A Small, One Frequency 40 Meter QRP Transciever Project

To fill some of my Winter hobby time, I decided to build a very portable QRP rig. I like to build some of my own equipment, and prefer to use kits when they are available. I have recently had my interest in CW rekindled, and enjoy being able to operate portable, usually in combination with other hobbies.

The Rockmite, from Small Wonder Labs, has been around a while, and combines a fairly simple and easy to build design with some very useful operating features. It is crystal controlled for one frequency, which is both a limitation and a design asset. You are limited to the one frequency, but by optimizing for that one frequency you get some very good receiver performance. There are simple ways to modify the kit to allow the crystals to be changed, and I may look into modifying later to do so. Initially, after consulting with a friend who may also be building one, I settled on a kit with 7030 KiloHertz crystals, which is the 40 Meter QRP calling frequency.

The Rockmite kit is just for the circuit board, with an accessory kit available with connectors, switches, and such. Mounting and enclosure are up to the builder. The basic Rockmite board will fit easily into an Altoids tin, but I wanted to include everything in one package. Of course, the next setp is to decide exactly what "everything" includes. At minimum for my project, that will be the transceiver board, suitable batteries, and an antenna tuner to allow use of end-fed half-wave antennas. I plan to build the whole thing into a flavoured instant coffee tin. I will update with more on the final installation as it progresses, but wanted to start this by detailing my experiences with construction of the Rockmite board.

Small Wonder Labs was run by Dave Benson, K1SWL. He had several interesting kits and products for hams, some of which have been sold or transferred to others when Mr. Benson retired. Several things drew me to the Rockmite, including a simple design, the time it has been on the market and the number sold, and the large online community of users, which has a large data base of construction and operating tips, mods, etc. I subscribe to the Rockmite group on yahoo, and have researched a number of sites with information and examples. I will add a list of relevant links at the end of this page.

One major advantages of building a Rockmite kit is the very affordable price. My kit was just $29 dollars, plus $16 for the optional "connector and control" accessory kit. I have a hard time opening Icom HF rig and playing around with the design, partly because of the cost and partly because of level of manufacturing detail. It just isn't meant to be tinker-friendly. On the other hand, a radio that I assembled myself, and that i understand thoroughly, can provide a lot of opportunity for applying mods, redesigning, and generally tinkering around with its circuitry. The worst case, if I seriously goober it up beyond repair, is to order a new $29 dollar kit and start over.

I ordered the 40 Meter Rockmite, for 7030, and the "connector and control" kit. These include the circuit board and all the parts mounted on it, plus the required connectors for power, headphones, paddles, and antenna. There is also a push button switch for controlling the PIC processor, a potentiometer for gain control, and a diode to put in series with the power connector to prevent frying anything by hooking it up backwards. The various parts are shown in their plastic bags in the photo above. The US nickel is included to give an idea of scale. This board is pretty small...

In spite of the small size, this little transciever has some pretty impressive features. An 8-pin PIC processor is used to control the T/R offset and provide an iambic keyer that will work with paddles, or a straight key. The reversable T/R offset is 700 hz, and provides a built-in sidetone, as a bit of the transmitted frequency is coupled back to the receiver. A crystal is used to provide the basic frequency for the oscillator, with a second crystal on the same frequency used to narrow the receiver front end. The single board provides a 500mW transmitter, a direct convertion reciever, and automatic full-QSK T/R switching.

The Rockmite board is 2 by 2.5 inches, which is fairly small. It will fit in an Altoids tin or other small enclosure. The most interesting installation I have seen is a complete Rockmite built into a computer mouse, with the left and right buttons used in place of paddles for iambic keying. At first glance the board seems to have places for a lot of parts, but actually there are not so many--it is just a compact design with everything in a small space. The board assembly took me two sessions of about 90 minutes each. If I still had the eyesight of a 20 year-old, it might have taken less time. I use a magnifying glass to augment my bifocals, and have gotten in the habit of double and triple checking component identification and placement.I have built a number of electronic kits, and found this one to be pretty easy and straightforward, with the exception of installing U1, which is a surface mount IC. It is not terribly difficult, but requires some care and very good soldering skills.

It is a bit of a challenge to solder down a chip smaller than Abe Lincoln's head on a penny. (I didn't notice the pocket lint in this photo until I was cropping it for this page--oops!) The surface mount package has four very small leads on each of two sides, and they lay down on very small pads on the circuit board and have to be soldered in place. I don't know why the surface mount package was used--the same chip is available in DIP packages just like the other two chips used in this kit. Then you would simply need to solder in three chip sockets, and not have the additional worry of a surface mount component. All the other parts mount conventionally, and it is not too onerous to install U1, but. it would be a more suitable kit for less experienced builders without the surface mount. It is not difficult for someone with decent soldering skills and good eyesight, but overall I would not recommend this as a first kit.

