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Re: Once again: Realistic BBQ Dish Gain with Original Dipole Feed, an



Almost...there's an assumption missed.

The illumination of the "original BBQ dish" is not the same as the illumination that most of us strive for on a "typical" circular dish (in the -10 to -13dB range at the edge).  So it is not correct to merely compare the ratios of the areas of the two antennas and conclude from that information alone that there is a gain difference; the combination of (spillover+taper efficiency) is quite different in this case.

I have calculated the gain of the 2'x3' BBQ using NEC-2 and get 25.5dBi.  This is using an "exact" physical representation of the reflector (meaning, the actual wire grid spacing and wire diameters).  

This result is highly dependent upon the accuracy of the dipole+reflector feed.  I also hand-calculated the spillover and taper efficiency using modelled patterns of the feed alone and got within 0.2dB of the 25.5dBi result above.

Certainly, there is a 3dB polarization mismatch when using said BBQ, so your realized gain would be at best 22.5dBic (that's why that letter -c- at the end is important!).

Even more important, and as pointed out by a few when I originally posted my calculated BBQ gain results a few months ago on this forum, is the antenna noise temperature Tant seen by the antenna side/backlobes.  Only then can one truly compare antennas, using their respective G/Tant ratios.

Without going into extensive detail right now, I did do the Tant calculation for the BBQ, based on the NEC-2 calculated patterns, and found it to be approx. 2.5dB worse than a 2' circular dish with helix feed.  This isn't terribly surprising given the higher spillover from the less-directive feed used by the BBQ.

So, in a nutshell, the approx. G/Tant of the 2'x3' BBQ is +1.5-2.5=-1.0dB worse than a "typical" 2' circular dish.  Obviously even worse for a "typical" 3' circular dish.

Another point which shouldn't come into play in our installations:  using a "noisy" LNA/downconverter can swamp out the G/Tant difference of these two antennas and make them perform comparably.  But for the typical noise figures in use, G/Tant is the thing to optimize.



> 
> From: "Eric Rawson" <ericrawson@rawsonoptics.com>
> Date: 2003/05/07 Wed PM 03:18:54 EDT
> To: <amsat-BB@AMSAT.Org>
> CC: <kn6kc@arrl.net>
> Subject: [amsat-bb] Once again: Realistic BBQ Dish Gain with Original Dipole Feed, and After It's Converted for Circular Polarization
> 
> I saw a posting citing a "24 dBi BBQ dish with original dipole feed".  As
> discussed on this BB before, this gain figure is misleading in two ways.
> 
> First, if you use a dipole-feed (linear polarized) antenna to receive AO-40
> or similar circularly polarized bird, there is a gain degredation of
> precisely -3 dB compared to the same antenna receiving a linearly polarized
> signal.  Second, a standard "BBQ" antenna is "race-track" shaped, about 2'
> wide x 3' long, so its area is ~25% less than a 3' circular dish.  That's
> about 1.25 dB less gain due to the area reduction.  A 3' dish with
> circularly polarized feed is generally accepted to have about 24 dBi of
> gain.  Using this number as a reference point, then a BBQ with dipole
> (linear pol'n) feed should have about 24.0 - 1.25 = 22.75 dBi receiving a
> linearly polarized source, but only 22.75 - 3.0 = 19.75 dBi RECEIVING AO-40!
> So calling such an antenna a 24 dBi gain antenna is a bit of a stretch.
> 
> BUT ... this is NOT to say that such an antenna, if modified for circular
> polarization, is not a useful antenna for AO-40.  I have one, and I covered
> the BBQ grill with aluminum door-screening material, made a 5-turn helical
> (circular polarization) feed, stuck a DEM LNA behind the helical feed's
> reflector, ran 6' ft of good coax via the 1" square BBQ axial boom to the
> DEM downconverter hung out the back as a counterweight.  The resulting
> antenna probably has about 22.5 dBi gain (allowing a bit for shading by the
> axial feed) and it works quite well on AO-40.
> 
> I'll have that homebrew, circularized BBQ antenna on demo at the SatEL
> indoor booth at the Dayton Hamvention, if anyone wishes to come by to take a
> look.  It's not for sale but you might like to see how easy it is to modify
> the BBQ dish for circular polarization.  With the extra 3 dB gain that
> circularization provides, and its portable nature, smaller size, lighter
> weight, and relative "air-transparency" compared to larger, solid dishes, it
> makes an attractive AO-40 option.
> 
> Eric, KN6KC
> 
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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> 

Scott Townley NX7U
Gilbert, AZ  DM43di

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