My Adventures in Satellite Communications - Part 1

I have dabbled with (FM) amateur radio satellites in the past, but without much success. Although they can be accessed with very basic radio equipment (a dual band VHF/UHF transceiver and a handheld dual band yagi antenna), two-way communications requires considerable operating skill. The satellite may only be within range for a few minutes as it passes over, and must be tracked with the antenna whilst also requiring retuning of the radio (to compensate for doppler shift), adjusting the polarisation, operating the transmitter, plus talking/listening and logging!

Commercial broadcasting satellites are put into a geostationary orbit so that they effectively remain above the same part of the earth all the time. Interestingly the concept of geostationary satellites was first proposed by Arthur C Clarke back in 1945 (Clarke, 1945), long before the ‘space age’.

In November 2018 the Qatari government launched a new DBS satellite into a geostationary orbit above the African continent. It is the first geostationary satellite to carry an amateur radio transponder, allowing communications over approximately half the globe. The satellite is called Es’hail-2 (designated QO-100 by AMSAT). At first I thought it might be difficult to use this satellite as it would require specialist microwave equipment for both the uplink and downlink. However, an article in Radcom (Read, Giles, 2019) inspired me to have a go. The uplink frequency is in the 2.4GHz band, which means that cheap equipment intended for WiFi applications can be repurposed to build a suitable uplink transmitter. The downlink frequency around 10GHz is close to the frequency used for satellite television broadcasting, therefore a receiving station can be built using a surplus satellite TV dish and LNB. Note that there is an SDR (software defined radio) located at Goonhilly earth station in Cornwall, accessible via the internet (https://eshail.batc.org.uk/nb/). This makes it possible to listen to the downlink without having any receiving equipment.

QO-100 (Es'hail-2) satellite

In the Radcom article mentioned above the author describes a simple design for the uplink which is essentially a frequency convertor, taking the 433MHz output from a 70cm transceiver and shifting it up to 2.4GHz. The basic concept is shown in the simplified block diagram below.

Simplified block diagram of a mixer-based 2.4GHz up-convertor

The design assumes that the operator already has an existing 70cm radio that can be utilised as the ‘prime mover’. After attenuating the transmitter output, the signal is mixed with the output from a 1966.5MHz oscillator to produce an output around 2.4GHz, which is then filtered and amplified before going to the antenna (in this case, a parabolic dish). The mixer will produce an output at two simultaneous frequencies, equal to the sum and difference of the mixer inputs. For example, if the 70cm radio is transmitting on 433.7MHz, the mixer will give outputs at 1966.5 + 433.7 = 2400.2MHz and 1966.5 - 433.7 = 1532.8MHz. The 1532.8MHz signal is out-of-band and therefore is filtered-out by a bandpass filter centred on 2400MHz.

Since I do not own a suitable 70cm radio, why not use my existing HF radio on, say, 28MHz, with the local oscillator adjusted accordingly? If we set the oscillator to around 2372MHz, the mixer would produce outputs around 2.4GHz and 2.344GHz. The two output frequencies are now quite close together, so it becomes more difficult to effectively filter out the unwanted frequency.

Some radio amateurs are using the mixing method described but also adding a transverter to up-convert from, say, 28MHz to 433MHz, before the mixer. This gets around the filtering problem, but is quite messy and involves having to build or buy the transverter. After doing a bit of internet research I found a neater solution. The ADALM-PLUTO SDR (software defined radio) is a development module for teaching engineering students about wireless technology, and is manufactured by Analog Devices Inc. When connected to a PC running appropriate software (such as SDR-Console) it becomes a very versatile radio transceiver operating up to 3.8GHz. The output signal needs amplifying before it can be connected into a suitable 2.4GHz power amplifier.

An Experimental 2.4GHz Uplink

My aim was to get at least a couple of hundred watts ERP (effective radiated power) pointing in the direction of the satellite. It is difficult to be too precise due to the unrealistic specifications of the components involved and me having no means to accurately measure power at microwave frequencies. Here was my initial power budget based on the published specification for each item.

Initial power budget for 2.4GHz uplink

Testing this setup in my lab, with the antenna pointing out the window, I could not hear my signal coming back on the WebSDR. After further experimentation I concluded that:

• The amplifier was not running warm enough to suggest an output of 4W
• The real antenna gain is a lot less than the specified 25dB.

By cascading another broadband amplifier for more gain I was able to drive the amplifier harder, but still not seeing any signal back from the satellite. I suspected some trees in line with the satellite causing some attenuation, so I set everything up in the garden, with the antenna accurately aligned with the satellite, pointing through a gap in the trees. Eureka! I could now hear my carrier at S7 signal level. The photos below show the setup. The bench supply unit provides power for the two broadband amplifiers. The tripod mounted antenna is a cheap 17 element yagi intended for Wi-Fi applications. The manufacturer claims a gain of 25dB (it does not say what the reference is), but I suspect it would be <15dBi in reality. It is also worth bearing in mind that the satellite uplink requires circular polarisation, so linear polarisation will lose another 3dB.

Experimental setup (with curious onlooker!)

The hardware including PLUTO SDR, two broadband amplifiers and the 2.4GHz power amplifier

Below is a screenshot of SDR-Console (V3) that I am using with the PLUTO SDR.

Screenshot of SDR-Console running on a PC

Further Work

• Improve frequency stability by upgrading the crystal oscillator on the PLUTO. As it is there is a frequency error (at 2.4GHz) of around -25KHz and a lot of drift as the radio warms up.
• Permanently instal the antenna, and perhaps upgrade it to a helix or a dish.
• Disable the auto Rx/Tx changeover in the EDUP Wi-Fi power amplifier.
• Build the 2.4GHz circuitry into an enclosure along with a 5V regulator. Include a 2.4GHz bandpass filter after the two broadband amplifiers to ensure a clean signal going into the power amplifier.
• Build a (Morse) keyed audio oscillator to feed into the mic input, thus producing CW in the SSB mode. Note that SDR-Console has no built-in provision for Morse keying.
• Rig up a satellite TV dish to receive the 10GHz downlink signal. The receive section of the PLUTO SDR can be tuned to the IF (intermediate frequency) coming out of the LNB.

Look out for further posts on this subject as the project progresses!
Part 2: https://www.myadventuresinengineering.com/2019/08/my-adventures-in-satellite_13.html

Health and Safety: It is advisable to stand well clear of the antenna when transmitting as high levels of RF can be generated, especially at the front of the antenna.

References

Clarke, Arthur C (1945). ‘Extraterrestrial Relays’, Wireless World, October 1945, pages 305-308.

Read, Giles (2019). ‘The Easy-100 No-tune uplink converter for QO-100 (Es’hail-2) sat’, Radcom, June 2019, pages 28-31.

Web Resources

QO-100 WebSDR - https://eshail.batc.org.uk/nb/
QO-100 band plan - ttps://amsat-dl.org/p4-a-nb-transponder-bandplan-and-operating-guidelines/
SDR-Console - https://www.sdr-radio.com/Console


Disclaimer: This is my personal blog. Views expressed in my posts are my own and not of my employer. The information provided comes with no warranty. I cannot be held responsible for the content of external websites. Any practical work you undertake is done at your own risk. Please make health and safety your number one priority.