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RE: BBQ Dish Gain



Eric said

> 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.

Pretty close. A 36" diameter dish equates to 91.4 cm. 2400 MHz is a
wavelength of 12.5 cm. The gain of a perfect antenna is defined as

      G = (4pi) * (antenna area) / (wavelength^2)
( see http://scienceworld.wolfram.com/physics/AntennaGain.html).

plugging in the numbers assuming an efficiency of 100% shows the gain to be
a factor of 527.6 which equates to 27.22 dB. BTW, if the circular dish is
uniformly illuminated, the resultant beam will about 9.5 degrees (FWHM)
wide.

Because it is hard to get an absolutely flat illumination from the feed onto
the dish without losing some power that doesn't strike the dish, it takes a
LOT of work to get more than half the power (minus 3 dB = 50% efficiency)
present at the feed to illuminate the dish. And there may well be some ohmic
losses in the reflection. And the dish may not be a perfect paraboloid. So
24 dB is a pretty good guess, but YMMV.

> 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.

Yes, because the typical BBQ dish feed is a dipole. But the dipole COULD be
replaced with a circular feed. For that to do any good, then the parallel
grid of the BBQ dish needs to be filled in, like covering it with chicken
mesh (while preserving the paraboloidal shape).

So the 3 dB polarization loss does bring the useful gain down to about 21
dB. The other "real world" factors you mention may well eat another dB out
of the gain.

In terms of what real counts -- the received S/N ratio -- it is really the
ratio of Gain to System Temperature (called G/T) the matters. If some of the
"lost" illumination leaks onto the warm earth because the feed is not really
right, then you take a double hit -- Gain goes down and Tsys goes up. This
is the reason that offset feeds work so well: much of the illumination
"spillover" loss hits cold sky instead of warm earth.

73, Tom


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