Es'hail-2 transceiver setup
QO-100, also known as Es’hail-2 is a geostationary amateur radio satellite.
It can be used by large parts of the world, including Europe, Africa, (parts of) South America and Asia.
The satellite works as a linear transponder. It receives a specific range in the 2.4 GHz (13cm) band and sends it back to earth on 10 GHz (3cm).
In order to generate such a 2.4 GHz signal and transmit it to the satellite conventional radio technology with transverters could be used. I chose a different route and planned a fully software-defined setup.
This has many advantages:
— Cost (PlutoSDRs can be bought for ~120€)
— Low power consumption (laptop, SDR, amplifiers), ideal for portable usage
— No SSB transceiver needed
— Software DSP processing for very clean signals
— I/Q samples can be transmitted over Ethernet instead of running long, lossy runs of coaxial cable.
I chose to use a PlutoSDR as it is modern, can be connected to an Ethernet link, full duplex and widely supported by popular software (including my own).
Other SDRs like the LimeSDR, rad1o and HackRF were evaluated and tested as well:
The HackRF had problems with LO leakage, while the LimeSDR could only provide -0.58dBm output power and had bad software support. The PlutoSDR can output a maximum power level of about 6.5dBm, which is enough to drive a PSA4-5043+ or the CN0417.
On the software-side of things, a DIY SSB modulator with GNU Radio was build and tested, but had high latency and suffered from occasional buffer overflows. This wasn’t ideal, especially for digital signals like FT8.
To operate with 3kHz wide signals at 2400 MHz you’ll need to have a very accurate and stable frequency reference in your setup. With a known good frequency reference, transponder operation can be as simple as transmitting on the RX frequency - SAT LO (8089.5 MHz). Es’hail-2 is located in geostationary orbit, so there is no doppler-correction needed. Correcting the TX frequency by whistling or CW carrier is frowned upon and disturbs other users.
In order to ensure a proper frequency reference I modified the PlutoSDR to add a reference input.
When initially planning the setup, I used a LNA4ALL (PSA4-5043+-based) as my first amplifier stage. Thankfully, around the same time AD released the CN0417 USB Powered 2.4 GHz RF Power Amplifier. It can deliver a lot of output power (almost +28dBm), which is useful to fully drive the second amplifier stage.
While illegal for their intended use, chinese WiFi booster amplifiers are great for usage in ham radio usage.
The EDUP EP-AB003 has a claimed output power of 8W and realistically delivers about 2W. I paid 30€ for a second hand unit. Because these devices are intended to be used with 802.11 signals, they have great linearity and stability. They do need to be modified though, inside the device is an RX/TX switch which is activated via HF-VOX. This is unusable for SSB signals.
The modification shown in my pictures worked for me and some other people, but wasn’t 100% reliable, which is why I would recommend doing the modification proposed by M1GEO: bridging the VSS and IN2+ pins of the AD4851-4 opamp.
The EP-AB003 could output +34dBm when driven into saturation. Pay attention to the connector types, both RP-SMA and SMA are used on the WiFi booster and in the supplied cables. Do not mix them up, as it could destroy the equipment.
In order to suppress LO 3rd harmonics (~6 GHz) and other out-of-band products, I’m using a band pass filter after the amplifier stages. The CN0417 already has an internal filter, but I wanted an additional filter after the WiFi booster PA. This filter has the same RP-SMA connectors like the PA and is called taoglas Airvu BPF.24 2.4GHz.
TP-Link TL-ANT2424B are cheap WiFi dish antennas with high gain. These are specified at 24dBi gain. QO-100 expects a right-hand circular polarised (RHCP) uplink signal, but this antenna is linearly polarized. This causes a loss of 3dB over an RCHP antenna, but is still good enough.
Dual band feed
With this antenna a regular satellite TV dish can be used for full duplex operation.
One extremly useful advantage of the PlutoSDR is it’s Zynq processor and FPGA core. It runs Linux and can be SSH’ed into, flashed with alternative firmware and modified in a lot of ways. It is possible to connect a USB-Ethernet adapter via an USB-OTG cable to the PlutoSDR. This makes it possible to use the radio over a long distance and even over existing network infrastructure. I can recommend adapters with an ASIX AX88179 chipset (CSL-USB3.0 brand).
Portable operation with battery power is easily possible, because all components need either 12V or 5V. I’m supplying the setup with a Li-Ion 3S (11.1V) battery pack and stepping the voltage down with a modified Aukey car USB charger.
In order to receive Es’hail-2, a regular Ku-band satellite TV LNB can be used. I used a Octagon OSLO PLL LNB, bought for about 10€. It’s internal clock is a 27 MHz crystal, which was replaced with some RG174 coaxial cable. To generate a very stable 27 MHz reference clock, I used the GPS reference again. The clock signal gets divided into 2 seperate signals, one for the PlutoSDR and one for the LNB. Normally the PlutoSDR requires a 40 MHz reference clock for the AD936x, but this can be changed with a bootloader environment variable. Both devices can now use the same supply signal. The LNB gets powered via a Bias-Tee, “Axing TZU 15-02” power injector. Receive polarisation can be switched by changing the input voltage from 12V (vertical) to 18V (horizontal). SDR Console can be set up with the correct LOs for TX and RX and will handle all the frequency conversion automatically.
Es’hail-2 is a fun satellite with lots of potential for experiments. QRPp operation is easily possible and a lot of fun. Alternative 2.4 GHz antenna designs can also be tested and used. I quickly worked 31 stations, all in SSB, some in India, Brazil and Mauritius. The 2W power output is sufficient for 59 signal reports and clear transmission.
Hans, OE8HSR has written an excellent guide about QO-100 with best operating practices and tips.