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Re: BBQ Dish Gain



I have a NEC-2 model of K5GNA's 38"x26" BBQ dish, including the integrated 
AIDC3733 dipole/reflector plate feed.  Interestingly enough it comes out to 
25.5dBi maximum forward gain.  This gain calculation would include 
spillover and aperture illumination efficiencies.  That's a nearly 70% 
reflector efficiency...hmmm...
Of course, in circular terms that's 22.5dBic.



At 01:40 2003-03-08, you 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.
>
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Scott Townley NX7U
Gilbert, AZ  DM43di

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