Octavian CRISTEA, Paul DOLEA, Paul Vlăduţ DASCĂL
66 TELECOMUNICAŢII ● Anul LII, nr. 2/2009
For antenna pointing, an AlphaSpid BIG RAS
AZ/EL rotator [1] was selected due to its high perfor-
mance (0.5° accuracy and high torque at low voltage)
– see Table 3.2 The remote control of the AlphaSpid
rotor is done by the digital interface Rot2Prog [2]
through 8 wires (4 wires for each azimuth and eleva-
tion). This interface is equipped with a serial RS-232
interface and allows steering the antenna beam toward
the satellite, using orbital TLE data.
Table 3.2
Rotator technical data
Azimuth angle of turn 360° ; +/- 180°
Elevation angle of turn 180° ; +/-20°
Azimuth rotation speed 120 sec. (12V), 60 sec.
(24V)
Elevation rotation speed 80 sec. (12V), 40 sec. (24V)
Motor voltage 24 DC/1.5A (12 DC/2.5A)
Weight 22 kg
A feed horn for the 2400 MHz communication
channel was mounted on the antenna tripod at the
focal point. It was especially designed to work in the
frequency domain of 2100-2700 MHz and it is suit-
able for parabolic antennas with F/D radio 0.4-0.5.
The feed horn is excited by two probes at 90 degrees
from each other through a 3dB hybrid coupler to
provide the appropriate right hand circular polarization
(RHCP). The feed horn provides an 6dB gain, more
than enough to cover a -3dB linear to circular po-
larization mismatch. An optional low-noise amplifier
mounted behind the feed can further increases the
antenna gain and correct for line losses.
A short notice on choosing a circular polarization
(CP) for the feed horn: the nanosatellite antenna is
usually a quarter-wave monopole which is linear po-
larized (LP). Using a linear polarized ground antenna
feedhorn, in an ideal case where the two antennas’
polarizations are perfectly aligned (co-polarized), then
there will be no polarization loss between them. How-
ever, a nanosatellite has attitude control limitations
and it is spinning with an unknown frequency. As the
angle between the two antennas increases to 60
degrees, the loss increases to 3dB, the same loss
as between CP and LP antennas. As the angle ap-
proaches cross-polarization at 90 degrees, polarization
loss can exceed 20dB, which would prevent a suc-
cessful communication link. Another problem which
can arise from this LP – LP configuration is that of
Faraday rotation as the signal passes through the
atmosphere. Near 2400 MHz, we can expect typical
polarization rotation of approximately 20 degrees,
which is added to the antennas’ polarization angles
mismatch.
3.2. The radio modem
The chosen radio modem for this station proto-
type is MHX-2400 since it was already tested for space
missions [3], [4], [5]. MHX-2400 is a frequency hopping
spread spectrum radio modem that works in ISM
band (2400-24835 MHz), and was developed by
Microhard Systems [6]. The unit can transmit up to
1 Watt (30 dBm, or 0 dB) of RF power, and has a
receive sensitivity of -108 dBm (-138 dB) with a bit
error rate (BER) of 1 in 10
-6
.
Other advantages of MHX-2400 are a relatively
slow frequency hop time interval, a compact size, and
an operational flexibility. The equipment includes built-
in features like addressing, retransmission protocols,
encryption and FEC. The module provides a theore-
tical maximum throughput of 83kbps and the over-the-
air data rate is fixed at 172 kbps. The modulation used
is Gaussian Frequency Shift Keying (GFSK). It has 20
pseudo-random user selectable frequency hopping
patterns and it is a full radio-modem that performs
packetization, modulation and demodulation.
The MHX-2400 module is designed to connect to
external logic circuitry through unshifted (0-5V TTL)
(20 pages)
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