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Re: BBQ Dish gain etc. follow-up



An excellent analysis.

It is interesting to compare the modeled numbers for the BBQ dish with some
measured values for my 18" offset DSS dish.  The measured G/t of the 18"
offset dish with a 5 turn helix feed and Down East Microwave preamp ahead of
a Drake downconverter is 0.75 db.  G/t is gain/(RXsystem_noise+ANTnoise).
Using the modeled values and assuming a 1.3 db NF AIDC downconverter:
Converting from dB and NF and using the modeled noise pickup by the antenna
gives, 178/(101+47) or 1.20.  Converting 1.20 back to dB yields a G/t value
of  0.79 dB.  So the larger 38x26 BBQ/AIDC is expected to have almost
exactly the same performance as my 18"/Demi setup.  Of course comparing
measured vs modeled isn't exactly an even playing field but I suspect the
two systems are very close together in performance.

It is also interesting to look at the RDP plot which clearly shows the
difference in beamwidth and pattern as expected between the two dish
orientations.  Mounting the dish for horizontal polarization is certainly
the way to go.

So yes BBQ and 18" dishes can be expected to work "adequately" for AO-40
reception.  However, doing so requires a truly optimized system.  Any dish
surface errors, mounting geometry, cable losses or other noise sources will
have things going down hill in a BIG hurry.

73,
Lee-KU4OS

----- Original Message ----- 
From: "Scott Townley" <nx7u@arrl.net>
To: <amsat-bb@AMSAT.Org>
Sent: Saturday, May 24, 2003 2:07 AM
Subject: [amsat-bb] BBQ Dish gain etc. follow-up


> As promised (threatened?), I have written up my NEC modelling results of
> the 38"x26" BBQ grid dish.  The analysis includes feed and total system
> patterns, plus a stab at antenna noise temperature (and therefore G/T
> comparisons).
>
> http://members.cox.net/nx7u/ao40/bbqdish/BBQGridNec.htm
>
> Any comments, questions, etc., on the analysis are welcome.  Probably more
> appropriate to make them directly to me rather than the list.
>
>
>
> Scott Townley NX7U
> Gilbert, AZ  DM43di
> http://members.cox.net/nx7u
>
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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