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Re: "Trash Can" dish antennas



RMckni8527@aol.com wrote:

>Most larger Navy and USCG ships have "trash can" antennas mounted on the
>flying bridge.  The best way for me to describe them would be to take a 
>metal
>trash can, cut half of the top off, and mount the parabola in the bottom 
>with
>the feed in the open end.  This in effect is like wrapping a "fence" around
>the outside edge of a normal circular dish.  Does this serve to eliminate
>background noise from over illumination of the dish, so the feed only sees
>signals from the front of the focal point?
>
>I don't know the official nomenclature of these antennas, but they're about
>4-5 ft in diameter, painted Navy gray, have gyro stabilized tracking, and 
>are
>always seen in pairs one port, one stbd on the flying bridge.  Not sure 
>what
>they're used for.  They do actually look like a trash can, or maybe more
>closely resemble a tub used for icing down a beer keg.

If the bottom of the "trash can" is flat, this sounds like
a short backfire antenna. It uses a driven element (often
a dipole) to excite a circular cavity, with radiation coming
out the open end. All the usual references.

>What are the implications for a dish used on our satellite
>freqs? Beneficial to explore a similar design?

Compact. More efficient than a dish of similar size, but
fixed gain.

I built and tested a 1296 MHz short backfire antenna at
school. The back plane and cavity walls were soldered
together from large sheets of surplus PCB material (I
used very thin flexible stuff for the walls), with the
feed being a wire dipole supported on a bit of UT-141 hard
line. I held the subreflector up with wooden struts
cut from small spruce sticks normally used for making
model airplanes.

It worked well, and showed once again that if an antenna
doesn't need to be particularly durable or weatherproof,
you can make it out of all sorts of odd things.

Laura Halliday VE7LDH     "Que les nuages soient notre
Grid: CN89lg                    pied a terre..."
ICBM: 49 16.57 N 123 0.24 W        - Hospital/Shafte

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