As recommended, I started by soldering on U1. Next I put on the two IC sockets and moved on to installing all the capacitors and three inductors. By installing a few components at a time, scattered around the board, it wasn't hard to make quick progress. I avoided putting in parts that were bunched up, rather putting in a few at a time that were spaced out a bit. This kept the leads far enough apart to solder and trim them easily.

There is no extra room on the board, so good technique and a bit of planning at each step helps a lot. Once you get things going, they seem to go quickly. A normal soldering pencil and basic hand tools are all you need. I have a 15 watt soldering pencil from Radio Shack that worked just fine. Too much heat and you might damage the board, so good technique and small wattage pencil are important. The photo above shows the board at the end of my first 90 minute soldering session. I worked at a fairly leisurely pace--it is much more important to get the parts in the right place, with the right polarity, and with a good solder joint than it is to get them in fast.

After a second session the next night I had the board completely populated.

It took a bit of coordination to make sure the crystals were elevated from the board as recommended and their cases grounded. I have never seen a ground applied to a crystal like this, and can see that it may complicate using socketed crystals to change frequency if it is actually needed.

Another area that requires some care is installing the PA transistor, Q6. You need to attach the heat sink before installation and then ensure as it is placed on the board that the heat sink doesn't contact any resistor leads. I added 10mm insulated standoffs to help with airflow around board and they came with screws and washers to mount them to the board and case.

I soldered in wires for the various connections to allow testing. Note the capacitor with the lead extending to the right, which is the antenna connection. This capacitor and an inductor were supplied on a supplemental sheet. they form a series resonant circuit to reduce spurious emissions, and you should ensure you install them. With the test leads soldered on it looked like the kit was having a bad hair day...

With power applied and hooked to a 40 meter inverted-V I was able to hear CW! The reversing function seemed to work also. With a dummy load I was able to hear sidetone, and transmit to my IC-718. I haven't hooked up a power meter sensitive enough to measure 500 mW reliably. I will likely have to use a 'scope and make voltage measurement to see how much power out. Since the testing seems to show normal operation, I just have to install the board in a case, with suitable batteries and tuner, and I should be ready for portable operation.

The enclosure I am going to use is a coffee tin about 2.5 by 2.5 by 4 inches. This should be plenty of room to include a small tuner, batteries, and voltage regulator and filter.

UPDATE April 12, 2012

I mounted the Rockmite board on the side, a bit more centered than it is in th photo above. The controls and LEDs are on the side opposite the one the main board is mounted on. This is the same side where I mounted the tuner, with its controls available. Antenna connection on top and the power connector on the end. The phones and paddles on the left under the Rockmite gain pot and push button. The opening in the tin is on the bottom with the plastic lid acting as a non-skid base.

I mounted a voltage regulator on the end above the power conector. I did drop the idea of internal batteries, but this will keep my power options open, including a small solar panel.
Since I plan to use a simple wire antenna, I built in a great little tuner, with SWR indicator bridge from Better QRP Kits.
I also built a second one of these tuners for general QRP use, and there is a separate article on my Ham Projects page for the tuner build.

The two 1/8" jacks are for the paddles ("P") and the headphones. Just above is the push button switch to select receive freq and enable paddle speed setting. The large knob is the gain control. The small knob is the tuner capacitor; the toggle switch is for tune/operate with the tuning LED just to the left. The power jack is on the left side, and the antenna terminals are just visible at the top. The photo above this one did not have the ground terminal for the antenna.

Inside the Rockmite board is mounted on the back side of the tin, and the tuner board on the front. the output of the Rockmite is connected tot eh tuner input by the series resonant circuit supplied to reduce spurious emissions. This is the bottom view with the tin on its back.

In the final form, the coffee tin contains the transceiver board, the End-Fed Half-Wave Tuner board (with SWR indicator bridge) and a basic voltage regulator to limit the voltage to 12 volts DC. This photo shows the 4.7 KOhm "dummy load" resistor still attached. With it the tuner tuned very easily, and the code sould be heard on my IC-718. I may modify the sidetone volumn, but that can wait for a few actual contacts, which should be coming shortly.

If you have comments, suggestions, or know of really good Rockmite information sites, please e-mail me! !