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Re: BBQ Dish Gain


Yep, pretty close.  My calculations for a 3-foot dish is 25 dBi gain,
assuming 50% efficiency.  

If this is a center-feed dish then you not need to consider feed blockage
as the BBQ will have about the same blockage.  I few tenth's of a dB as you
say.  I assume 60% efficiency for my 85cm offset feed dish to give you
comparison.  Theoretically that could provide a 0.8 dB increase of gain
over the equivalent center-feed dish.  So the gain of my 85cm offset feed
dish is roughly the same or more than that of a CP 3-foot (91cm) center
feed dish.  Actual efficiencies are less than theoretical calculations in
all cases and subject to skill and accuracy of construction.  Paul Wade,
W1GHZ, avowed expert in ham microwave dish analysis, est. theoretical eff.
of offset dishes as high as 71% but cautions to derate actual performance
to 50% or less.  Center feed dishes are correspondingly lower in
efficiency, perhaps only 40% eff.

The 3 dB penalty of using linear polarization on AO-40 is considered a loss
rather than an dish efficiency parameter but the result is the same.  The
LP BBQ dish will be the equivalent of using a smaller CP dish.  My guess is
that 24 dBi is about right for the BBQ dish gain and it performs like a 21
dBic gain CP dish.

Using A040_v2.0 you can see the effects of dish gain and receiver NF on
overall signal to noise performance.  The BBQ dish with a good < 1 dB NF
system will have a hard time seeing the satellite noise floor at near
apogee distances.  The additional 3 dB from using CP makes up that gap.

73, Ed - AL7EB

At 12:40 AM 3/8/2003 -0800, Eric Rawson wrote:
>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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