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M0SKM

Tuesday, 31 March 2020 14:20

150 Watt Power Amplifier for 23cms

Following my recent amplifier build for QO-100 I decided to scrap the power hungry Spectrian board. It wasn't faulty but I had decided to put the XRF 286S transistors to better use in a 23cm amplifier.

Nervously I fired up my hotplate and raised the temperature to 235 centigrade, the trick is to turn the plate off when you drop the board onto it. If you leave the power on the hotplate it will continue to rise in temperature to a destructive level. Using a pair of ceramic tipped tweezers I removed the transistors very rapidly and put them to one side to cool naturally.

I had heard stories about the XRF 286 being damaged during removal/re-soldering in this way, so I wasn't too surprised when I found one of the three read short circuit gate to source. Luckily the two remaining devices had survived; reading fine when tested with a multimeter. My plan was to use a PCB which I bought from Jim W6PQL, fixing holes were sized in US inches but 3mm screws fitted nicely.

My engineer friend milled some 6mm thick copper for me; he also drilled and tapped all fixing points including those for mounting to the heat-sink. I planned to reuse the aluminium chassis and heat-sink from which I had just removed the Spectrian board. Using a nibbling tool I removed a bit more of the chassis to get the new larger PCB assembly to mate with the heat-sink.

So the assembly sequence is important; once the PCB is screwed to the copper heat soak It's difficult to get enough heat onto the board to solder. Initially the board is fitted to the copper with a few screws, at this point I slid the transistors into position and marked the position for soldering. After soldering the transistors to the copper and checking for any bridges I placed the PCB sections under the drain and gate tabs to check for correct alignment. When satisfied the fit was good I populated the two sections with the rest of the components including power cables, bias feed and coax. Ensuring the bias pots were adjusted for minimum/no bias I screwed the boards onto the copper heat-soak and soldered the gate and drain tabs. These tabs are thin and usually solder to the board quite easily.

Using 6mm copper allowed me to power up the amplifier prior to housing, I was able to check voltages and set the bias without generating much heat. I set the bias at one amp each transistor as this was the setting for the Spectrian amplifier, I'm still pondering whether to experiment further with the setting. Up till now I have run it at over a hundred Watts and a friend monitoring my DATV on a spectrum analyser said it was clean with spectral regrowth 30dB down.

On this occasion I only took these two pictures. You can see a good video showing the hotplate soldering process on YouTube, look for W6PQL.

 

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

73 Steve

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.

1) Ampleon Modules

2) BATC Wiki QO-100 Transmitting

3) Groups.IO DATV

4) W6PQL LDMOS Soldering Video

5) Digikey UK

Wednesday, 15 May 2019 10:47

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 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.

On the first day of this event I received no signal on the first couple of passes so assumed it was delayed or cancelled. At the time I was just using my dual band vertical so not the best setup. Saturday the 9th I was tied up with other things so didn't give SSTV a second thought. Then early Sunday I saw some good SSTV pictures posted on Twitter and heard there had been a technical problem in the Russian module; all it seems was sorted now.

Keen to see what I could download I went into the shack and fired up the RX and PC; during the next four passes I got some reasonable results given my less than ideal antenna. Signals were very strong but I lost out when the ISS was directly overhead and in the antenna null point.

I've put some of the pictures in my photo gallery and you can see them here.

If you're interested in SSTV pictures sent from the ISS during previous events you can view the ARISS Gallery here.

One of my friends recently showed me an interesting YouTube video; a tutorial on receiving digital amateur TV pictures with SDRangel. All I've done is follow the instructions in the video and tested SDR devices which I own. Both RTL dongles worked fine; with the newer metal cased version suffering less drift. My LimeSDR Mini also performed faultlessly; and with the increased frequency range over the RTL devices is a better choice for me. The last test has not worked out well so far and that's using an ADALM Pluto, I understand USB performance may be an issue with this device. I'm still looking at the Pluto setup and I'll add some notes here if I can get it to work. What I'm seeing is an odd shaped constellation with the four points looking like four tear drops.

Below here are a couple of screen grabs showing results at 437MHz and symbol rate 333kS/s.

Link to the SDRangel download which has the DATV plugin included.

Link to the YouTube tutorial video. Below the video you'll find links to ffmpeg and a script which you run to add ffmpeg to your Windows path.

