630m (475kHz) 100 watt transmitter

Building a 100 watt power amplifier for 630m band.

Starting here...

The circuit for the 100 watt PA is driven by the Ultimate U3 kit provided by Hans Summer from his website. (See link below.) This small micro kit provides a multitude of digital modes over most of the amateur radio bands and is well worth investigating. I have attached an external GPS to give both frequency and timing stability to this kit. Both important for reliable WSPR detections and other modes.

Driver details.

To provide isolation from the U3 kit, as previously as with the 20 watt 630m transmitter, a relay was used to provide rf isolation as opposed to the FET switch arrangement supplied in the U3 kit. The driver circuit is also the same as previously used. A single transistor is provided as a digital switch followed by a rail to rail current source. This stage is supplied by a 12 volt linear regulator.

PA Final details.

The PA final transistor is an IRF640 FET and operates in a typical class E configuration. This FET has a Vds of 200 volts and should be capable of giving us our required 100 watt RF output. The power supply for this project was anticipated to be around the 48 volt mark. The output FET voltage rating selection tends to be 4 to 5 times the anticipated dc supply voltage. The FET needs to be isolated from the heatsink and/or ground as the tab is at rail volts. A fan is used when using extended digital transmissions.

Final results.

The DC power switched by the output FET (alone) is around 121.4 Watts. (43.5 Volts x 2.79 Amps DC) The calculated RMS output power is around 103.8 Watts, produced after the LPF connected to the output of the PA. This gives us an efficiency of more than 85%.
So, there you have it. This is what I currently use for WSPR-ing and digital modes on the 630m ham band. I've included some photos of the current test setup and screen shots on the DSO. I've also included a PDF of the schematic below. E&OE. YMMV.

Bench testing

 Here you can see the DSO, PA,  LPF, fan, RF ammeter and micro.

WSPR micro.

 Showing the micro display during  WSPR transmission.

PA output monitoring

 RF ammeter follows DSO rms  current value from scopematch.

Nearly there ...

 Output waveforms shows antenna  needs tuning. We want both  waveforms to be the same ...

PA into 51 ohm antenna

 Relatively good match of PA to  antenna. Purple trace shows Vds  volts of final FET.

Tweaked final output.

 Antenna tweaked to match PA  output. 100+ watts!

Useful Links


VK5FQ website
12/02/14 Start build of new website.


Steph - VK5FQ
Adelaide, South Australia,