BOB SEYDLER
K5GNA http://www.k5gna.com/ bob@k5gna.com
TWO METER IF VERSION
-- FREQUENCY: The middle beacon is at 2401.323 MHZ (plus or minus Doppler),
it will be found at 145.323 MHZ on your
receiver. Doppler can move it plus or minus 50 KHZ or move. Remember: 2400 MHZ = 144 MHZ and 2401 MHZ =
145 MHZ. Check with the schedule, the
beacon (and translator passband) may or may not be on and the squint angle may
or may not mean that AO-40’s antenna is pointing at you. Keep listening. The downconverter needs to be left on all the time so it will be
at operating temperature when you use it.
IT WILL NOT BE ON FREQUENCY UNTIL IT WARMS UP. I set the 2 meter version to within less than 1 KHZ at room temperature after
the downconverter has warmed up for 2 hours or more.
123 MHZ IF VERSION --
FREQUENCY: The middle beacon will be
found at 123.323 MHZ on your receiver.
These are set at the factory at an accuracy of plus or minus 25 parts
per million which translates to about plus or minus about 60 KHZ.
POWER FOR THE
DOWNCONVERTER: The downconverter
requires about 15 to 28 volts to for operation, it draws 165 MA under normal
circumstances. The downconverter uses a
7812 three terminal voltage regulator
-- 12 volts or 13 volts from the receiver is not enough voltage for
proper operation and the gain will be
lower. Supply power for the downconverter
through the F connector using a bias TEE
(power inserter for $10.00). The circuit of the bias TEE is simply a 2
meter RF choke to keep the RF out of the DC power supply and a capacitor to
isolate the DC from the receiver input.
RECEIVER PREAMP POWERED
12 VOLT OPTION ($20.00) -- I remove
the 7812 voltage regulator and replace it with a low drop out regulator that
will run from about 12.2 volts up to about 28 volts. It will run directly off 12 volts using the preamp power in your
RX. The Pacific Monolithics power supplies have too much ripple to power
the LDO regulator.
CONVERTER GAIN AND
CABLE LOSS: The downconverter was
made to feed a long run of 100’ or more of RG-59 cable into a multiset TV
coupler and into the house TV wiring – this adds up to a lot of losses. It has excess gain at the 2 meter IF
amplifier frequency to overcome these losses and the signal should be
attenuated into your communications receiver. THE GAIN OF THE DOWNCONVERTER HAS ALMOST NOTHING TO DO WITH THE SIGNAL
FROM THE SATELLITE – ONLY THE NOISE FIGURE AND DISH GAIN WILL AFFECT THE SIGNAL
FROM THE SATELLITE. It is much
easier and cheaper to improve the gain of the dish rather than purchasing a
lower noise preamp. You will get more dB’s per dollar of improvement by putting
your money into a dish.
With 50’ of cable, the downconverter will produce an S - 7
to S – 9 meter reading in your
receiver, with no signal. Use an inline 75 ohm 6 db attenuator (Radio Shack
15-1257, $3.49) to reduce the S meter to a more practical S -- 3 or so. I supply a 6 dB attenuator for $3.00.
You need some S-meter reading, so you know it is working. Remember, the AGC in your receiver is
treating this S – 6+ noise from the downconverter like a signal. So, along with some other conditions, an
excessively strong noise signal into your receiver could end up masking weaker
signals
THE ATTENUATOR PAD IS
HIGHLY RECOMMENDED FOR PROTECTION. This
pad also provides some protection if you accidentally key up into your
downconverter with your 2 meter transceiver – it takes ¾ of the power and will
burn up first.
THE 75 OHM CABLE:
The RG-59 cable, with F connectors, provides for a cheap and very easy way to
connect the downconverter. The small
mismatch from the 75 ohm cable to the 50 ohm receiver input translates to a
very small amount of loss, or attenuation, and is of no concern in this
application, since we have excess gain (OVER 20 dB) at this point. Using a low loss cable here, or too little
cable, will not buy you anything, it will result in too much noise into your 2
meter receiver though.
FINDING YOUR TX IN
THE TRANSLATOR PASSBAND Tune your
receiver to hear the MB beacon, and then set your TX frequency to 435.665 MHZ
and lock the TX to the RX (reverse). DO
NOT TRANSMIT. Tune your RX up the
band away from the beacon and unlock your TX from your RX. Transmit and tune your TX to find yourself
in your RX. Lock them back and they
will be good for the session.
FREQUENCY CORRECTION
After the downconverter has warmed up sufficiently, you can tune in the beacon
and record its frequency. Before the
Doppler has moved it too far, go to your tracking program and set the
parameters so that the beacon frequency in your program will come out to the
actual frequency that you received it.
This way, you will always know where the beacon is on your receiver and
your frequency will be accurate in the program for the beacon and the middle of
the band.
NOTE: THE
POPPING IN YOUR RX IS PROBABLY A 2.4 GHZ WIRELESS LAN OR 2.4 GHZ CORDLESS
PHONE. They use spread spectrum
transmission and are actually transmitting at 2401 MHZ. Turn it off and it will go away, if it is at
your neighbor’s house, then you will need to work with them.