Getting ready for amateur radio’s first geostationary satellite

June 30th, 2016, Published in Articles: EE Publishers, Articles: EngineerIT

 

If all goes according to plan the Qatar Satellite Company’s second satellite, Es’hailSat-2, will be placed in a geostationary orbit by a Space-X Falcon-9 rocket in early January 2017. The Qatar Amateur Radio Society (QARS) managed to secure the privilege to have an amateur radio payload as part of Es’hailSat-2. Discussions regarding the payload were held with experts from AMSAT-DL, AMSAT-OH and AMSAT-UK.

Hannes Coetzee ZS6BZP speaking at the AMSAT SA Space Symposium about an inexpensive way to operate the first amateur radio geostationary satellite. Hannes Coetzee is the project leader for the AMSAT SA CubeSat project “Kletskous”. He holds a B.Eng. Electronics (Pretoria) and a M.Sc. Space Physics (Rhodes).

Hannes Coetzee ZS6BZP speaking at the AMSAT SA Space Symposium about an inexpensive way to operate the first amateur radio geostationary satellite. Hannes Coetzee is the project leader for the AMSAT SA CubeSat project “Kletskous”. He holds a B.Eng. Electronics (Pretoria) and a M.Sc. Space Physics (Rhodes).

In a paper presented at the 2016 AMSAT Space Symposium, Hannes Coetzee, ZS6BZP explained that the gain of an antenna on a geostationary satellite is limited by the area that needs to be covered (the satellite’s footprint). With too much gain the beam width becomes very narrow leading to areas not being illuminated by the antenna on the satellite. This is the case for spot beams. In order to cover all the visible Earth from geostationary orbit the beam width may not be less than 17,4° leading to a maximum gain of ~20 dB.

Es’hailSat-2 will be “parked” 35 786 km above the equator at 25,5°East, nearly due north from Pretoria and Johannesburg (which are both at 28°E).

The published uplink frequency is 2400,050 to 2400,300 MHz with the centre of the band being 2400,175 MHz. Total bandwidth is thus 250 kHz and right hand circular polarisation will be used. The downlink frequency is 10 489,550 to 10 489,800 MHz with the centre of the band being 10 489,675 MHz. Vertical polarsation will be used.

The preferred modes of operation will be SSB and CW,5 W uplink power to a 60 to 75 cm offset dish should be more than sufficient for the uplink. The transponder will also be fitted with a “LEILA” (LEIstungs Limit Anzeige) input power limiter to ensure fair play and that the AGC of the transponder is not triggered (hogged) by a single, high power transmission. In short, running higher uplink power than necessary will be counterproductive.

Many satellite enthusiasts already possess an all mode 2 m radio. This radio can be used to drive a linear up converter to generate the required 2400 MHz signal. A modern approach making use of easily obtainable microwave components is considerably friendlier to the home brewer than previous approaches.

10 W is a very common output power level for many all mode 2 m radios. This is way too much to drive a microwave mixer and an attenuator is required to reduce the power level to the mixer to the 0 to 10 dBm level. The low power 2 m signal is mixed with a +7 dBm, 2 256 MHz local oscillator signal. Passive diode ring mixers covering the 13 cm band are now available from manufacturers such as MiniCircuits. The mixer is used as an up converter requiring that the 2 m signal is fed to the IF port.

The output of the mixer is at the RF port and includes the sum of the LO and IF components at 2400 MHz as well as the difference of the LO and the IF at 2112 MHz. The wanted 2 400 MHz signal is selected by a bandpass filter. The 2400 MHz output of the bandpass filter is amplified by a low cost microwave monolithic integrated circuit (MMIC). The output of the MMIC is once again filtered by another bandpass filter to ensure spectral purity. The output of the bandpass filter is once again amplified.

The up converter should ideally be capable of delivering +20 dBm (100 mW) at 2,4 GHz. Low cost MMICs start to compress before the +20 dBm level. For that reason two of them are combined to ensure linear operation at the required 100 mW output level.

Coetzee said that many high power amplifiers are now available for the 2,4 GHz WiFi band. “In South Africa these amplifiers are illegal for WiFi use but 100% legal for amateur radio applications. Specified frequency range is 2,4 to 2,5 GHz which suits us fine. Required drive power is typically +20 dBm with a gain of 17 dB. This translates to an output power level of 5 W or +37 dBm. Automatic T/R switching that senses the input port is also incorporated to further simplify matters.”

The up converter requires a 2256 MHz LO signal at +7 dBm. This can be supplied by a crystal oscillator driving a MMIC into saturation and the required harmonic once again selected by a copper pipe cap filter.

The 10,4898 GHz signal from Es’HailSat-2 needs to be down converted to a much lower frequency for filtering and demodulation. It is possible to construct your own 10 GHz down converter but a much easier (and cheaper) option is to make use of a commercial Ku-band satellite low noise block (LNB) down converter such as used for DSTV reception. The IF output of the LNB is fed to a suitable VHF receiver than can demodulate CW and SSB.

DSTV dishes with a diameter as small as 60 cm will suffice for both the 2,4 GHz uplink as well as the 10,4848 GHz downlink. On the receive side a high stability Ku-band LNB in the form of the Avenger PLL2 that feeds a RTL or FunCube dongle-based SDR is all that is required. The full paper with the necessary references is available on www.amsatsa.org.za

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