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On top of the calculated link budget, we have a number of parameters
that will further weaken the signal. Atmospheric losses losses are an
estimated 0.15 dB/km for water vapour and 0.01 dB/km for oxygen. Rain
is obviously another major cause of loss. A good estimate for
absorption on 24 Ghz is 1 dB/km in the case of 5 mm/h rainfall, and
approaching 10 dB/km in the case of 30 mm/h.
This box sits on top of the next one.
The mixer delivers about -10dBm. From there we amplify with HEMT's to
11 dBm. This signal is amplified by two modules to 26 dBm and by the
final to 30dBm. The drivers are HEMT's from Toshiba. They are actually
specified up to 18 GHz but work well at 1.5 cm. Depending on the
device, they give between 5 and 10 dB amplification. The modules are
FMC2223P1-02 and FMC2223P5-01 from FUJITSU. They are internally matched
devices that have internal power supply biasing networks. They will
deliver 21 dBm at 12 dB gain and 28 dBm at 9 dB gain. Since they are
matched for the 22.4 to 23.6 GHz region, we lose some of the
performance. Fuji was so nice to test a batch at our band for us, and
have come up with an actual decrease in gain of 3 dB. This can be
partly compensated by external tuning. The final stage is the prototype
of a new type of transistor from RAYTHEON. It's a 1 Watt PsHEMT, with a
typical gain of 7.0 dB (at 2 dBc) from 20 to 25 GHz. We mounted the
devices on standard 5880 Rogers substrate and obtained very good
results.
A horn with a 13.7 degrees beamwidth at -3 dB, a gain of about 23 dB,
would have an aperture of 7 by 10 cm and a length of 22 cm. We have the
flight version of the horn. It is a 26.5 dB gain horn. It is
compensated for equal E and H planes and reduced sidelobes. Because of
this the - 3dB points are close to those of the 23 dB horn. Also, with
40 cm in length, the feed point is close to the bottom of the module
where the output is. This saves on coax.
How much power do we really need ?
Because of Doppler shift, the transponder will be switched on at
apogeum only. So the path loss needs to be calculated for 47000 km.
From this distance and the diameter of the earth we know that we need
an antenna with a -3 dB beam width of 13.7 degrees. Converted into
gain, this gives us 23 dB. We assume a receiver system noise figure of
2 dB. By the time this project will be operational, we will be able to
get a noise figure well below 2 dB with amateur means. The receiver
bandwidth was assumed to be 2400 Hz. Reducing the receiver's bandwidth
to 1 kHz reduces noise power by 3.8 dB. The sky temperature is assumed
to be 50 Kelvin. Some documents claim a sky temperature of 10 Kelvin at
Ka-Band. This would result in 0.9 dB less noise power. The receiver
antenna gain can be higher than the 35 dB used. 40 dB seems reasonable.
Calculating a link budget with these parameters tells us we need 800 mW
of power.Concept
The transponder is built the same way as our current mode L and S
transponders on OSCAR 13. Basically these are HELAPS type transponders
without the amplitude restoration. The circuit is divided into four
pieces:
Oscillator
The oscillator generates a 65 MHz LO signal. The circuit sits in a
separate box to avoid heat transfer from other components.Lower Half of the Transponder
Here we come in with the 10.7 Mhz from the IF matrix and the 65 MHz LO
signal from the oscillator module. The outputs are 470 MHz IF and 1200
MHz LO. This pcb also contains the clipping circuit and the bandpass
filter for the IF. This circuit is on standard epoxy pcb. Most
filtering is done with helicals.Power supply
In the same box as the lower half of the transponder, also sits the
switched power supply. This converts the 28 V from the spacecraft to
the several voltages needed. This power supply will also do the proper
power on sequence for the finals. The power supply design is from
Werner Haas, DJ5KQ.Upper Half of the Transponder
This part is fed with 470 MHz IF and 1200 MHz LO. The output delivers
800mW on 24.048 GHz into the antenna. The circuit is partly built on
pcb and partly in waveguide. The part from 470/1200 MHz to 7000 MHz LO
and 2800 MHz IF is done on 0.79 mm Teflon. All filtering is done in
microstrip. The tripler from 7 GHz to 21 GHz is partly microstrip -
partly waveguide and the mixer is waveguide. Both the 21 GHz LO and 24
GHz output filters are waveguide. The final stages are again in
microstrip: 0.25 mm Teflon with 6 mm aluminum on the back side to
facilitate the mounting of the connectors and to allow sufficient
cooling for the power transistors. These pcb's are fixed to an aluminum
plate that covers the whole back of the module. This plate radiates the
heat. All connectors at 24 Ghz are SUHNER 3.5 mm. All others are SMA.Antenna
Although a dish looks promising, there is a problem with the feed. Both
horn and waveguide feeds are mechanically weak and might not survive
the launch. A Cassegrain feed is good but not suited for this small a
dish. The hyberboloid would block too much of the primary reflector and
thus reduce efficiency.
Last updated: Feb 6, 1996
by Ralf Zimmermann, DL1FDT