Linear Build

 

This is actually my second Linear build and what is being documented here is based on the mistakes ^h^h , sorry lessons learnt from the first one,

 

My first SSPA was OK, but I found I was hearing better than people could hear me. Analysing the result of about 100 QSO's I noticed on average I was giving reports 2-3dB's better than I was being given. My old Amp ran at about 400W and the extra few dB by increasing it to 1Kw should come in handy. At the same time I realised that there was a limit to how long I am going to be able to have the 2m array in the garden. So I negotiated another year and then I need to go to 70cm.

 

The time for 70cm has now arrived so It's time to fit the 70cm PA. It shouldnt be too hard - as it was designed from the ground up to take both - so its mainly a case of soldering up the PA PCB and filter board and adding in the 70cm wiring. The sequencer , heatsink , spreader and power supply are shared between the two sections.

 

Since I'm highly unlikely to want to run 70cm and 2m tx simultaneously, it makes sense to share the case and power supply. At the moment only the 2m Section is fitted.

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If you are building your own linear , I hope a few of my choices help you avoid the mistakes I made first time round.. Also I have given suggestions for the suppliers I use to save you some searching.

 

Most of the ideas here are already recycled, but hopefully you might get one useful bit of information from it.

 

Just about everything is going to be documented - but not in the normal way. Each lesson I learnt is shown by at least one example - that is the documentation

There seem to be quite a few linear amp builds on the net

This is my one!

 

A combined 1kW 2m / 500w 70cm

Like most Amateur linears, this one is based on the w6pql PCB. Mainly because he is one of the only two people who sell them. I fear for the homebrew aspect of the hobby if anything happens to him! The PCB's and circuits are based on the Freescale sample circuit but use fairly esoteric PCB materials that are hard to get hold of at a reasonable price so having my own Amplifier PCB made was not possible.

 

However a complete set of components from w6pql would be incredibly expensive. Adding 20% VAT on the components, postage plus the import handling charges makes it quite expensive when you consider you need to have two 1Kg copper spreaders sent across the atlantic.

 

So I went for the PCB and a basic component set. His combination filter and coupler is also a good deal.

 

For the opther parts, I either made them , had them made or bought from suppliers within the EU.

 

 

Size Is Everything

(choosing an enclosure)

 

The first lesson I learnt is make it big, make it so you can get both hands in. Make sure you can adjust every adjustment, tighten every screw with just the lid off. Whilst my first try was a marvel of minaturisation, having to dissasemble it to change the bias was not fun. It's solid state not valve - 50V is only going to tickle you so leave room for both hands a pair of pliers and a screwdriver!

 

I bought a case from RS components. It's basically a 5U 19" sized case. Big enough for both amplifiers, filters and power supply.

 

Stock no.: 7546036

Leave it To the Experts

(Front panel and spreader)

 

There are some jobs it is just better getting someone else to do. Not only are the result better - but it works out cheaper in the long run. Frankly I never believed that - I like hammering metal and there is still a lot of bashing for the internal metalwork to be done. However when it comes to front panels, I end up with scratched panels with badly filed holes and covered with peeling letraset. ThenI came across frontpaneldesigner from Schaeffer in Germany. There is an American outlet too if you are that side of the pond.

 

I am in love - beautiful panels routed and engraved for basically very little money in the grand scheme of things. I saved money by buying a case with panels from RS components and sending them for routing and engraving rather than paying them to supply the materials.

 

The front and rear panel cost me about £80 (or 100 euros) for the pair including shipping to and from Germany.

 

For the spreader I bought a 75mmx300mmx10mm copper plate off ebay for about £30. This is big enough to fit both the 70cm and 2m amplifiers on.

 

frontpaneldesigner can handle plates up to 10mm so I sent it to them to drill and tap the holes and mill out the recess for the devices. The charge was £38 plus postage - so I saved about £180.

Time isn't money

(SWR metering and Relays)

 

Time is infact the opposite. Take more time and spend less money and get a better result.