Hope you enjoy playing with this as much as I have. Lastly thanks to YouTuber "Corrosive" KR0SIV.

 

Friday, 04 January 2019 08:45

4 Metre DATV Upconverter

This simple to construct project uses a PCB designed by Mike (G0MJW); the circuit consists of an amplifier and mixer with filtering on the input and output. While I have used the board for 4 metres; it is possible to use the same board  for other frequencies with adjustment of component values. BATC members can purchase this PCB from the BATC shop. Circuit details can be found here.

For this project I have used one of the Chinese ADF4351 boards to produce a local oscillator at 400MHz. The 4 metre input at 71MHz is mixed up to 471MHz and the output fed to a MiniTiouner.

Another board available at the BATC shop is used for the PIC which sets up the ADF4351 output frequency and RF level. The board designed by Ron (G7DOE) will fit on top of the square Chinese ADF4351 boards. For this project I used the darker rectangular ADF4351 board so mounted the PIC Controller board on a small sub panel. You can see the board in the pictures below here. Code for the PIC is available on the BATC WIKI.

Local oscillator level for the mixer is specified as +7dBm but I found the +5dBm output from the ADF4351 board to be just perfect.

Builders have the option to use the mixer board with or without the PGA-103 MMIC amplifier, I included it on my board as I have no external antenna amplifier.

Below are some pictures taken during the build. The LED on the front panel is connected to the PIC board and illuminates on PLL lock.

 

See here the PIC Controller board top right; temporary wiring is in place to check that oscillator frequency and output levels were correct.


This picture was taken during a test prior to final assembly; note the hi-tech tinned copper wire antenna.



Now with all the plumbing and connectors in place. To the left of the PIC Controller board is a Chinese buck converter which I used to lower the input volts to a 5 volt linear regulator. This is only for the ADF4351 board as the mixer board will accept 7 to 15 volts DC.


The finished item with not so posh labels; it works a treat so I'm not too bothered about looks.

Lastly a screen grab of the output.

 

 

Friday, 23 November 2018 11:17

My Mobile Shack and Four Metre DATV

In an earlier blog my silver Ford van can be seen in a drone picture; so here's a little more detail about it.

Named Kath after my late mum the van is my mobile shack; I drive a car for trips out so the van is pretty much dedicated to amateur radio and TV. For security reasons I load the kit just before driving to the location. You'll see from the picture of the interior that some storage has been built in by one of my sons. Noticeable also is the outing was a rush job and therefore very untidy. On this particular occasion we were just granted an NOV for DATV in the four metre band so all was prepared in a hurry.

For power I use a 110 Amp leisure battery and a Honda 1000Watt generator; the battery is isolated from the vehicle electrics. During cold weather the generator can power a 600Watt heater. On the van roof bars I have added some clamps for mast support on windy days. The clamps secure a crossbar which overhangs either the offside front or nearside rear of the van. You might imagine using a van allows plenty of space for both carrying kit and operating, I found whatever size vehicle you use you'll always fill it.

Here are a few pictures.

Kath on Dunstable Downs

 

Hasty 4m setup September 2018

 

Rack and shelving in the workshop

 

 

Wednesday, 31 October 2018 21:46

ARISS SSTV Award

ARISS issue an award certificate for submission of SSTV pictures received from the International Space Station. When your pictures are uploaded to the ARISS gallery http://www.spaceflightsoftware.com/ARISS_SSTV/index.php you can claim an award by filling out a questionnaire. I'm not sure exactly what is gleaned from the data but it must give some clues to the ISS radio coverage. Here is my certificate received today for submission of the pictures in my previous blog.

To celebrate the 60th birthday of NASA the international Space Station transmitted a series of twelve pictures; of those I received ten which were fair. I'm quite pleased with them given my antenna is a fixed turnstile about eight feet above ground.

For those new to Slow Scan TV (SSTV) the pictures are converted to audio tones and transmitted over a radio link. In this case a frequency of 145.800 Megahertz. While the frequency is allocated to licenced amateur radio use; anyone with a radio scanner could receive the pictures and decode them with a mobile phone app.

Here are five received this weekend 27 to 28th October 2018. Interference on the pictures can be due to a number of issues; typically movement of the ISS or electrical noise at the receiving station.

 

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