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Re: Once again: Realistic BBQ Dish Gain with Original Dipole Feed, and After It's Converted for Circular Polarization



Wow!  Eric.

OK, I confess not to correcting the description of the BBQ dish from "24
dBI" to x-inches by y-inches!  I copied the list of items FS directly from
the e-mail of the person from which I acquired them.  I figured this was an
adequate description to list it.

The dish is one of the BBQ style originally marketed by Myers (in the
1990's) for AO-13 mode-S.  I  hope that clarifies it some more.  I will
take tape measure to it and report exact dimensions...with NO claim for
gain....OK?

Now permit me to clarify and rebut your comments a bit more...:-)
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At 12:18 PM 5/7/03 -0700, you wrote:
>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.

OK you are getting to an issue, but may I correct your terminology...just a
bit?  The gain of the dish is not "degraded".  A linear polarity dish has
perfectly good gain.  It suffers cross-polarity loss used with circular
polarized signals.  I know...I'm nit picking!  

A dipole suffers the same 3-dB loss with circular pol signals, but would
you say its gain is "degraded"?  No a dipole has the gain of...a dipole!

  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.

A 3-foot circular dish has about 24 dBi gain (one may argue the exact
amount but I will not quibble here).  But it has the same gain independent
of polarity.  Polarity "Loss" only comes into the question if not
polarization matched.

  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!

I'm OK with this analysis.  This BBQ dish with dipole feed probably
performs equivalent to a circular polarized dish with ~20 dBi gain when
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.

This is fine and so is the theoretical discussion...but next is the actual
performance.  The Ham who used this dish had a DEM preamp and the
"old-style" DEM 2.4/144 converter which he was able to receive the AO-11
s-band beacon.  That is a quite an accomplishment compared with the
considerably easier AO-40 beacon.  You are correct that a solid 3-foot dish
with circular-pol feed will out perform this dish.  But if you use a fairly
good preamp/conv lineup this dish is quite usable!  Sorry I called it a 24
dBi dish.

>
>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.

I won't be there this year...but certainly will recommend anyone attending
Dayton check out Eric's set up.

73, Ed - AL7EB
My mode-S:  33-inch offset fed dish (helix), MKU-232 A2 preamp, Drake
convertor.  I hear the AO-40 transponder noise floor out to squint of
10-degrees!  I consider it equivalent to a 3-foot dish in performance.  

My mode-LS setup will use a 8-foot dish, DEM preamp, and Drake (435 IF).
Wonder how well this will work? ;-)

PS: at $50 the BBQ dish is not a bad deal is it?  Now that the preamps and
convertors are sold it will be harder to sell.  Maybe I will just use it to
cook steaks on this summer?  LOL! 

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