The W7JI QRP/QRO 40 Meter Transmitter

By Lou Burke

Homebrew Philosophy:

            I’m sure most of you will find my homebrewing method quite extravagant to say the least.  I am a firm believe in using printed circuit boards for my projects.  Of course this drives up the cost on the front end of the project but provides some lasting rewards.  The first and most obvious advantage is the ability to sell the pc boards or to actually kit the project and sell the kits.  I personally do not have any desire to sit around all day counting and packaging little bags of parts.  So why would I go to the expense of using pc boards?

            I love all aspects of homebrewing, including the design of the pc boards. I use ExpressPCB software which is FREE.  You can download a copy at http://www.expresspxb.com  After finishing the board layout you simply upload the file to ExpressPCB and in a few days you will receive you professionally manufactured circuit boards with plated through holes, tinned backplane, ready to build.  Just to set the record straight, my only affiliation with ExpressPCB is that of a satisfied customer.  Aside from the fact that I enjoy designing the parts layout on the pc board, the real advantage is that the circuit is 100% repeatable every time you build it.  This is not always true with free-style construction methods.  So if I want to build a transmitter on several different bands I know the layout will work if I use the proper parts.  If your club wants to build a transmitter or receiver project, a circuit board would certainly go a long way towards a successful completion of the project.

            By the time you actually get to mount and solder parts on your board, you feel as though you’ve built the project so many times, that you know it inside and out.

The Project:

After building and using a very basic crystal controlled QRP transmitter, I realized how much I missed not having a VFO along with several other features like a CW keyer and automatic antenna switching.  So, it was time to begin planning a 2nd QRP transmitter that would incorporate these new features.  I wanted to keep the design straight forward and use through-hole components in order to generate interest among potential builders in the QRP community. It seems like the only people who do any building any more are the QRP enthusiast’s.  With these new features in mind I began the design process. 

The first step is to browse through every article available on QRP transmitters and check out what others have done so I wouldn’t have to re-invent the wheel.  Usually after finding an idea from one guy, another from someone else, along with some of my own, I end up at a starting point in the design process.  Before you know it I’m sitting at the computer drawing up a schematic.  I wanted the VFO to be as stable as possible and decided to bread board several different circuits before arriving at the circuit which would be incorporated into the design.        

            After several days of experimenting, using a Colpitts Oscillator for the VFO circuit appeared to be the best choice to achieve reasonable stability; the optimum frequency to operate the VFO is 2 MHz.  VFO stability is achieved by using a combination of NPO disc ceramic caps and polystyrene caps along with the proper toroid core.  The use of a VFO spot button is a throw back from the good ole days when most transmitters had a spot button to aid  in tuning the VFO to your receiver frequency without putting the transmitter on the air.  I've always like this feature when using separate transmitter and receiver and decided to incorporate it into this rig. 

Having achieved a stable, drift-free VFO  it would be necessary to use a transmit mixer in order to arrive at the designated output frequency of 7.0 MHz..  The SA612 is especially appealing as a mixer since you can also incorporate a crystal oscillator circuit with just a few parts and it’s all contained in one single package

.The Mixer:

            In order to generate a 7.0 MHz signal the VFO must be tuned to 2.000 MHz. The mixer oscillator is crystal controlled at 5.000 MHz.  The output of the Mixer contains the VFO frequency and the complex frequencies generated by mixing the 5.000 MHz crystal with the VFO.  It is necessary to filter out our 7 MHz signal and this is accomplished with a simple band pass network resonant at 7 MHz.. 

The output of the band-pass filter is fed to the input of the IF amplifier.  The output of the IF amp is a broadband transformer which feeds the input of the RF Driver.  It is easier to use a broadband transformer in the output if you want to build the transmitter on different bands.  This way I wouldn't have to bother with different component values for tuned circuits.  At this point you have a choice of making the transmitter a 1.5 watt or 7.5 watt version.

The RF driver (final) depending on the power level you decide to build utilizes a simple 5 element low pass output network designed for 50 ohms impedance To use the transmitter as a 1.5 watt rig, simply run a short piece of wire from the output of the low-pass network to the output connector and you’re ready to call CQ. 

