[NLRS] 10GHz feed horn design & question

[Baker, Donn B]

Hi Jon,
The feed horn affects the "edge taper," or the "illumination" of the dish.  Ideally, the feed horn will uniformally place all the energy on the surface of the dish, and none beyond the the surface.  In practice, this is impossible:  the rule of thumb is for -10dB edge taper... i.e., the edge of the dish is illuminated 10 dB down from the center of the dish.  If the pattern of the feed horn does not provide the correct beamwidth, it cannot provide the correct illumionation.  The higher the gain of the feed horn, the narrower the pattern.  Since your dish is slightly larger than the (usual) DSS dish, you would want a slightly lower gain feedhorn, just like the patterns produce.  (Funny how that works out, isn't it ?)

That said, it may not be worth the effort to change.  A few tenths of a dB at best.  On the other hand, making upo a new feedhorn isn't hard... maybe a hour's work.

My TRW "Antenna Communications Calculator" shows slightly smaller numbers than your calculations:  33 dB gain @ 50 % eff.; 33.8 at 60% eff.  Have you measured the dish as it is ?  If its within a dB of the either of these, I'd be tempted to leave it alone.

The place of the phase center of the feedhorn at the focal point of the dish is more important that having exactly -10dB, etc. illumination.  At 10 GHz, we have maybe 1/8" to play with.  Thats in 3-space: Left-right, up-down, and in-out.  I'd bet (at least a quarter !) that you'll get more gain out of the dish playing with this than you will with a new feed horn.

73 Donn
WA2VOI/0



> -----Original Message-----
> From: nlrs-bounces at mailman.qth.net
> [mailto:nlrs-bounces at mailman.qth.net]On Behalf Of W0ZQ at aol.com
> Sent: Wednesday, November 24, 2004 1:30 PM
> To: nlrs at mailman.qth.net
> Subject: [NLRS] 10GHz feed horn design & question
> 
> 
> I finally measured the dimensions of my "larger" 10gigs 
> offset dish that I  
> have been using this summer.   Here are the stats:
>  
> Largest axis (top to bottom) = 654.05 mm
> Shortest axis (left to right) = 596.90 mm
> Deepest spot along largest axis = 58.42 mm
> Deepest spot from bottom along axis = 292.10 mm
>  
> Using hdl_3b3, this dish, at 10368.1 has the following attributes:
> Focal length = 357.9 mm
> Full parabola f/D = 0.3
> Focal point is located 357.92 mm from bottom edge and 611.73 
> mm from top  edge
> Large axis tilt (to get beam on the horizon) = 67.2 degrees
> Use a feedhorn design of f/D = 0.68
> With 50% efficiency, gain is 33.2 dBi
> With 60% efficiency, gain is 34.0 dBi
>  
> So then I punched the f/D = 0.68 data into the horn design 
> and  generated a 
> template for an 11.29 dBi horn (at 10368).   The horn that  
> is currently on 
> this dish is a RCA DSS horn with a gain of 11.49  dBi.   When 
> I hold the two 
> templates up against each other over a  strong light source 
> to compare them, they 
> are indeed just a little bit  different.
>  
> So, for the dish guru's out there, will changing the feedhorn 
> from the  
> current RCA DSS feed horn (11.49 dBi) to a more appropriate 
> feedhorn (11.29  dBi) 
> provide better illumination for the slightly larger dish and 
> hence a bit  more 
> gain ?   My thinking is yes, but the improvement may be on 
> the  order of one 
> dB or so (its not the difference in the gain of the two  
> feedhorns ..... is it 
> ?), certainly not a BIG deal, but a deal none the  less.   
>  
> 73,  Jon
> W0ZQ
>  
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