Amateur Satellite Communications (3)
Since I first blogged about my QO-100 setup I've made a couple of changes, it was always going to happen.
First to be upgraded was the dish, I had chosen a 1 metre because I didn't think the shack manager would authorise anything bigger. When she heard I was getting mediocre results the outlook changed and I soon took delivery of a nice new 1.2 metre aluminium Gibertini. Actually it wasn't nice because the courier laid it face down and stacked every heavy box he could find on top of it. Having looked forward to installation I was gobsmacked.
My supplier immediately set about packing a replacement, this time he got some wood and made some bracing for it. How annoying that the dish had survived the trip from Italy and the UK based courier had destroyed it, I was so sorry for the seller.
For a while I remained custodian of the bent one while the seller negotiated with the courier. Photo's were sent and I don't know what the outcome was except that my local scrap collector had the biggest grin when he arrived to collect it.
So the new one arrived in perfect condition in a white finish, it's the standard colour but you can get grey or brick finish to special order. As you can see in the picture below I made an awful job of spraying it brown, I chose matt brown because it doesn't reflect the light like white.
Results are much better than the 1 metre but my dual feed is a compromise, I wish I could use the 1 metre for receive and the 1.2 metre for transmit, I'm told this isn't possible because it will interfere with the rotary washing drier. Say no more.
On a good day the QO-100 beacon has an MER of 8dB so I don't do too bad. I also manage a pretty good 2 M/s signal into the satellite using the new PA in the last blog.
So the other change was to fit a switchable attenuator to the front panel of the driver section. This is a used 10dB step attenuator I bought through eBay, manufactured by Tamagawa. Excellent value and there's a load of them at just over £15.00 each at the time of writing. There's a picture of my spare one below.
An attenuator was needed because I’m now able to go overpower on the satellite. A click or two of the attenuator keeps me out of trouble. Pity so many others on the satellite can’t conform.
Credit to Tony at Hisat for his efforts to get a dish to me undamaged. Look him up for a good deal. Hisat.com
My 1.2 metre Gibertini. Changes since the photo are LNB clamp modified (ground) to allow fitting of a straight feed connector, previously right angled. There's also a cover over the LNB / Feed assembly using a piece of inverted plastic gutter, picture below.
This is the enclosure for the PA driver and filtering seen in an earlier blog, now with the addition of a step attenuator.
Tamagawa 10dB step attenuator, came complete with lock washer, washer with locating tab, nut and collett knob. Some of the ebay sellers were selling the attenuator without fixings or knob.
Guttering is clamped to the 22mm feed tube using easy to buy plumbing fittings from Screwfix.
During the early part of 2019 some DATV operators on QO-100 were looking at bigger power amplifiers. Many were using the very inefficient Spectrian boards obtained through eBay, heat output from these boards is huge.
Initially a couple of amateurs that I know of tried a pallet from Ampleon. There are two varieties; one with a single LDMOS device and another with an additional driver stage, both intended for ISM use. Of the two built one is still in use and working well; the other expired fairly quickly I understand due to poor earthing during tests. (1)
Then we heard that Jim G7NTG was carrying out tests with the Ampleon transistor BLC2425M9LS250. After some smoky tests he eventually came up with a finished PA which worked very well. I won't get into Jim's story any further because it's all written up on the BATC wiki; see the references at the bottom of this page. (2)
Realising it wasn't going to be that cheap; and there was some risk of damaging an expensive transistor I decided to go ahead. Luckily Jim had several PCB’s made so I purchased one from him. Jim no longer supplies the boards but I understand the Gerber files and material specifications are available.
Now I'll be honest and say that I borrowed the mechanical layout from a German amateur DC2TH Wolfgang. Pictures of his build were published on a newsgroup; I could see he had access to a workshop with milling facilities but I wasn't going to be deterred from going ahead. The only tools available to me were simple hand tools, drill, taps files etc. (3)
An old pal I used to go shooting with; an engineer and gunsmith has a milling machine and he milled a trench in the copper for me. I supplied a paper template and showed him how I would have to solder the transistor into the trench. He had a lot of work on but I received the copper back finished in two weeks; the rest was up to me.
Along the way I took photographs; some stages were missed due to not having enough hands. One part I should have pictured was soldering the LDMOS device to the copper. There is an excellent video guide on YouTube made by another Jim (W6PQL), I copied his technique which entails the use of an electric hotplate and temperature gauge. In the video flux is applied to the foot of the transistor and a length of solder placed in the trench. Not being brave enough to guess how much solder I used the hotplate and a 25 watt solder iron to tin the trench; just enough solder to the area where the transistor foot would seat. (4)
Now a series of pictures showing various stages in my build. Later I will describe the bias arrangement and one or two other points worth a mention.
Checking threads and alignment after drilling and tapping.
Luckily there had been plenty of space to locate the PA enclosure.
Tapping the copper for sidewall fixing. Yes I know my workmate looks crap but the new one's are not as good.
Input SMA socket in and well lined up, some components soldered on the board.
Output "N" connector mounted. It was necessary to overhang the heatsink to get a plug on.
This is my hotplate used to solder the LDMOS to the copper circa 440 degrees F. Since then I used it to remove some XRF-286S from a Spectrian board. The black rectangle is exhaust paint which is where the temperature is measured. The IR thermometer doesn't like reflections.
Getting there. The RF pickup probe has since been shortened and terminated to the board ground.
The lid was cut from an old diecast box, following test and setup it was sealed with some RF gasket kindly donated by Phil G8XTW.
This was a nervy moment with power applied for the first time, this was just to set the bias at one amp...OK 999mA. More about the bias later.
The big day, unfinished mechanically but ready for big RF out. Took it to my pal Arthur G4CPE because he has test gear which works 13cms and way higher. That's him on the right.
Well it worked no problem, well apart from some sealing and firmer fixing to the lid. We tested it to just above the limit of Arthur's attenuator 158 Watts; and for a brief moment 200 Watts. Gain was 19dB with 33dBm drive and 52dBm out.
So there it is in it's home next to the dish. The dish has been changed to a 1.2 metre Gibertini since an earlier blog, Gibertini call it their 1.25 metre because it measures 1.3 x 1.2 metres.
Summary and references.
Well given the tools I had and the workshop (kitchen) facilities I'm quite pleased with the result.
Some of the passive components and the LDMOS transistor were purchased through Digikey. In the UK buy through the .uk website with free postage on purchases over £33.00. Goods are supplied from the USA and normally arrive in about 3 days, there's no additional tax. (5)
The LDMOS bias is not temperature compensated at present. I used a buck converter down to 8 volts from 28 and then a linear regulator to 5 volts. I also incorporated a crowbar circuit and a glass fuse. At the time of writing this I have just received a temperature compensated board from Patrick ON1BTE, over the coming days this will replace my temporary board.
So a great project provided you are cautious and check where all those Watts are going.
5) Digikey UK
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 https://uhf-satcom.com/blog/patch_antenna
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.