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Re: Double Dishes



A quick array calculation shows that you could just get away with it. 
The edges of the two dishes will have to touch so that the focus-focus
spacing is 60cm.
You should be able to get +3dB out of the lashup.
You'd end up with a 2.8 degree half-power beamwidth (plus or minus from
maximum)!
You'll want an honest power divider to combine the feeds rather than a
simple coaxial tee.
The two feedlines from the two dish feeds to the power divider will need
to be matched to < 30 degrees electrical length error to avoid squinting
the beam more than 1 degree (which may be a bit high to design to).
30 degrees at S-band with velocity factor=0.78 is .32 inches of cable.
Plus, any mechanical placement error in the phase centers of the feeds
will...be bad.  Can't do *all* the engineering for you!


David Rush wrote:
> 
> In light of the calculations and observations about S-band receiving setups, I expecting that my 60cm dish + modified Drake (changed xtal,
> removed 2 coils & 2 caps) may not be "good enough".  (Pictures at http://www.draig.org/gallery/200105/20010520a.html if you want to see
> yet another G3RUH-style helix-fed dish.)
> 
> As luck would have it, I do have two dishes, and am considering building a second feed and running two dishes "in parallel".  Any thoughts on
> the additional gain vs. the hassle and additional loss of teeing the two helices together into one Drake?
> 
> Other options I'm considering:
> (1) Looking for a bigger dish, cheap.
> (2) Plunking down the cash for the DEM preamp.
> 
> David, ky0dr
> 
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org


David Rush wrote:
> 
> In light of the calculations and observations about S-band receiving setups, I expecting that my 60cm dish + modified Drake (changed xtal,
> removed 2 coils & 2 caps) may not be "good enough".  (Pictures at http://www.draig.org/gallery/200105/20010520a.html if you want to see
> yet another G3RUH-style helix-fed dish.)
> 
> As luck would have it, I do have two dishes, and am considering building a second feed and running two dishes "in parallel".  Any thoughts on
> the additional gain vs. the hassle and additional loss of teeing the two helices together into one Drake?
> 
> Other options I'm considering:
> (1) Looking for a bigger dish, cheap.
> (2) Plunking down the cash for the DEM preamp.
> 
> David, ky0dr
> 
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



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