After doing a SOTA activation recently with VK3ZPF using only 5 watts from the FT817, thought it would be a good idea to build a lightweight amplifier that can be backpacked around. For power, will use a lithium polymer RC type battery back of around 3000mAh capacity with 3-4 cells. This should give up to 30 minutes of transmit with SSB duty cycles at a power level of around 50Watts. The design is based on a design by W6JL and uses low cost IRF2Z24N hexfets (~$1 ebay). The binocular core was also obtained from ebay. The change over relay and other toroids came from minikits.

The finished amplifier after a coat of primer and flat black:

Here is the amp with the FT817 mic for an idea of size:

I couldnt find a box of the right size for the amp, so ended up making one from aluminium angle and sheet, its a bit rough but tough enough. The amp has a low pass filter for 7MHz installed and puts out about 60W from 13.8V and 45W from a 3S 2200mAh RC Lipo pack. It stays quite cool considering I used minimal heatsinking to keep the weight down. The 5mm thick aluminium base plate of the amplifier is the heatsink. A 60amp RC connector is used for DC power and a 3.5mm jack is used for TX key from the radio.

The amp delivers good power at 14MHz (45W @13.8V)but requires another filter. For display of DC volts, PA temperature and RF out, an LED bargraph is used. Was initially going to breadboard up a PIC processor to drive the LEDS, but a small arduino board was only $9, so this was used. The board is very flexible with ample analogue inputs. Arduino is an easy platform to write software on.

Metered mode is selected from a pushbutton and an LED indicates the current mode with on/slow flash/fast flash/off. I was worried that the Arduino might cause noise on the HF bands but it was not a problem.

The power on/off switch kills the bias to cut the fets off and also disables the relay and Arduino (not shown).

This is the binocular core for the output. The primary is made from RG-58 braid and the four turn secondary is wound through the braid.

The primary connects directly to the tabs on the FETS with solder lugs.

It is a bit tricky to put together using normal insulation on the secondary as the heat from the soldering can melt the secondary insulation. A better approach would be to use silicon insulation on the secondary. Several toroid cores could be used to replace the binocular core.

Another view looking down at the amplifier. The two toroids at the right are part of the low pass filter. The changeover relay is top right.

The Arduino controller is not connected up in this photo. It has connections to RF sample of BNC connector, temperature sensor on one PA FET and DC supply.

Amp alongside FT817. The unit is a pretty good size for portable work, and could be made a bit smaller if need be. In this build, I wanted to make sure there was enough heat sink in the chassis traded off with weight and have enough room for the Arduino controller.

Connectors at rear of amp. note that a couple of adaptors are used on the DC connector.

Update 2016-05-07:

Its been about 4 years now and this amp has been dragged around a few mountains. Amazingly, it is still on its original set of Mosfets, despite some bad antenna matches. It now has a slide switch on the top that selects either the 40 metre or 20 metre low pass filter. Many of the recent activations with it have used a 4200 mAh LiFe pack which holds up at 12 volts nicely.

The switched 40/20 metre low pass filter is a bit untidy but it does work pretty well. Output post filter @ 13.8V is about 55 Watts on 40m and 45 Watts on 20m.

The filter is mounted on standoffs from the base of the amplifier and straddles the output transformer. A slot in the top cover allows the switch to change band. The filter caps are SMD, the 40m caps are TDK 500V 470pF at either end and 2x 470pF at the centre junction. The 20m caps are AVX 250V with 180pF at either end and 330pF at the centre junction of the inductors.