SA ready for Qatar geostationary satellite

May 21st, 2018, Published in Uncategorised articles

Hannes Coetzee (ZS6BPZ): “SA is ready to communicate through amateur radio’s first geostationary satellite”.

If all goes according to plan, the Qatar Satellite Company’s second satellite, Es’hail-2, will be placed in a geostationary orbit by a Space-X Falcon-9 rocket within a few months. It will carry an AMSAT linear transponder as a secondary payload, giving radio amateurs access to a geostationary satellite for the first time. The transponder uplink frequency will be 2,4 GHz, downlink frequency 10,489 GHz and the bandwidth 250 kHz.

The Qatar Amateur Radio Society (QARS) managed to secure the privilege to have an amateur radio payload as part of Es’hail-2. Discussions regarding the payload were held with experts from AMSAT-Germany (DL), AMSAT-Finland (OH) and AMSAT-UK. The flight model of the transponder is being built by AMSAT-DL in Germany. Es’hail-2 will be “parked” 35 786 km above the equator at 25,5°East, nearly due North from Pretoria and Johannesburg (which are at 28°E).

Speaking at the recent AMSAT SA annual space symposium in Pretoria, Hannes Coetzee (ZS6BZP) said that South Africa is one of the first countries ready to communicate through the satellite.

Previous high altitude amateur satellites were all in highly elliptical or eccentric orbits to increase the area covered and increase the duration that the satellite is visible to the target area. These satellites were referred to as Phase 3A (lost at launch), Phase 3B (AO 10), Phase 3C (AO 13) and Phase 3D (AO 40) which operated until 2004.

Satellites in geostationary orbits are referred to as Phase 4, with Es’hail-2 being the first. It is referred to as Phase 4A. There are currently other Phase 4 projects in development such as the one by AMSAT-North America (NA) but it will most probably not be visible from South Africa.

The gain of an antenna on a geostationary satellite is limited by 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.

The published transponder uplink frequency is 2 400,050 to 2 400,300 MHz with the centre of the band being 2 400,175 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 polarisation will be used. The preferred modes of operation will be single-sideband modulation (SSB) and continuous wave (CW). Transmit power of 5 W 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 automatic gain control (AGC) of the transponder is not hogged by a single, high power transmission, thus reducing the sensitivity of the satellite. Running higher uplink power than necessary will thus be counterproductive.

A satellite transponder makes it possible to work full duplex. This simplifies the design of a ground station considerably as no transmit-receive switching, sequencers, etc. are required. Things are even further simplified by using a separate transmit exciter and receiver.

For the 2,4 GHz uplink, Coetzee proposes a 2 m all mode radio to drive a linear up converter to generate the required 2 400 MHz signal. Ten watts is a very common output power level for many all mode 2 m radios. This is too much to drive a microwave mixer and an attenuator is required.

The 10,4898 GHz signal from Es’hail-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 option is to make use of a commercial Ku-band satellite low noise block (LNB) down converter, such as used for DSTV reception. The intermediate frequency output of the LNB is fed to a suitable very high frequency receiver that is capable of demodulating CW and SSB.

The universal LNB incorporates a circular waveguide to effectively illuminate the offset-feed parabolic dish antenna. Inside the waveguide are a vertical waveguide to coax and a horizontal waveguide to coax converter. These converters are simple waveguide probes directly connected to very low noise, GaAsfet amplifiers (LNAs). The noise figures of these LNAs are typical < 0,5 dB with values as low as 0,1 dB claimed by some manufacturers. Supplying the LNB with 12 V will select one of the polarisations and 18 V will select the other.

Although the LNB is designed for an input frequency range of 10,7 to 12,5 GHz, the filter preceding the mixer is usually wide enough to enable reception of signals as low as 10,250 GHz with minimal loss. This is very useful for operation on the 3 cm (10 to 10,5 GHz) amateur band.

A 22 kHz tone imposed on the supply voltage selects the 10,7 GHz local oscillator. With no tone present the 9,7 GHz local oscillator is selected. A dielectric resonator (DR) is used for the local oscillator.

The full paper with construction details and reference is available for download from www.amsatsa.org.za.

 

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