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*Subject*: Re: [amsat-bb] BBQ Dish gain etc. follow-up*From*: "Lee" <lee-fl@xxxxxxxxxx>*Date*: Sun, 25 May 2003 21:41:12 -0400

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. > Not an AMSAT member? Join now to support the amateur satellite program! > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org ---- Sent via amsat-bb@amsat.org. Opinions expressed are those of the author. Not an AMSAT member? Join now to support the amateur satellite program! To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**References**:**BBQ Dish gain etc. follow-up***From:*Scott Townley

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