GW4ALG's QRP Radio Pages

[ GW4ALG went QRT in February 2007 ]

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A QRPP VXO Transmitter for  5 MHz


1.   Features Summary
2.   Introduction
3.   Overview
4.   Variable Crystal Oscillator (VXO)
5.   Pin-out Diagrams
6.   Introducing 'Ugly' Construction
7.   Component Sources
8.   Feedback from other constructors


Features Summary

This transmitter has been designed for use on 5 MHz and has the following features:
- 12 v operation
- VXO transmitter, with 2 kHz tuning range;
- key-click filter;
- 50 milliwatt output power;
- built-in antenna changeover.


This transmitter was built for a bit of fun.  I was astounded when GW3UEP, at a distance of 120 km, replied to my CQ call.  This QSO took place on 6th November 2005 -  just 130 minutes after I started designing and  breadboarding this little transmitter!



The transmitter uses a bipolar VXO using a 2N2222 transistor.


Variable Crystal Oscillator (VXO)

For the transmitter, channel ‘FB’ (5.280 MHz) was chosen and the 50 pF tuning capacitor provides a tuning range of approximately 5.2795 - 5.2833 kHz.   This little TX may be expected to draw about 9 mA at 12 volts.  The core of the tuned circuit should be adjusted slightly HF of resonance.

Pin-out Diagrams


Introducing 'Ugly' Construction

Those with lots of experience of building homemade rigs will have no trouble building the transmitter into a smart box.  Those with less experience would, perhaps, benefit by starting with a simpler approach. A 'bird's nest' version can be constructed quickly on copper-clad board using 'ugly' construction techniques.  This is exactly what I did when I made the prototype transmitter section of this little transmitter (see picture below).  It didn't look very pretty, but that really didn't matter - the important thing was that it was possible to use the prototype to test the design - and also work GW3UEP - my first QSO on QRPP!


And click this thumbnail for a close-up of the main components:


For more information about ugly construction techniques, click here.

More testing

On 12th November, I carried out some more tests using the following set-up:

The photo shows the K2 receiver; the receive converter (the receive section of a Super-Sixty); a much-modified ETM-8C electronic keyer; 12v battery supply; and the bread-boarded QRPP TX.  Using a G5RV antenna, I worked the following stations around mid-day:

Richard, G3CWI in Macclesfield (185 km).  Sent 599, received 569
Ian, G3TLH in Exeter (? km, 5 watts).  Sent 579, received 579
George, GI4SRQ in Armagh (396 km, 5 watts).  Sent 56/79, received 339
Peter, G3TJE in Highbridge (68 km, 5 watts).  Sent 569, received 449
Ian, G3TLH in Exeter (? km, 500 mW).  Sent 579, received 559

I later received an emailed report from Norman, GM4KGK (IO68VE, 761 km) to say that he had also heard my signals.  Not only that, but Norman had also recorded some of my 'CQ' call! 

Click here for a .WAV file (370 kB) of Norman's recording of my CQ call, and the reply from Ian G3TLH (5 watts).  (You will need to turn the volume up to hear my weak signals - but they are definitely there!)


Component Sources
The transistor may be obtained from either of the following suppliers:
1) Grandata Limited, K.P. House, Wembley, London, HA9 0HB
Tel: 020 8900 2329    Fax: 020 8903 6126

2) Sycom, PO Box 148, Leatherhead, Surrey, KT22 9YW
Tel: 01372 372587    Fax: 01372 361421
(Also a good source for loads of specialist components for the home-brewer.)

The HC25 crystals and sockets may be obtained from:
QuartSlab Marketing Limited
PO Box 19
Tel: 01322 330830
The crystal used was made to 'Specification E', 5.28000 MHz.


Feedback from other constructors

None yet!