KD1V Project Page

Homebrew QRP Transmitter

Here's a brief descripton of my 400 mW homebrew VFO transmitter for 40 Meters. It has clean keying and a very pure CW tone. I will later post a schematic of this rig. Here are some pictures of it.

This transmitter is an experimental work in progress using readily available parts as well as parts I just happened to have lying around the shack. It incorporates some of my own ideas with varying degrees of success.

This transmitter utilizes bipolar transistors throughout. The VFO is a Hartley oscillator, using a 2N3565 NPN transistor. The buffer stages (using 2N3904 NPN and 2N3906 PNP transistors) provide isolation, and they drive the primary of the toroid transformer for the power amplifier. The power amplifier consists of a single 2N4401.

Now here are some of the "features" which make this a little different than most.

Perhaps the most unique feature is the method of keying. I've inserted a third winding in the toroid to the final amp. This third winding, which is bifilar-wound with the secondary, is shorted out during the key-up condition (by two saturated 2N3904 transistors), and open during key-down. When shorted, the drive to the final amp is reduced to the point that the class C amplifier transistors are cut off, drawing no current. The keying circuit provides adjustment of the time constants to control the shaping of the keyed waveform. It should be noted that the toroid is driven by a current source - the 2N3906 collector

Another interesting feature is the absence of variable capacitors or varactors for the oscillator. The oscillator is tuned by means of a 10-32 brass screw (fitted with a knob) inside the tank coil. As the screw travels into the coil the inductance is reduced, raising the oscillation frequency. The tuning rate is about 4.5 kHz per turn, and the tuning range is from 7.000 to about 7.070 MHz. The screw travels through nylon washers in the coil, making tuning very smooth, with virtually no backlash.

I recently made some new mods to this transmitter, rebuilding the oscillator, buffer, and keying circuit on a homemade PCB. I also changed the output stage from parallel 2N3904's to a single 2N4401, since the parallel transistors gained me nothing - the single transistor is hardly warm to the touch. (I really should be able to get more output - maybe the output transformer needs a tweaking).

I recently added another feature - a temperature compensator for the oscillator. A single transistor senses the temperature, amplifies that sense voltage, and biases a diode, which acts as a varactor. My uncompensated VFO drift was about -300 Hz/degree F. It is now less than one tenth that figure, and total drift from a cold start is under 50Hz.

There is one IC in this rig - that being the +5 volt regulator for the oscillator. The raw supply for the transmitter is +12 volts DC.

Unfortunately, this rig is a transmitter only, so I can't really use if as a portable rig. I'm now using an NRD-515 general coverage receiver when operating.

Antenna

I decided to put up a vertical antenna for 40 meters, to replace the inverted vee that fell a while back. Using EZNEC, I designed what I think is a pretty good one, fairly simple and not very visible. It has two "tall" supports, much like a horizontal dipole, between which is strung the horizontal "top hat" end load, from which hangs the vertical wire itself. The four radials are 8 feet high, with the two "cross radials" sloping down to the 5 foot tree stakes. The modelling suggests this height is just fine, and it's up out of the way (except near the tree stakes). Here's what it looks like (overhead oblique view).

EZNEC predicts this antenna (including wire loss and ground loss) should perform better than a horizontal dipole at the same height of 25 feet for takeoff angles below about 18 degrees (single hop night distance of ~900 miles). That's assuming "medium" ground conductivity. I can't vouch for the exact details, but it seems to perform very well. Note that an antenna using such small wire isn't very permanent, but on the other hand it'll be pretty easy to repair after a storm brings it down..



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