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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.

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