[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Once again: Realistic BBQ Dish Gain with Original Dipole Feed, and After It's Converted for Circular Polarization



I saw a posting citing a "24 dBi BBQ dish with original dipole feed".  As
discussed on this BB before, this gain figure is misleading in two ways.

First, if you use a dipole-feed (linear polarized) antenna to receive AO-40
or similar circularly polarized bird, there is a gain degredation of
precisely -3 dB compared to the same antenna receiving a linearly polarized
signal.  Second, a standard "BBQ" antenna is "race-track" shaped, about 2'
wide x 3' long, so its area is ~25% less than a 3' circular dish.  That's
about 1.25 dB less gain due to the area reduction.  A 3' dish with
circularly polarized feed is generally accepted to have about 24 dBi of
gain.  Using this number as a reference point, then a BBQ with dipole
(linear pol'n) feed should have about 24.0 - 1.25 = 22.75 dBi receiving a
linearly polarized source, but only 22.75 - 3.0 = 19.75 dBi RECEIVING AO-40!
So calling such an antenna a 24 dBi gain antenna is a bit of a stretch.

BUT ... this is NOT to say that such an antenna, if modified for circular
polarization, is not a useful antenna for AO-40.  I have one, and I covered
the BBQ grill with aluminum door-screening material, made a 5-turn helical
(circular polarization) feed, stuck a DEM LNA behind the helical feed's
reflector, ran 6' ft of good coax via the 1" square BBQ axial boom to the
DEM downconverter hung out the back as a counterweight.  The resulting
antenna probably has about 22.5 dBi gain (allowing a bit for shading by the
axial feed) and it works quite well on AO-40.

I'll have that homebrew, circularized BBQ antenna on demo at the SatEL
indoor booth at the Dayton Hamvention, if anyone wishes to come by to take a
look.  It's not for sale but you might like to see how easy it is to modify
the BBQ dish for circular polarization.  With the extra 3 dB gain that
circularization provides, and its portable nature, smaller size, lighter
weight, and relative "air-transparency" compared to larger, solid dishes, it
makes an attractive AO-40 option.

Eric, KN6KC

----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home