The Transponders of the Next Amateur Radio Satellite: Phase 3C
by Werner Haas, DJ5KQ
Translated by Don Moe, DJ0HC/KE6MN
Updated and Edited by Vern Riportella, WA2LQQ
20Feb88
With the launch of the AMSAT OSCAR 10 satellite in the summer of 1983, a new
era in amateur satellite communications was ushered in. It now became
possible for the first time to carry on interference-free contacts via
satellite throughout the world for several hours at a stretch. Even those
radio amateurs whose licenses restrict them to VHF/UHF operation are able to
make use of this possibility, since the up and down link frequencies are in
bands corresponding to their license classes. Atmospheric interference and
frequently unreliable propagation are no longer problems for the typical HF
operator. As soon as the satellite is theoretically visible, contacts can be
made.
After nearly five years of operation, a number of possibilities for
improvement have become apparent, especially in the case of the L-transponder.
Due to the advances made in component development, a redesign or modification
of the transponders is now worthwhile. The transponders presently used in
AO-10 still represent the state of development nine years ago. Additionally
there is a new mode of operation, call RUDAK (Regenerating Transponder for
Digital Amateur Radio Communication), a digital transponder module, which
requires new inputs and outputs.
The following is a short description of the transponders, as they will be used
in the next satellite, AMSAT Phase IIIC (to be launched in May, 1988).
The L-Transponder
In general:
The L-Transponder is a linear translator with a bandwidth of 250 kHz. The
input band is 1269.575 to 1269.325 MHz. The output of the satellite occurs
between 435.725 and 435.975 MHz. The general beacon transmits on 435.650 MHz.
The beacon sends general information for the satellite users and transmits in
CW, RTTY as well as in PSK telemetry at 400 bps. An additional beacon on
435.675 MHz is only modulated with 400 bps PSK and serves as the RUDAK
transmitter. The corresponding data input frequency for RUDAK is 1269.675
MHz. The RUDAK transponder channel is completely unaffected by normal QSO
operation and constitutes its own information channel.
For those users of the linear transponder who are not set up for 23 cm
operation, there is an additional input on 2 meters available, which however
has a bandwidth of only 40 kHz. Two possible inputs are under consideration:
1. Input 145.84 MHz +/- 20 kHz - Output 435.95 MHz +/- 20 kHz
2. Input 144.46 MHz +/- 20 kHz - Output 435.95 MHz +/- 20 kHz
The 2 meter input should be reserved as much as possible for stations in the
east-block countries, since it may be very difficult for them to achieve 10
watts on 23 cm. (10 watts and a 15 dB circularly polarized antenna are
necessary for normal QSO operation.) The maximum output power of the
satellite is 50 watts PEP. The L-Transponder needs for the uplink 25 dBW
EIRP. This corresponds to 10 watts into a 15 dB antenna. For reception, an
antenna with a gain of more than 13 dBi should be used (assuming a receiver
noise figure of 3 dB and a bandwidth of 2.4 kHz. A satisfactory receiving
station should receive general beacon with a signal to noise level of 17 dB
minimum.
The circuit:
The input section of the L-Transponder consists of a printed circuit converter
with two mixers. Despite extensive efforts in the input section, the noise
figure amounts to 1.9 dB! The front-end is followed by a variable gain,
wide-band, linear IF amplifier at 10.7 MHz which feeds into a radar limiter.
The RUDAK and command signals are then separated out at this point. The
bandwidth of the transponder is now limited by a filter to 250 kHz. In a
subsequent mixer the 10.7MHz signal is converted up to 53 MHz. After that,
both beacon signals (GB and RUDAK) are fed in. The signal is now separated
into its phase and amplitude portions. The phase portion is converted up to
435 MHz by a limiting IF amplifier and a subsequent mixer. The amplitude
portion is required for control of the HELAPS modulator (High Efficiency
Linear Amplification by Parametric Synthesis). Phase and amplitude portions
are combined again in the power amplifier and result in an output power of 50
W PEP.
This elaborate technique in the HELAPS modulator makes possible a transmitter
efficiency level at moderate drive levels of 40% (envelope elimination and
restoration). A classic linear amplifier achieves at best 12-15%. As can be
seen, the satellite's output power could be thereby virtually tripled.
Frequencies of the L-Transponder Phase III C
Input 1269.575MHz - 1269.325 MHz
Output 435.725MHz - 435.975 MHz
General Beacon = 435.650 MHz
RUDAK Output = 435.675 MHz
RUDAK Input = 1269.675 MHz
Additional input to the linear transponder is to be determined by IARU -
Region 1.
a) 145.84 MHz +/- 20KHz --- 435.950 MHz +/- 20 kHz
b) 144.45 MHz +/- 20KHz --- 435.950 MHz +/- 20 kHz
The U-Transponder (also called Mode B)
The U-Transponder is a linear translator with a bandwidth of 150 kHz. The
input band is 435.425 MHz to 435.575 MHz. The output of the satellite occurs
in the band 145.825 MHz to 145.975 MHz. Additionally, the U-transponder
contains a beacon on 145.8125 MHz (General Beacon) as well as a second beacon
on 145.975 MHz (Engineering Beacon). The maximum output power amounts to 50 W
PEP. For the uplink on 70 cm, the transponder needs 21.5 dBW EIRP for a 20 dB
signal-to-noise ratio for the downlink. This corresponds to a transmitter
power of 10 watts into a 12 dBi circularly polarized antenna.
For reception, an antenna with more than 10 dBi circular gain should be used
(assuming a RX noise figure of 5 dB and a 2.4 kHz bandwidth). A satisfactory
receiving station should receive the general beacon with a signal-to-noise
ratio of 17 dB, minimum. The U-transponder corresponds essentially in
sensitivity and power output to the U-transponder in the AO-10 satellite.
U-Transponder Frequencies Phase III C
Input 435.425 MHz - 435.575 MHz
Output 145.975 MHz - 145.825 MHz
General Beacon = 145.8125 MHz
Engineering Beacon = 145.975 MHz
The S-Band Transponder
For Phase III C there is an additional transponder installed. It is a
hard-limited FM repeater with input on 70 cm, output on 2.4 GHz and a
bandwidth of 30 kHz. The receiver section is already contained in the Mode U
transponder. The IF signal will be fed to the S-band transmitter section,
where it will be up-converted and amplified to 1-2 watts of output power. A
detailed description will become available later on. The Mode-S transponder
will use a small helix antenna with approximately 13 dBi gain. The exact
frequencies will be announced prior to the launch.