ANTENNA STRUCTURE. The antenna is made from a recycled
C-band (4 GHz) satellite television (TVRO) receiving
antenna. These antennas were popular a few years ago but
have largely been replaced with Ku-band (12 GHz) systems
with 18-inch diameter antennas. These antennas can often
be found no longer used and obtained for simply removing
them. The most common sizes are 10- and 12-foot diameters
but there are a few larger ones around. The typical TVRO
antenna mount is similar to a telescope polar mount and
could be used in this fashion if desired. However, I
wanted full computer-controlled azimuth/elevation control
so I built an entirely new mount (probably a few hundred
dollars at a local welding shop). Three 3-inch steel
angles are set in concrete and provide support. The main
mount assembly consists of a tripod structure which
includes the azimuth chain drive mechanism.
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DRIVE SYSTEM. I am using the drive mechanism from a 16-foot
"horizon-to-horizon" TVRO antenna as the
elevation drive for my project. This drive has the
advantage of a built-in worm drive which keeps the
antenna locked in position when not powered. Many C-band
antennas used telescoping linear actuators for
positioning. These actuators use a motor/gearbox which
typically operates on 36-volts D.C. They are often
available as part of a complete obsolete satellite TV
system. The worm drive used in my project was designed to
use one of these readily available motor/gearboxes.
However, I decided to buy a new actuator motor/gearbox (~$180) because I wanted the additional
horsepower provided by a 90-volt D.C. version. Power for
this motor is provided through a simple Variac and bridge
rectifier supply which allows me to adjust the motor
speed. Approximately 400 lbs. of steel and lead are used
as a counterweight to assist the motor in lifting the
antenna and feed. The azimuth drive uses a roller chain driven by a sprocket
attached to another worm drive. This second worm drive is operated by a
standard 36-volt linear actuator motor/gearbox. The
sprocket is sized such that it turns about 9 revolutions
for a single rotation of the antenna.
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POSITIONING SYSTEM.
The elevation position of the antenna is measured using a
digital inclinometer (~$100)
module mounted in the upper connection box. Although the
module provides data output in serial ASCII format the
electrical interface is TTL. A TTL-to-RS-232 converter (~$35)
module is also located in the upper connection box and
provides the elevation data to an RS-232 COM port on the
286 PC. A 10-turn potentiometer is attached to the azimuth worm drive
and rotates as the antenna turns. The azimuth position of
the antenna is indicated by the voltage returned by the
pot to a general purpose analog-to-digital converter card
(~$300) installed in the 286 PC.
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FEED SYSTEM. A
simple 1420 MHz feed horn was built
from sheet brass and provides both horizontal and
vertical polarization outputs. A commercial equivalent is
available (~$160) from Radio Astronomy Supplies.
These outputs are connected to a 4-position coaxial
switch which is can also be used for switching between
reference load resistors (new surplus, ~$20 each). The
resistive loads are operated at 77K using liquid nitrogen
cooling and at 273K using ice water, and are used for
equipment noise figure checks and as references for
quantitative antenna temperature measurements. The
coaxial loads were obtained at an electronics flea market
at the Dayton, Ohio Hamvention held
annually in May. The preamplifier is a high gain, low
noise (~20K) design using a PHEMT device. Commercial
units are available from Down East Microwave (1420LNAH,
~$140) and Radio Astronomy Supplies (~$185). Standard
RG-213 coaxial cable connects the preamplifier to the
receiving equipment in the radio room.
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