Items filtered by date: Wednesday, 15 May 2019
Wednesday, 15 May 2019 11:47

Amateur TV through Satellite QO-100

With the launch of the Es’hail-2 satellite on November 15 2018; radio amateurs now have access to a geostationary bird.

Thanks must go to the Qatar Amateur Radio Society for the use of two transponders, one for narrowband; and the other for wideband or digital television.

Explore these links to find out more, the launch on YouTube and some technical info on Wikipedia.

This blog is an outline view of the equipment I'm currently using to transmit through the wideband transponder. I say currently as there are plans to increase the dish size and modify the power amplifier. Some of the equipment will be shown in more detail in later blogs.

My first picture is the transmitter which is based on the BATC Portsdown and incorporates a Raspberry Pi computer and a LimeSDR Mini. This particular one includes a 7 inch screen; Pi Camera and built in SMPS which supplies 5.2 volts from a 12 volt input. The Lime Mini is plugged into one of the USB ports and not seen in the transmitter picture below here.

With transmit level adjusted in the Portsdown software there is no added attenuation between transmitter and driver amplifier. In the driver amplifier enclosure which resides in the shack there are several components; I will list the key items here.
•    Isolator to present a load to; and protect the LimeSDR Mini.
•    Bandpass filter to reduce spurious inputs to the amplifier.
•    PTT circuit to key the amplifier from the software delayed Raspberry Pi PTT out.
•    Ex commercial class A amplifier which produces a clean 35dBm output; gain is 47dB.

Using a delayed PTT is absolutely essential as the Lime SDR goes through a calibration process at power-up; output is very big and out of band for a few seconds.

My shack is on the first floor so the output from this driver amplifier is routed to the main PA through approximately 23 feet (7m) of HDF400 cable.

Note in this next picture I have intentionally made space in the left side of the enclosure for the Lime Mini which may end up being installed here.

Same driver amp with the lid on.

Moving downstairs to the back garden I have kept the power amplifier as close to the dish as possible. Based on a repaired Spectrian board it lives in a cupboard on the patio. In a previous life the board overheated and the copper output track burnt, I wired the coax inner direct to the coupler and it works.

There's not a lot more I can say about the PA except that it probably uses more power heating the cupboard than it does generating RF power. On the heatsink I fitted a 60deg thermal switch for fan control. The Sanyo fan works extremely well but the fitted clutch makes an awful screech on start-up.

See the less than exciting picture here.

Home Sweet Home.

Several LNB's have been torn apart and tested for best results; this is the Technomate which gave an MER of 8 on the QO-100 beacon. The dish is a 1 metre Gibertini with an aluminium face. With luck this will be replaced with a 1.25 metre Gibertini.

This to me is the most frustrating element of the setup; the dual feed. Don't get me wrong I think the guys who came up with the feed done a great job on the transmit side. Credit to Mike, Paul and Remco

For someone who is restricted on dish size and quantity receive is very disappointing; there's just so much loss. On receive the Technomate LNB produced an MER of 8dB on the beacon, using it with the dual feed it drops to 5dB. I'm currently using a Venton EXL S which performs about the same but I find it easier to mount.

If I could get away with it I'd have a second dish for receive but that's not going to happen. Perhaps some sort of Helix/LNB combination will perform better.

Anyway here's a picture of the current assembly.

73 for now.