 

I looked at the old linear and saw the sheer amount of connections running everywhere. I really needed to put a pcb in to make all the interconnects and for holding the relays and temperature sensors.

 

So I downloaded a copy of EAGLE PCB software which is free for home use and small projects. Spent a couple of evening learning that

 

I then sent off for a PCB - £8 for 10 PCB's plus £5 postage. I used Seeed Studio fusion service - impossibly cheap for a two layer, solder resist and silk screen board.

 

OK it took 2 weeks to come - but it tidied up the design nicely - I was so impressed I ordered another PCB for mounting the indicator LED's and dropper resistors on.

 

For metering I didn't like the bargraph indicators - and on my last linear I used an 16 x4 line LCD. This time I decided to use a 1.8" TFT screen and an arduino. However the pin outs between the two aren't compatible. So I used the relay PCB to act as an intermediate connector. The forward power is a meter display and the reverse power is the red bar underneath. In the event of overtemperature It replaces the 'Forward Power' with the temperature and a warning.

 

Also I needed some potentiometers to adjust the output from the directional couplers into the microprocessor. Since I was designing a PCB I thought I might as well put the pots on that , oh and the resistor needed for the temperature probe..

 

My combined SWR/Power Relay board /temperature board/ emergency cutout board cost me £24 including the display and Arduiono Leonardo

 

 

5U x 279D x 432W Stock no.: 7546036

Shopping Can be fun!

(Sequencer, relays,power switching, meters)

 

Apart from the obvious ebay, there are some great small businesses out there.

 

One I came across whilst looking for preamps is vhfdesign.com. They sell a sequencer board with the normal pull down outputs plus a switched high side to drive the masthead preamp and relay based on the w6pql circuit. This is a Ukranian company - and for some reason this seems to mean there are no import duties. If you contact them directly by email, they offer very reasonable prices and postage. The 2m preamp much to my suprise really was 0.2dB NF and well made - considering the price I wasn't expecting it to be that good!

 

A lot of the other components like the solid state relays, Arduino processor and TFT screen came from china via ebay. You can pay more and buy them from the UK - they will come quicker - but they are the same units - just marked up. The chinese suppliers care about their feedback. Whenever I have hadan issue they have refunded without question and sent me tested replacements.

 

Yes they are clones, however if you use common sense you can get a bargain. Firstly buy two of everything. Test one to destruction. Secondly avoid anything which is being run close to its rating or dependant on using the right materials.

 

For instance the SSR's are rated at 60A. It's pretty obvious they aren't. My tests showed that they would do 30A comfortably without too much drop - but 60A - no way!

 

Secondly N connectors - especially teflon ones aren't often made of teflon. Chinese manufacturers often substitute cheaper materials after the first run - so don't buy them! If you are putting 1KW through them buy name makes from known suppliers.

 

And Never EVER buy semiconductors from China!

 

I use www.dutchrfshop.nl for critical components like RF devices, teflon coax and connectors.

 

Last time I monitored the voltage and current on the LCD display. This time I chose big analogue ones. It is much easier to spot a problem. (see under 'noise' below). Again dutchrfshop.nl came to the rescue.

 

Trust no one

(Input attenuation+ thermal protection)

 

Sadly all Rigs suffer from overshoot to some degree or another. Whilst SSPA's really can stand 50:1 VSWR - trust me I've forgotten to plug the antenna in more thsn once.

What will blow them up in a second is too much input. Why do you think my 600+W linear only gives 400W!

 

They do seem to be able to survive a 3dB input overload so the best bet is to run you rig/transverter at 50% and then use an input attenuator to drop that down to the 2W or whatever is required to run the module. So even if it spikes at 100% you aren't going to be buying a new mosfet.

 

You can buy 100W attenuators that mount on the heatsink and occupy little space.

