[NLRS] Feedhorns for 902 and 1296

[Scott]

If I did the calculations correctly, according to the 1988 ARRL Antenna 
Book, for 902 I would need a (circular) feedhorn of about 9.8" in 
diameter and 42-63" in length (6.9" by 28-42" in length for 1296).  That 
is following the advice to make the feedhorn 2 to 3 guide wavelengths 
long.  My question:  Is it necessary to have this much length or would 1 
guide wavelength be OK?  What are the effects of a shortened length? 
Something to do with dish illumination perhaps?  The dish is either 8' 
or 10' in diameter.

-- 
Scott N0EDV
http://corbenflyer.tripod.com/
Building RV-4
Gotta Fly or Gonna Die