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Re: Dish Question



George,

This is one of those "trick" questions!  The answer yes and no:

Dish size changes with frequency since the wavelength is shorter at higher
freq.  This results in a narrower main beamwidth as well.  But the
effectivity to receive a weak signal is tied to the capture area of the
antenna...in this case, a dish's physical area is nearly the same as its
radio aperture (capture area).  This ignores the dish illumination (feed
design) which is another issue that affects dish effectiveness.  {note that
a yagi has a capture area bigger than its physical size, and that is why
you must space/stack them properly to maximize gain and keep a clean
pattern}    
-------------------------------------------------------------------
Some dish formulas:
G = 4*pi*Area (in WL),  Gdb = 10Log (4*pi*Ae),  where Ae = e*(Diam/WL)^2,
e = the effectiveness or efficiency coefficient.  For "real" dishes we
usually chose e = .55 or lower.  An ideal dish (no one has one) e  = 1 (it
gets better when the dish is very large in WL).   {the new dish at NRAO
Greenbank is over 100m diam; at 100 GHz how many WL is that?  what is the
gain?  how narrow is the beam?  extra credit questions}
-------------------------------------------------------------------
Realise that path loss goes up by 6 dB everytime frequency doubles, but so
does the dish gain (directivity), so the effectiveness is the same at all
frequencies for a given size dish.  On EME this has the result that a
ten-foot dish is the smallest "ham-standard" size from 1296 MHz to 24 GHz.
It doesn't hold below this frequency because the dish stops working like a
parabolic "mirror" when it is smaller than 8 wavelengths in diameter.
(i.e. at 1296 MHz the wavelength is 23 cm and a 10-foot dish is 3 m diam
(or 13 WL in diam).  At 432 MHz (70 cm) the dish is only 4.3 WL and begins
to perform not very well near the edges (this is more important for
low-noise reception than for transmitting).

So yes bigger is better (at any frequency).

I don't know if this was what you were asking or wanting to know about.

73, Ed - AL7EB

At 09:53 PM 5/9/2003 -0500, George Humphrey wrote:
>This is probably a really stupid question, but does dish size have any 
>relationship to frequency, or is it just "bigger is better"?
>
>George KC5WBV
>
>Life is a grindstone, it will grind you down,
>or polish you up, depending on what
>you are made of.
>
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