 

The other protection required is thermal. For this I used a DS18B20 probe bought for £3 and fed that to the processor to monitor the heatsink temperature and cut the power if required.

 

They had to buy it from somewhere.

(Power supply)

 

One good source of 50v power supplies is server supplies. Dell sell ones that are rated at 1.5KW and 2KW that can be picket up as NOS (new old stock) for silly prices on ebay - or dutchrfshop.nl

 

However they don't come with connectors. Soldering directly to the pins is a bodge and I've tried to avoid that.

 

Now everybody has to buy connectors from somewhere so track down the manufacturer of the supplies and get the datasheets. They give the part numbers for the connector on the unit.

 

The odds are that either Digikey or Mouser will sell them and they happily sell to individuals.

 

approx 10mmx30mm 10dB from DutchRFShop or Mouser (581-RP10975A06DBFPBK 6dB 100W) £13

DS18B20 probe from Ebay. Library photo as mine is stuck in the heatsink

This one is a bit beaten up after much experimentation on the old linear

72mm meters from dutchRFshop.nl

This is the 1.5KW PSU - when I was just going to rebuild my old linear. The 2KW psu is the same physical size

 

Keep cool.

(heatsinks and cooling)

 

This is actually a reverse lesson. My last amplifier put out 200W of heat - and out of paranoia I fitted very powerful fans that were far too loud when transmitting. The temperature rise was tiny. Since this amp is designed for EME with a 45% duty cycle I decided to go for some quieter fans.

 

The heatsink I found on ebay is one from farnell.(4106015) It is 300mmx150mm with loads more fins than are normally associated with a heatsink.

 

it is rated at 0.04degrees per watt with two papst fans.

assuming 75% efficiency, to get 1kW out means I'm putting 1.33kW in. so 330W wasted heat which corresponds to 13 degrees run continuous. With the thermal mass of the heatsink and the huge copper spreader and a 45% duty cycle, I could cut the flow rate down by 50% and still keep the temperature rise very small and when run continuous still be within specifications for the output driver.

Bigger Isn't always better

(cable)

 

I used LMR400 and Huber+Suhner Succoform 0.25 flexible hardline on my previous amp. This was a mistake.

 

Terminating big stiff cables to a PCB means you tend to pull the tracks off if you move them. They have huge bending radii and are a pain to work with.

 

This time I went for teflon RG303 - it has a power handling capacity of 2.3kW at 100mHz so there is a bit less headroom but it is easy to work with.

Noise is bad for you.

(Screening)

 

My last linear had an issue when the power kept dropping, It seemed to be related to SWR. The culprit wasn't easy to find. Actually it turned out that the RF was making it back into the sense lines of the power supply and the supply voltage was dropping. Since I was taking my voltage reading from the I2C bus on the PSU the display was still showing 50V. In reality it was dropping to 35V.

 

Secondly I hadn't put enough filtering on my 12V lines and on the outputs from the diretional coupler. It took me two blown arduinos to discover this. It was easily solved with a few ferrites on the lines - but again hard to track down.

 

One other thing that I'm doing again (well I'm going to steal the parts off the old linear) is fit a really good filtered chassis mains socket to clear up any RFI out. This is so the SMPS doesn't put any RFI down the mains to spoil my reception.

digikey part A114247-ND for the 1.5kW . Can't remeber the 2kW connector - sorry.

fan assembly with heatsink removed - SSR's mounted on exhaust ports to side of case.

Note the aluminium Screening round the power Supply

A couple of nearly completed ( pre tidying ) shots

 

(These shots have the old 500W amp fitted rather than the 1KW module and the filter is on the main heatsink but has now moved onto the psu cover.

Some links

FrontpanelDesigner (Schaeffer AG)

Metalwork

Seeed Studio Fusion

PCB manufacture

Eagle PCB Design

PCB LAyout

VHFdesign.com

Sequencers LNA's etc

dutchrfshop.nl

RF Components - coax, meters etc.

By M0VKK

hope it helps.