[NLRS] Feedhorns for 902 and 1296

[Dr. Gerald N. Johnson]

On Wed, 2006-10-11 at 11:07 +0000, Scott wrote:
> 
> 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.
> 

Depends on the F/D of the dish. Most of the effective feed horns have
had some length, and the better ones for longer F/d (0.5 to 0.7) came
from  K2RIW Richard Knadle <k2riw at riwproducts.com> designs.

I think the longer horn gets a more flat phase front and so more gain.


-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer