WEEKEND PROJECTS FOR QRPERS
By George Dobbs G3RJV
First offered as part of the Four Days In May Symposium at the 1999 Dayton Hamvention.

Introduction
Experience has taught me that relatively few radio amateurs build complex equipment. Rather more of them are inclined to build small projects, which can be completed in an evening or a weekend, cost very little in parts, and will probably work first time.

It is satisfying to go to bed knowing that you have built something with your own hands and it worked. Few of us get the chance to do that these days. Here are four simple projects, all of which could be built in a couple of hours or less. None of them is very original but all of them should work first time if built with care. Hopefully they all have practical applications for the QRPer.

A Universal VXO

I recall having a long conversation with the late Doug DeMaw, W1FB, who was an advocate for the use of the VXO in home built equipment. His contention was that the VXO is a very viable option for home built amateur radio equipment especially for miniature or portable stations. Some of the ideas below come from the scribblings I made at the time.
What is offered here is a utility VXO circuit that can be used as a frequency source for a transmitter, or even a receiver, on a range of amateur bands.

The circuit for the VXO is a bipolar transistor oscillator followed by a bipolar tuned buffer stage. My prototype used 2N2222A transistors for both TR1 and TR2 but many similar types would work. I used 2N3904 devices in an earlier version. The oscillator has a stabilised supply derived from an 8.2 volt zener diode. I had intended to use a 9.1 volt zener but could not find one. Other constructors might like to use a three-pin voltage regulator chip of similar voltage.

The oscillator is based on the popular Colpitts circuit. The capacitive feedback is via the capacitive divider provided by C2 and C3. These values vary according to band (see the band table). C1 is used to minimise the effects of the parallel capacitors, C2 and C3. Without C1, the upper frequency range of the oscillator would be restricted. By using an inductor, L1, and a variable capacitor, VC1, the crystal should pull slightly above its nominal frequency. The value of C1 will depend upon individual crystals. I found that around 100pF served very well for 40m. Try 100pF as a starter value on other bands and experiment.

Suggested values for L1 and VC1 are also given in the band values table. The suggested inductors are standard commercial moulded inductors. These values can offer considerable frequency shift. I make no numeric promises.
The oscillator stage is coupled to the buffer via C4, the value of which is ideally changed to suit the band. The table shows suitable values for C4. The buffer stage is tuned for the band in use using a tuned transformer, T1. These values for T1 and VC2 are chosen to resonate at the frequency of the crystal. The buffer amplifier operates in Class A to encourage a spectrally clean output.

Notes:
T1 for 40/30/20 - 5.2uH = 36 turns on T50-6 core. Link winding 10 turns

T1 for 80 -10.3uH=45 turns on T50-2 core. Link winding 12 turns

VC2 is a trimmer capacity Value given is theoretical capacitance to resonate T1 on the QRP calling frequency for the band in question

VC1 may be a mix of fixed and variable capacitance. The values for T1 and VC2 can be taken from the band table. To save additional calculations, I used the same value of inductance for all the bands from 40 to20m. For 80m L1 is a larger inductance tuned with a larger capacitor. The figures in the table show the calculated values for VC2 to hit the International QRP Calling Frequencies on the 4 bands. These are 3.506MHz, 7.030MHz and 14.060MHz and 14.060MHz

The higher frequency version of T1 (40, 30 and 20m) can be made using about 60cm of 26swg (about 25 awg) enamelled copper wire wound to occupy about three-quarters of the core. The version of T1 for 80m will need 32swg (about 29 awg) enamelled copper wire.
T1 and VC2 should peak the output of the VXO at the desired frequency. This resonance ought to be fairly flat over the whole range of the VXO. If this is not so, the bandwidth of the tuned circuit can be increased by damping it with a resistor. Connect a resistor, try 4.7K or perhaps 10K, across the tuned winding of T1.

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• The G-QRP club was formed by Rev. George Dobbs G3RJV in 1974 to cater for those interested in low power communications after a group used to meet around 3.560MHz. In the year 2000, the club celebrated its 25th birthday. To visit the G-QRP website, please click here
  
  
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