[NLRS] Modifying DirecTV feedhorns?

[Baker, Donn B]

'Morning, Scott.

In general, the backshort should be about 1/4-GUIDEwavelength from the probe.  At 10.368 GHz and 0.75" dia. waveguide, that works out to be ~0.6".  What I've found to be the best is to make it adjustable between about 0.3 and 0.7" if at all possible.  The position can then be adjusted for best return loss (SWR).  I'd shoot for 0.375 for a start.  It may not be the BEST, but it should work.

I'm not sure if you'll be able to optimize with another signal or not.  Never tried it.  You may not be able to get a weak enough signal to notice much difference. Sometime this summer, we'll have an antenna tune-up party, I'm sure.  Try to make it over for (or at least send your feedhorn over).  At last years', I saw feeds go from 6-10 dB to >30 dB of return loss by adjusting the backshort.  (6db RL is 3:1 SWR; 10dB RL is 2:1; 30 dB is <1.1:1 )

(Oh.  Check with one of the other guys who had their horns tested last summer.  If the feedhorn you're using is the same type as one of theirs, measure the distance and use that.  You'll be very close.  Many people are using DirecTV dishes and horns, so you may have the same one as Charlie or Ray, etc.  Make sure both are drilled to 0.75" dia.)

Good luck, and I look forward to working you sometime this summer.

73 Donn
WA2VOI/0

> -----Original Message-----
> From: nlrs-bounces at mailman.qth.net
> [mailto:nlrs-bounces at mailman.qth.net]On Behalf Of Scott
> Sent: Monday, April 25, 2005 10:30 PM
> To: Chippewa Valley VHF Group; NLRS List
> Subject: [NLRS] Modifying DirecTV feedhorns?
> 
> 
> 
> 
> I'm in the process of modifying my feedhorn for 10 GHz.  I 
> have it bored 
> out to 0.750" and I plan to make the plug for the back end 
> adjustable in 
> and out.  About how far from the antenna probe should the plug be?  I 
> think something like 1/4 wavelength?  That might be somewhere 
> a bit over 
> 1/4" ???  Without proper test gear, what's the best way to tune the 
> plug?  I would guess that having someone transmitting a carrier to me 
> and then adjust for highest S-Meter reading?  Would this be a 
> good way 
> to insure a good match on transmit?  I don't have an SWR 
> bridge for 10 
> GHz...
> 
> Got the tuning tool for that tiny capacitor on the DEMI L.O. 
> turned out 
> on the lathe tonight.  Let me tell you, it's a bit hard for a 
> complete 
> lathe novice like me to turn the end of a 3/16" brass rod 
> down to 0.050" 
> and grind a blade on that for adjusting the piston in the capacitor 
> without breaking it off (that happened on the first try -- 
> must not have 
> had the cutting tool bit exactly centered)...now I'm in the mood to 
> finish up the rest of the 10 GHz transverter :)
> 
> -- 
> Scott N0EDV
> Check out our latest promotions at...
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