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A 1 Watt transmitter to upgrade you from ZR to ZS

This project was developed to meet the criteria for upgrading from a ZR to ZS call sign, ie a 1W CW only transmitter. It was designed to be as simple and with as few components as possible, and yet still actually function with reasonable performance. I called it the 1 hour TX because that's how long it took me to build it! Put another way, all that stands between you and a ZS call sign is 1 hour of real amateur radio stuff. This is a low tech project suitable for novice constructors and can be made with a minimum of equipment. I intend to offer a talk on the project at the CTARC and host a Saturday morning workshop for constructors once they have all the parts.
The tricky parts are: the 7MHz crystal, Toroids ,and the PA transistor.
The crystals you can buy if you can find them, bum off other hams, or have made by STC, there are tons floating around.
The toroids are a dream to work with. T-50-2 types are readily available everywhere except here:(but can be ordered if you club together - from Amidon USA) Micrometals make equivalents and are sometimes available from Avnet Kopp. I have a lot of T-44-2 and am donating enough to CTARC to make 5 kits. (the T-50 is the size in inches (1/2 inch) so T44 is the same core just a wee bit smaller.
The transistor is also readily available everywhere else except here , but any of the following will be perfect: 2N3866, 2N5109, 2N3553, 2N5320 should all be ok or anything that can handle 30V, 5W with an Ft of above 500MHz will probably work. Shop around, I have seen 2N3866 at R24 and R60. Please let me know if you find a cheaper available equivalent eg a 2SK type etc.

Some years ago (Nov 1983) ZS6BKW and I published an article "Design your own QRP CW TX" in Radio ZS which proved quite popular and the need for such a circuit is still apparent.
[Read original article 413kb]

The Specification

  1. At least 1W across the 40m band
  2. Crystal control for sheer simplicity
  3. Low radiated spurii - at least 30dB down
  4. No more than 3 transistors
  5. PCB available
The design below adequately meets these, it has a power output of 2W at 12V and still provides 0.8W with a 9V battery. The radiated harmonics are more than 45dB down, and only 3 transistors were used. Although it is unusual to key the oscillator, being a crystal, it didn't mind, and keys cleanly. A power FET such as a IRF510 would have been cheaper in the PA, but would have required an additional gain stage to drive its high gate capacitance.

The Schematic

Click on the schematic (to the left) to get a A4 size printable PDF. If you have Accel PCB, Tango, Pcad or other equivalents you may be able to see the schematic source file. here. If you edit this, you are infringing my copyright. You may do so, but I would like to be informed as well as get a mention where you use it.

Q1, with tuned drain via L1 and C3, is the crystal oscillator. This is a very robust FET circuit and starts and runs without issues unlike some other transistor oscillators. The output is hard coupled into the base of the PA Q3, and so if you were to look at it with a 'scope it would look terrible. Quite normal. Do not try to avoid R5, you will destroy Q3 without it. Q3 conveniently lacks a tuned load, but 'looks into' a 2 stage half-wave filter which does a great job of cleaning up the class C PA output, and provides a spotless sine wave to the output terminal. The RFC L2 is simply 8 turns on a little ferrite bead. The zener D1 is to protect the 30V PA transistor if the load becomes open circuit or high SWR develops. You can short the output so long as Q3 has a small heat-sink. Diode D2 looks odd, it's there to shunt (and probably burn) if you connect the battery the wrong way. Why not in series? for QRP every milliwatt counts and the 0.6V loss would be too much to bear. Q2 is a switchto provide power to the crystal oscilator, and provides ground-referenced and slightly shaped keying.
L1, 3, and 4 are not critical, one counts the turns as being the number of times the wire passes through the center of the core. The wire size is irrelevant, just keep it to one layer - I use about 0.35mm diam enameled wire. L1 secondary is just wound 4 turns over the 30 turn primary. L3 and 4 are 13t each. The trimmer C3 just has to resonate with L1, anything 80 - 200p will probably be OK.

The PCB

Click on the overlay to get a A4 size printable PDF of the component overlay.
If you have Accel PCB, Tango, Pcad or other equivalents you may be able to see the PCB source file here. If you edit this, you are infringing my copyright. You may do so, but I would like to be informed as well as get a mention where you use it. The PCB is 120 X 31 mm and was designed to fit a standard box size available from most component shops. Here is a PDF of the Copper side of the PCB, note that the text is reversed as you need to contact print it onto the copper PCB.
See the links to article elsewhere on this site on how to make you own PCB's. It really is easy and you should give it a go.

Setting up

See that all the transistors are in the right way round. Set C3 mid way. Connect a 50Ω load (2 100Ω in parallel) to the output. If you have a current limited power supply, set it at 300mA and turn on. There should be less than a few mA current and nothing getting hot. Make sure you have a heat-sink on the PA transistor. Key the oscillator. The current will jump to about 200mA if the PA is working, if not you might need to tune C3. If you have a 'scope check that the output is over 20Vpp (1W). It is normal for the PA transistor to get a little warm on key down.
That's all there is to it! You can now head for your nearest SARL assessor and ask for your ZS!

First ZR builds the kit

Rodney Wahl ZR1RKW became the first ZR to build the project on Sat 24th November. Rodney (seen soldering at left with me keeping a safe distance) had no construction experience, and had never soldered before, yet he completed the project in 3 hours and is now well on his way to being a ZS. Rodney's TX ran 2.2W into 50Ω and had all harmonics greater than 35dB down. This also proved the repeatability of the design.

How do you get involved with the project?

As you can imagine, I can't individually mentor every candidate that wants to build one every now and then. Also, as some parts are better available overseas, we are going to do this as a CTARC club project under the auspices of the committee. Dave ZS1DFR is taking names, and you can contact him at zs1dfr@telkomsa.net or 083 270 6843 to get your name down. Tom ZS1TA is collating parts for the kits, Tony ZS1TN is going to be buying parts in the UK in December 2007 so get your name in soon. Please work through these channels and not directly to me. We expect the kits to cost about R100 but this may change. Thank you to G0GSF for a donation of 5 crystals.