10m QRPp CW TX

What is QRPp?

While QRP stands for "low power", operating QRPp means running even less power than QRP. Internationally, QRP levels are 5 watts for CW and 10 watts for SSB. QRPp is commonly meant as operating with less-than-one-watt RFout.

My QRPp design

My QRPp design is a reproduction and adaptation of a circuit found after extensive Internet searches on easy QRP and QRPp projects. I first encountered the Pixie CW RTX and then discovered a simple QRPp TX with a one-chip PA stage: 74HC240, apparently designed by N7KSB.

My lazyness, inexperience and lack of instrumentation (oscilloscope), forced me to experiment with a canned oscillator from the junk box, operating at 28.322 MHz. The choice of 10 meters is forced by my antenna setup, that works only on that band.

With reference to the original schematic, I dropped the XTAL oscillator in favour of a direct connection to the output of my 4-pin canned oscillator.
Pay special attention to the correct voltage supply of the canned oscillator, if you are using one from the junk box! While pinout is most probably the same for every model, they might work at 5, 3.3 or even 2.5 volts!
I assembled my oscillator and powered it with a variable voltage, starting from 0 volts. At the same time I was listening on 28.322 MHz for the oscillator signal. I measured that my junk box oscillator started working at 3 V.
I went up to 5 V without burning my oscillator, but I did notice it was heating up a bit. Another noticeable effect (varying the supply voltage) is the output frequency drift of about 200 Hz that is useful if you want to couple some sort of DC receiver to this QRPp TX.

N7KSB suggests to power the 74HC240 at 7-8 volts, and that it holds up to 10 V. But the datasheets I found on Internet say that 7 volts is the absolute maximum rating for the supply voltage, so I decided to run it at 5 V.
At the same time I used a 3,3 V regulator to drive the 28.322 MHz canned oscillator.

Construction method is dead-bug, so that I have a good ground plane for RF. On my local receiver the keying sounds good, and harmonics seem to be attenuated properly by the output filter.

It works!

The time for a field test finally came. After in-house experiments, dummy load, attempts of power measurements, I plugged my 10m car antenna,
I knew I could count on the cooperation of Angelo IK1QLD, so I called him on the phone. My QRPp TX was all wired up, including the vertical key. I instructed Angelo to tune on 28321 USB, and I could hear the huge noise coming from his loudspeaker. I know he lives in a noisy place, even at night.
My IC-706 was tuned on the same frequency, so that I would have a sidetone for my keying. Angelo was there as well and... YES! I had no need of a sidetone! I could hear my keying out of his loudspeaker, over the phone!
Then he moved to 28322 CW and added the 500 Hz filter, plus IC706's DSP, and my note was clear. Report: 519.

As a comparison I switched the antenna to my IC-706 and sent some dots and dashes with it: the report on Angelo's 706 was S7 at its minimum power (should be 5 W). Then he switched to his Kenwood R5000 and the signal dropped to S5 (50 microV). Unless IK1QLD has some losses in his antenna switch, this proves that many parameters in a receiver can influence the meter reading. My QRPp signal was readable with both receivers. Actually on the R5000 without any filter was like 706 with CW narrow and DSP.

Hearing my transmission coming back through the telephone was like being Marconi when he heard the shotgun shot from behind the hill. It is a great feeling indeed!

QRB is about 7 km, line-of-sight.
My setup:
TX: 10m QRPp described above
Ant: shortened CB whip with base load, about 1m long, on a magnetic base standing on my balcony fence

Additional Comments

July 2003.
While surfing on Internet I found a project named "Fireball 40", which uses the same setup of mine, with the addition of a logic divider to obtain 14.161, 7.080 and 3.540 MHz as well as 28.322 MHz.
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