Radio frequency (RF)
system. After emissions are
received by the antenna and amplified at the feed by the
low-noise preamplifier they are usually routed through a Telonic TTF-1500-5-3EE tunable bandpass
filter (used ~$20).
This provides some
protection from out-of-band interference sources such as
from RADAR sites, etc. Filters similar to this one can
often be found at electronics flea markets or from
surplus electronics dealers. Additional filtering is
available by inserting a 2-stage tunable (1-2 GHz) YIG filter. I bought
the YIG filter elements at an on-line auction site (new
surplus, ~$60 each) and built the power supplies. I am
planning on using the filters as part of a 1420 MHz total
power receiver in the future. Following the filters is a
GaAsFET "gain block" amplifier which increases signal levels by 25 dB.
The signal is then split using a NARDA 4321-2 0.5-2.0 GHz power divider, which feeds the R-7000 receiver. Following
the power divider another gain block amplifier is used to
further boost the signal for the Tektronix 2710 spectrum analyzer. The gain
block amplifiers have noise temperatures of ~200K and are
available at electronics flea markets such as the Dayton,
Ohio Hamvention each
May. Both the R-7000 receiver (used, ~$700) and spectrum
analyzers similar to the one I have (used, ~$2000) can be
found on internet auction sites.
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Data acquisition.
Hydrogen line spectra are currently recorded with a
Tektronix 2710 digital spectrum analyzer. This instrument
can record up to several hundred individual spectra and
average them together for improved noise reduction. Data
can be downloaded from the spectrum analyzer via its
IEEE-488 port. Currently I am using an old
Hewlett-Packard HP-87 computer as an interface for the
analyzer. The HP-87 has both IEEE-488 and RS-232
interfaces and is used as a low-cost (used, ~$25)
translator. Data from the HP-87 is sent on to a Macintosh
computer for collection and storage. Simple software was
written for both the HP-87 and the Mac to perform this
data transfer. While the use of the spectrum analyzer is
convenient an ICOM R-7000 receiver is also available.
This receiver is used for noise figure measurements and
will also be used for hydrogen line spectra when a
suitable detector is built. Audio from the receiver is
measured with the A.C. voltmeter, and its D.C. output can
be fed to an analog-to-digital converter
board (~ $300) in the PC. Using the old HP-87 is an
awkward method of converting the IEEE-488 data of the
spectrum analyzer to the more common RS-232 format but it
was inexpensive. A National Instruments GPIB-PCII
card (used, ~$75) has recently been obtained and will be
used to control the analyzer in the future.
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The positioning system.
The antenna is controlled by software currently running
on an old 286 PC. A program has been written that can be
used for tracking the sun, moon, satellites or any point
in space. The program continuously calculates the
position of an object and this information is compared
with the actual antenna position. If the antenna is off
target by more than a set amount, the drive motors are
operated until a correction is completed. The antenna
uses a digital inclinometer (~$100)
to feed back elevation position data to the computer and
a potentiometer connected to the drive
mechanism provides a voltage ratio which is
proportional to the antenna azimuth position. The voltage
is read by the analog-to-digital converter board in the
PC.
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