GW3UEP 100W MF CW TX Description

 1 April 2013



This 100W MF CW transmitter and the 25W QTX [Quick-TX] were developed from earlier versions used on 160m and 80m in the 1980s.  The original 160m transmitter used a 7.6 MHz FET VFO, CMOS divider and VN88 VMOS PA with 5W CW output.  This had good frequency stability with freedom from pulling and chirp. Higher power [25W] versions followed on 80m and 60m using the IRF510, with the simple current-boost circuit providing the extra gate-charge current.  All versions had high PA efficiency. When the 500 kHz SRP/NOV arrived in 2007, the solution was to hand and the QTX was born!  Thanks to GW4HXO and EI0CF for their collaboration in construction, evaluation and on-air-testing of the transmitters.


VFO circuit

A CMOS 4049 Hex-Inverter [IC1] with 2 MHz ceramic-resonator provides a temperature stable VFO. The buffered 5Vpp output is ac-coupled to a 4024 divider [IC2], which delivers 12Vpp output at 501-504 kHz. Frequency stability is optimised by low power / continuous operation of the oscillator circuit.The VFO box is separate from the PA unit in order to avoid thermal coupling and temperature change.  The 472 kHz version operates at 3.8 MHz and divides by 8.


PA circuit

The IRF540 [TR1] was chosen for operation at 500 kHz. TR3/4 form a zero-biased complementary voltage-follower, buffering IC2 output-stage and providing adequate source/sink current for the IRF540 gate-charge. The gate is ac-coupled and dc-restored to ground should a dc-drive fault occur. The PA operates in switched-mode with drain efficiency in the 80% range. The output circuit provides matching and LPF functions, presenting a clean sine-wave to the 50R load. C1/L1 form a resonant MF tank circuit. L-section C2/L2 transforms the 50R load into the drain circuit load and is optimised for best efficiency.


PA keying is achieved with P-FET TR2, which also shapes the keyed RF envelope and eliminates key-clicks.

R6/R7 switch TR2 and along with C10 set the rise and fall times. R5 ensures stability by rolling-off the frequency response of TR2, forming a LPF with its input capacitance. TR2 drops 1V at 5A / 100W RF output.


Simple CMOS keying is achieved using the VFO divider 4024 reset-line. TR2 is replaced by RLA2 contacts, which remove the PA drain supply on receive for key-down netting. The keyed RF envelope is free from spikes and glitches, minimising local key-clicks.



Regulated 24V PSU with current limiting set to ~5A.  The LM338T provides a simple low-cost solution - Schematic

Operation over the range 14-25V is recommended.  For QRP antenna matching see note on Schematic & QRP Meter


Setting-up  [Typical values at 24V dc using DVM & Scope/10:1 probe for RF tests]

Terminate TX output with 100W 50R load & MF power-meter.

VFO: check RF & dc values are as shown on VFO schematic

PA: check <2mA dc TX/RX [no CMOS input, excludes relay & regulator current].

With CMOS input: check >10Vpp 501 kHz across R4 [Vpa= 0V].

TX key-down: check 100W RF output / 5A dc [30W RF / 3A dc with 14V supply].

Check PA drain waveform is a clean pulse as per scope shot and that efficiency is >80%.



1] L1-3 values measured with simple test-oscillator see schematic InductanceTestOsc

2] TR3/4 alternatives: BC549/559, BC337/327, or BC109/BCY71, 2N3904/3906.

3] Antenna system: ensure that a matched-load of <1.25:1 is presented to the TX.

4] TR1: if other devices are used [e.g. IRF640], fit R10 directly on the PA Gate-pin

5] For simple RF test circuits see MF Test Gear & QRP Meter

6] For 25W versions - see 25W QTX or 472 PA

7] 472kHz operation - see  100W Schematic


GW3UEP Index - updated 1/1/14