[NLRS] Feed horns for 902 and 1296

[Donn Baker]

Hi Scott.
The numbers sound about right.  The length of the horn does need to be at
least one guide-wavelength long... longer is better.  This length allows
unwanted field modes to die out before they reach the end of the horn and
corrupt the pattern.  Important if (as in you're case) you're going to feed
a dish with it.

One source you really should look at is Paul Wade's (W1GHZ) On-line Antenna
Book.  Don't know the URL exactly, but try <www.w1ghz.org> or maybe
<www.w1ghz.cz>  I'm pretty sure that the ".cz" domain went away, but there
may be a forward there.  Paul has done a great job.

W2IMU feeds have been around for a long time... VE4MA-designs also.  Both
excellent.  Try a search for either/both.  What design, and exactly what
dimensions you use depends on the f/D ratio of your dish.  Seemingly minor
differences can make a big impact on the over-all efficiency of the feed
and dish.  Again, take a look at W1GHZ's stuff to get a feel for what f/D
ratio means for the feed method, etc.  Ask questions.  The only "dumb
question" is the one you don't have an answer for, AND WON'T ASK.

Good luck.

73 Donn
WA2VOI/0

At 11:07 11-10-06 +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.
>
>-- 
>Scott N0EDV
>http://corbenflyer.tripod.com/
>Building RV-4
>Gotta Fly or Gonna Die
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