            The RF driver/final is a 2N5109 which generates enough drive to turn on an IRF510 to 7.5 watts output.  This final stage RF amplifier is derived from the “Mini-Boots” amp that Wayne, NB6M published some time ago.  The output network is a typical low pass filter designed for 50 ohms input/output.  The output impedance of the IRF510 is approximately 12 ohms at the 7.5 watt level so it is necessary to use the 1:4 transformer to get the impedance close to 50 ohms to effect a good match to the output network.  Here is a copy of page 1 of the schematic.

CW Keyer:

For the CW Keyer, I use the PK-4 chip and used the entire keyer circuit from Jackson Harbor Press in my design.  I have used their circuit with the PK-4 chip in other projects and have always been pleased with the performance.  The PK-4 keyer is very powerful with a wide range of menu functions and the keying action feels very solid and smooth.  Here is a schematic drawing of the transmitter.

Building the Board:

To keep the cost as low as possible on the pc boards I never buy them with a parts legend silk screened on the top of the board. Simply print up a full-page size drawing of the board with the holes and parts shown and use this drawing as a guide for mounting the parts.  Here is a sample of the board: (Not to scale) Here is the schematic drawing.

        Below is a complete parts list with Mouser Electronics part numbers.  For those who would like to build the transmitter from parts available in their junk-box, Included is a Zip File available for downloading that contains the schematic drawing and the complete parts list with Mouser part numbers.  Please note that some parts at the bottom of the list are not from Mouser. 

Parts list with Mouser Electronics parts numbers

page 1

Here is page 2 of the parts list.

             This is a picture of the completed pc board with all parts mounted for the 10 watt power level.

            This is a picture of the transmitters enclosure with it configured as a 1.5 watt rig.  The 7.5 watt version uses a heat sink on the RF final transistor which is to tall to allow the pc board to be packaged in a 1” tall enclosure.  This size enclosure makes a nice small unit for portability and is the same size as the receiver.

I like to build one stage at a time and get each individual stage working properly before advancing to the next stage.  With all parts soldered to the board you can apply 12 VDC or 13.8 VDC as most power supply's output, to J1 the 2.1mm power jack.  You should see the output of the oscillator on pin #2 of the SA602 socket.  Simply push a small piece of wire such as a lead from a 1/4 watt resistor into pin 2 and put your scope probe onto the wire and observe a sine wave.                   

You will have to experiment with the values of capacitors necessary to bring the oscillator to 2 MHz.  You may also have to remove turns from L1 depending on your combination of capacitors and coil.  Always make sure that you have the VFO pot full counter clockwise in order to display the lowest frequency of the VFO.  I set mine at 2.0 MHz.  When mixed with the 5 MHz crystal oscillator you get 7.0 MHz.  You may wish to adjust your VFO to a different frequency depending on what portion of the band you want to operate.

In order to key the transmitter on, while making the following adjustments, it will be necessary to take a short length of wire approximately 4 inches in length and ground one end of it to the board. Use the other end to temporarily ground the end of the 10K resistor farthest from the base of the 2N3906. Any time you need to key the transmitter simply touch the ground to the resistor or you could tack solder the ground wire to this point during testing 

Once the VFO is operating on the frequency of your choice, install the SA612 IC and connect your scope probe  to the base of Q3.Key the transmitter while observing the signal as you tune the variable capacitors of the band pass filter.  Once you have peaked the signal, remove the scope probe and connect your frequency counter to make sure you are reading the correct frequency.

At this point it's a simple matter of installing the remaining parts and doing the final test.  The 7.5 watt version will produce 55 volts peak-peak across a 50 ohm load and the 1.5 watt version will produce around 25 volts peak-peak across the 50 ohm load. 

Connection to Receiver:

Using a short length of RG58 cable connect the receiver BNC jack to the antenna input of the receiver to be used.  You will be able to push and hold the SPOT button while tuning the VFO to the frequency of the receiver with in the 65 KHz limits of your transmitter.  Tune in 7040 KHz and then hold the SPOT button in while tuning the transmitter VFO until you hear a beat note in the receiver headphones or speaker.  The tuning ratio of the VFO is rather sharp, so tune slowly or you will not hear the beat note.

            In the first two weeks of use I have worked approximately 25 states with the 1.5 watt version of this transmitter using a Inverted Vee at 55 feet..  I hope you enjoy building and operating your QRP transmitter as much as I have enjoyed this project.

Best 72/73 and have fun.  Remember, anybody can do it with QRO but it takes a dedicated ham to do it with QRP!  For more information my email address is w7ji@wildblue.net.