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Re: BBQ Dish Gain



Hello Eric,
   true a 3 foot dish is around 24 dbi gain, and true a prime focus dish will have some blockage due to the feed, but its not a
large amount, that coupled with the fact that it is a little harder to make sure that your offset feed is in the right spot, makes
it
a wash to me... YMMV

  A BBQ grill on the other hand is no where close to 24 dbi on gain...  if you cover it with window screen then its better.

The real test is on an antenna range, 3 foot dishes come out at around 24 dbi gain, BBQ grills come out between 18 and 20 dbi
gain... its really hard to make that up that 4-6 dbi loss  in a preamp or dc... gain at the antenna itself is always better than
amplified gain...

73's
Kevin
WA6FWF

----- Original Message -----
From: "Eric Rawson" <ericrawson@rawsonoptics.com>
To: <amsat-bb@AMSAT.Org>
Cc: <kn6kc@arrl.net>
Sent: Saturday, March 08, 2003 12:40 AM
Subject: [amsat-bb] BBQ Dish Gain


> I see gains claimed for BBQ dish antennas that do not seem credible.  What
> are the facts?
>
> As a benchmark, a 3-ft diameter, solid-reflector dish antenna with off-axis,
> circular-polarization feed is often said to have a gain of about 24 dBi.  Is
> this correct?  I assume this is true, or close to true, in this note.
> Whether this is true or not, I balk when I see some BBQ dishes that also
> advertise 24 dBi gain when used for an AO-40 2.4 GHz downlink.
>
> What is the correct gain of a typical BBQ when used as a 2.4 GHz AO-40
> downlink?  Here is my estimate.
>
> Firstly, the BBQ is advertised for use as a 2.4 GHz AO-40 downlink, so it's
> receiving a Circularly Polarized ("CP") signal.  But a BBQ, by its nature,
> is a linearly polarized ("LP") antenna.  When an LP antenna is used for
> receiving a CP signal, its efficiency is ~3 dB less than an equivalent-area
> CP antenna.  If the BBQ were used to receive a LP signal, it would not
> suffer this loss, but we are talking about the CP signal from AO-40, so the
> loss is there.
>
> Secondly, the size is smaller.  Typical BBQ antennas are about 2 ft x 3.4
> ft, with an area of about 85% the area of a 3-ft circular dish, so the
> reduced area causes a further 0.7 dB loss.
>
> Thirdly, there is a small loss associated with the shadowing due to the
> on-axis feed of the BBQ, as opposed to no shadow loss with an off-axis feed
> antenna.  Perhaps another few tenths of a dB.
>
> So the total losses compared to the reference case is essentially 4 dB.
> That makes the BBQ, when used for an AO-40 2.4 GHz downlink, essentially a
> 20 dBi antenna, not a 24 dBi antenna as is sometimes advertised.
>
> Is this analysis correct?  If so, a 20 dBi antenna will be noticably
> inferior to a 24 dBi antenna  when copying AO-40 near apogee.
>
> Eric Rawson, KN6KC.
>
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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>


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