SOTA 144MHz Amplifier

The FT-817 has a power out of around 5 watts, probably not quite enough for some Aircraft Enhancement propagation which has been a recent topic on SOTA.    I have been looking around for a fairly simple way to get around 30W from a light weight amp that can be carried easily in the field and powered from LiPo batteries.

Mini-Kits have an amplifier kit that uses a Mitsubishi module for a fairly simple linear and FM mode amplifier.  There is a 30W module and a 60W module that can be used with the kit.  I have a couple of 30W modules so I bought a kit to experiment with and see if I could make a small amplifier.

Testing Amplifier

Testing Amplifier

The kit arrived pretty quickly and on Sunday night I put it together and ran a few tests.  I made a 20dB attenuator for the input and found this about right for the 817 running 2.5W and the amp producing about 33W with 12.5V.   The amp would put out more, but wanted to keep it out of saturation.   The amplifier has a bias control that can be used to keep it relatively linear for SSB operation, although the power has to be backed off a little.  The question is how linear is the amplifer?

Next job is to do some IMD tests.

One question was how would it go from a 3S Lipo pack.  The amp was generating 33W on 12.5V from the fully charged LiPo and 27W at about 11V.  Probably fine for what I need.

The amplifier heatsinking is a lightweight compromise.  I chose a piece of 5mm aluminium plate 10cm x 7.5cm.  The module variant I am using has a step on each flange of a few thou that makes it more suitable for a non machined heatsink.  The idea is to fill the small void with silicon grease.  The standard version of the module is very sensitive to surface tolerances on the heatsink and will crack if not treated carefully.

After some “typical” SOTA overs, ie 20 seconds on 20 seconds off for a few cycles, the heatsink was pretty cool (SSB), so I think this is all the heatsinking it will use.  The case will be made from some aluminium angle bolted to the plate and a folded cover screwed over the base.  This will also add some thermal mass.

Next to measure IMD and put in a change over relay.

Relay in

Relay in

A little more done.  Changeover relay is in, next to do some switching control from the 817 and a DC connector.

Will either use an XT60 DC connector panel mounted like on the HF amplifier or have it on a flying lead, although I dont like flying leads.

The top cover will be a piece of aluminium sheet with 20mm angle bolted to it forming a C section that will go over the base and screw into the 5mm base plate edges.  Size is working out to be 10cm x 7.5cm x 2cm high so far.  Pretty compact.

Spent a bit of time on it this morning and now it is pretty well finished except for the top cover.  The XT60 power connector is glued to the front face with teflon tube over the positive lug that goes through the aluminium front face.  On the other side, a piece of PCB with two holes to match the pins on the power connector slides over the pins and is glued to the rear of the front face.  The PCB is then soldered to the pins.  Feels pretty solid.

The negative going key signal from the FT817 goes to the relay via a 47 Ohm resistor and also to a PNP transistor (2N4250) base via a 4k7 resistor.  The emitter of the transistor goes to supply and the collector goes to the bias input to the module, switching the bias on during transmit.

A flat Omron relay is used to switch the RF.  It seems to be OK at 144MHz.

It works

It works

A 10 Amp fuse protects things but this has about 0.25V voltage drop at 5 Amps which is annoying.  This is a nuisance in a battery powered system.  Might try an automotive mini blade fuse for drop.

Power output has dropped a couple of watts to between 25-30W out due to fuse and relay losses (~1dB).

This is still fine for now.  Next is a low pass filter as second harmonic is around 50dB down.  The antenna is resonant, so I could cheat a little and assume it is going to take at least another 10dB out.

Had a little time to make a top cover.  Instead of fixing two lengths of angle to a piece of sheet, I got out the not so good Ebay metal folder and folded a single piece for the cover.  It worked out OK.  Now for a led or something to indicate RF power out.

Now with a lid

Now with a lid

All up weight is 230 grams.

The datasheet for the module says maximum voltage of 17.0V so I tried a 4S lithium pack (16.8V fully charged).  It worked fine, the linearity should be better on the higher voltage, must check this later.

Fully saturated, the output is 45W (FM) on the 4S pack.

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3 Responses to SOTA 144MHz Amplifier

  1. Ian says:

    Very nice work! I have considered building an amplifier with one of those kits (http://vk7is.wordpress.com/2013/05/29/amplifiers-for-2m/), it’s great to see one done!

    Am I correct in understanding the relay is being switched by the PTT from the FT-817? This is one bit I’ve been wondering about – especially seeing Mini-Kits also have an RF relay kit.

    • vk3yy says:

      Hi Ian,
      yes the relay is keyed by the FT-817 PTT line from the accessory connector on the 817. The relay used is from Mini-Kits as well (Omron), but you could do better with a purpose designed RF relay. There is a little loss through the relay used, although the price was right!

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