Microwave Modules 2 Metre Linear, MML144/100P

 I can't remember where I got my linear amplifier from but it must have been over 30 years ago.

There were several variants and mine is a basic one carrying an RF amplifier for receive and a power amplifier driven from a transmitter with an output of typically 10 watts. It's designed for amplifying not only FM but SSB as well hence the term "Linear". It's rated at 100 watts output and its got a big heatsink....

 Back in the days when I worked for Plessey in Liverpool there were two enterprising young lads, both called Richard and both keen to better themselves. I noticed a colleague carrying a parcel one day as he went into our laboratory and engaged into conversation with one of the engineers sitting at a bench. An ernest discussion followed during which I noticed printed circuit boards and bags of components changing hands. It was the start of Microwave Modules. Engineers short of cash were assembling circuit boards at home and these were being sold by the two Richards. Eventually, when turnover became large enough, they left Plessey and continued their business, even branching out into non-amateur fields. In those days there had been a large increase in enthusiasm for ham radio and new amateurs without the benefit of morse code, confined to VHF, were looking for equipment other than Japanese black boxes. Because the market was so good and because of very little competition from elsewhere, Japanese manufacturers were raking in profits. Prices were astronomic because of price fixing which worked thus... "Dear Retailer, if you want our products you have to sell them at our RRP (recommended retail price) otherwise we won't supply them to you".

British manufacturers couldn't make stuff anything like as good as the Japanese, so very little was on offer but, there was a niche market for cheaper things like VHF converters for HF receivers, transverters and linear amplifiers. Although these items were relatively exotic circuitwise in those days, their component parts were cheap and, given low labour costs, there was a steady market for them. Low labour costs=impoverished Plessey engineers and their wives.

Years ago I twiddled the trimmers in my MML144/100 and increased its power slightly. I recall being warned that I really needed a spectrum analyser, but as I didn't have one continued anyway.

The other day I decided to investigate the linear and see what it did exactly. A wattmeter showed 100 watts output. A spectrum analyser showed two signals, one at 2 meters and one at double this frequency. Today I decided to investigate further.

The first thing one notices under the lower screening panel is the power transistor is a single SD1477 surrounded by chunky tuning capacitors. This transistor provides a minimum of 6dB gain. This is only a factor of 4 so with 20 watts of drive from a transceiver you might get 80 watts output, but typically, the transistor will provide 100 watts from 20 watts drive, a gain of 7dB. Of course the UK license regulations have changed over the years, but basically one was allowed to run 150 watts of DC power into a power amplifier valve anode (or the same total amount into two valves or more) and produce no more than 100 watts of power output. This assumed some 66% efficiency. The 100 watt figure was key. If you realised more than 66% efficiency you needed to reduce the DC input to see 100 watts output.

AM or CW were of course the only modes around when the original figures were produced. FM is a useful mode but overall not as good as AM for communication, which is pretty obvious if you consider plate modulation which will consume as much power again as the DC input to get your RF carrier power. As an aside.. you can also produce AM by using grid modulation, such as that used in the Wireless Set No.19. This demands masses less power but clearly won't be as good for communication watt for watt. To achieve the best 100 watts of AM will often need around 300 watts of DC power. FM on the other hand will require half of this.

SSB is an ideal mode for transistors as the latter don't particularly like getting hot. The MM 2m linear has an enormous heatsink. In an ideal world power goes to the aerial but in practice lots of power is dissipated as heat. With FM the heatsink gets hotter and hotter because the carrier is always present at full power, but with SSB you only dissipate the quiescent power needed to keep the amplifier at the correct bias point plus any inefficiencies due to matching problems and instantaneous losses in the transistor. In theory you could use a smaller transistor or use more DC supply voltage for the SD1477 for SSB transmission. However this would demand more circuit complexity and you would be infringing your license conditions.

The UK license has kept up-to-date by using dB instead of DC power input, however it uses dBW rather than dBm. dBW is the ratio of output power referred to 1 watt rather than 1mW. It mentions 400 watts output being equivalent to 26dBW. This is 56dBm or 400,000 times 1 milliwatt. The change also mentions PEP rather than the carrier output from 150 watts of DC input at 66% efficiency. PEP is regarded as 4 times the RMS power, in other words you can measure the peak to peak voltage of the RF output on an oscilloscope and if you see 200 volts peak-to-peak across a 50 ohm load, this is all you're allowed to use in the UK for the main amateur bands. This represents 70.7 volts RMS (70.7 x root 2 x 2 = 200) which was the voltage across a 50 ohm load when running 100 watts RMS output.

Lots of changes but nothing new, just the way it's expressed. You're still allowed only 100 watts of RF, no matter how it's put. Of course you may misunderstand the various terms and think you're doing better than you are. For example dealers now often refer to a 100 watt transmitter but this is a mere 25 watt transmitter in AM terms. Sounds a lot better though, "I'm running a 100 watt transmitter". The Microwave Modules transmitter is described in old-fashioned terms... it's a real 100 watt transmitter or if you wish, it puts out 400 watts PEP. Drive it a little harder and you're running nearly half a kilowatt. That sounds really good.

 A single transistor was probably cheaper than two, but has the problem of potentially producing a sizeable second harmonic, especially if operating in a non-linear fashion. Better would be a pair of transistors operating in push-pull mode which would produce only odd harmonics. Given a decent matched load the MML144 would not produce problematic levels of harmonics, but into a typical aerial the harmonic levels could be tricky. To help combat the 2nd harmonic a notch filter is incorporated in the drive circuit of the SD1477. Now, here's the problem. If one tunes a power amplifier (by just twiddling) using a typical wattmeter the indication will be that of total power (including harmonics), not just power at the desired frequency.

Below is the PA stage. On the left is a matching trimmer set at minimum capacity and above it a rejection circuit having a small coil and trimmer which are set to 290MHz. Below the PA is the bias cicuit for linear operation. "Stripline" is the term sometimes given to the VHF circuit design where inductors are calculated lengths of track. Chokes are recognisable by their meandering track. This form of circuitry lends itself to broadband amplification because the quality of the inductors is less than perfect making tuning less critical and easier to cover the 2MHz of the UK 2 metre band.

 Years ago I twiddled trimmers to obtain maximum power output. What I ended up with was power at 145MHz AND at 290MHz. Using a spectrum analyser one can see the power indicated on the Bird wattmeter fall as the 290MHz notch filter is tuned. The power level at 145MHz indicated on the analyser remains substantially level as the spike for 290MHz disappears into the noise. I suppose an alternative would be to use an absorption wavemeter tuned to 290MHz and use this to indicate minimum power as the notch is adjusted.

Below is the RF amplifier circuit which provides a decent amount of aerial signal amplification. I understand about 18dB (or an increase of 8x the signal voltage at the aerial)) is the typical figure. The idea is that with 100 watts of output you'll need extra receive sensitivity to hear low power stations that can hear you. The two trimmers positioned one either side of the transistor are adjusted for maximum signal amplification at 145MHz (or if SSB is your preferred mode then 144.5MHz is best).

 During preliminary tests I operated without a dummy load and substantial second AND third harmonics were present. Now, with a good 50 ohm load and the 290 MHz notch adjusted, there is only the 2 meter signal. Power consumption is 9 amps at 13 volts with around 80 watts RF output.

Below is a circuit diagram of a similar model using an SRF1397 power transistor

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