a uniformly illuminated circular aperture!  So 
while G does take a bit of a hit, G/T will take much more of a hit.

I have been planning on writing a bit of code to convolve the resultant 
antenna patterns across warm earth to calculate their resultant antenna 
noise temperatures, but haven't gotten a round-tuit yet.  Hopefully all my 
backlogged writing will break free over the holidays while AO-40 is resting up.

I had a 75cm offset dish for awhile, helix-fed.  Anecdotally I found it 
equal to very slightly better than my 60cm center feed (with no blockage), 
which I found noticeably better (again, anecdotally) than my 60cm, 4" 
blockage dish.  Sadly it died in a windstorm just last month.

At 21:03 2003-11-13, Anthony Monteiro wrote:
>At 08:02 PM 11/13/2003 -0700, Scott Townley wrote:
>>My $0.02 is that I don't agree with either of your points :-)
>>1. With a 4" diameter ground plane, the blockage on a 60cm dish is only 
>>(2/12)^2=1/36~3% of the total dish aperture.
>>2. A properly wound 5-turn helix has very little sidelobe 
>>structure.  Front-to-back may not be the greatest, but it doesn't look 
>>anything like a longer helix where the 1st sidelobe is only 8 to 10dB 
>>down from the main beam.  Besides, what's the alternative for a dish with 
>>a f/D of nearly 0.6 (most offsets are around this value)?  The patch is 
>>clearly too broad-beamed.
>>And 1a. there is a way to feed a prime-focus dish with nearly zero feed 
>>blockage...I just haven't written it up yet (but I am using it!).
>Hi Scott,
>The blockage loss doesn't work that way. It is much
>worse than just the area blocked because the feed
>signal is at its max in the center and tapers off
>at the edges. In other words, blockage in the
>center results in a lot higher loss than blockage at the
>edges for the same size obstruction.
>I realized when I sent this out though, that I was thinking
>of an 18" dish where the feed blockage is horrendous
>on a prime focus dish. For a 60cm dish it is only about .7
>to .8 dB - not that bad.
>I agree with you that a patch would be a poor feed for
>an offset fed .6 f/d dish. A better feed would be a horn.
>1a. - OK, I'll bite! How do you get nearly zero feed
>blockage on a 60cm prime focus dish?
>Tony AA2TX

Scott Townley NX7U
Gilbert, AZ  DM43di

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