Page 1 A QRP Paddle for the Hendricks PFR-3 Transceiver by Richard Meiss, WB9LPU Introduction The Portable Field Radio by QRP Kits is an excellent addition to the variety of QRP rigs that are available in kit form or as commercial units. As a ham who likes to invent things, especially keys, I was challenged to come up with an accessory key for the rig. Naturally, I had to buy the neat little kit (purely in the interests of science) in order to test out the ideas that became the little key presented here. The radio set itself goes by the name of PFR-3. This name suggests a name for the key that uses these letters hence the name PuFfeR, or Puffer. (If this is too silly, it could be called something like accessory iambic key. ) The first challenge was to make a key that would fit to the rig using its existing mounting fixtures without the necessity of making any modifications to the radio cabinet. Other design options centered about the basic iambic mechanism (whether to use pivot bearings or flexing spring elements), the tensioning mechanism (spring or magnetic),
Page 2 and the contact arrangement (where to pivot the levers), and what kind of adjustments to provide. The basic structure of the key uses a frame member that bears the fixed contacts and tension mechanism, along with a threaded rod used in the contact spacing adjustment. The machined parts were made of C360 brass, and stainless steel screws were used throughout. Upper and lower plates which were attached to the frame member and provided the fixed part of the bearing assembly. A rear plate provided a means of electrical and mechanical mounting of the key to the PFR-3. (This design is an extension of a previous paddle, the KXer, which was made for use with the Elecraft KX-1 transceiver.) During the course of development several types of bearing mechanisms were tried, and the most satisfactory (and elegant) proved to be single brass balls mounted above and below the paddle arm. These run in polished holes in the upper and lower plates of the key mechanism; the geometry of the constructions insures that they run without binding. Some versions used the smooth heads of stainless steel cap screws running in polished holes. This also worked well but did not have the self-aligning feature. To provide paddle tension, both springs and opposed magnets were tried. Both schemes provided a good feel, but the magnetic system allowed for easy selection of the amount of tension desired. (The fixed-position magnet used was a rare-earth type the 1/8 diameter rod type comes in lengths ranging from 1/8 to 5/16, allowing for a range of repulsive forces. The 1/4 length appears to provide the best restoring force.) The contacts are lathe-polished brass-on-brass, which provides good low-current service and which are easily cleaned, if need be, with a strip of typing paper pulled between them. Attaching and Adjusting the Paddle The series of photographs on the next page details the attachment of the key to the rig. The threaded rod extending from the knurled nut is screwed into the right-side 4-40 hole on the front of the PFR-3. The key itself has a hole in the rear plate with a diameter of 3/8, which is matched to the diameter of the knurled nut. To install the key, its 1/8 phone plug is inserted into the key jack and the key is pushed forward until the knurled nut is fully inserted into the rear-plate mounting hold. Tightening the vertical screw in the upper plate of the key engages it with the groove cut in the knurled nut. This action draws the key tight against the front panel of the transceiver.
Page 3 The installed key will fit to the transceiver without blocking the earphone jack, and it is held tightly without any wobble. Now the key can be adjusted to the user s touch and key the transducer. The knurled nuts at the side of the paddles are threaded onto a rod that extends through the main frame of the key. The smaller hex nut is for setting the adjustment drag and for locking the adjustment. A special carbon-steel curved washer is between the two nuts. To make the adjustment, the dot-side knurled nut is turned inward (clockwise) until the contacts just close. Then the hex nut is turned until it is snug, and the knurled nut is backed out until the paddle movement is satisfactory. The hex nut provides drag on the adjustment and prevents lost motion the relative amount of drag is set by how closely the two nuts are turned against each other (against the resistance of the curved spring washer). For subsequent adjustments, both nuts are held with the fingers so that they turn together. To lock the adjustment more securely, the hex nut is more fully tightened against the knurled nut. This arrangement allows for very close and stable spacing; the key has been tested at 30 wpm (as fast as I can go) without falsing.
Page 4 Stand-alone Paddle Operation In order to operate away from the PFR-3, the paddle must be supported. A design for this purpose is shown below. It consists of a brass base with fold-out legs and a mounting fixture that mimics that of the normal transducer mounting. Connection to the transceiver is made through a stereo cable with male and female ends. The base is rather heavy, and with the self-adhesive plastic feet and a reasonable keying touch, movement has not been a problem. The weight of the key alone is 5 ounces. The base adds another 12 ounces. Some construction details The next page shows two stages of disassembly of the key. The key is built around a center frame, to which are attached an upper and lower plate. These have precisely-machined holes into which the brass bearing balls are fitted. The bearing balls are mounted to the end of short brass cylinders to which the plastic paddle arms are attached, and nylon washers limit up-and-down movement of the paddles. Because the bearing balls run in cylindrical holes, alignment is not critical and there is no binding or drag on the paddles. Each paddle bears a thin brass strip that conducts from the paddle contacts to the phone plug via an insulated wire.
Page 5 The magnets for paddle tension are mounted behind the brass paddle strips and interact with the fixed magnetic mounted in the center frame. Further disassembly shows more details of the paddle construction.
Page 6 In addition to the key described here, a number of other paddles were built during this project in order to test out various approaches to bearing and tensioning design. In the design above, the electrical connections are made via brass strips on the inside of the paddles, with wires extending to the phone plug. (The brass strip also serves as a retainer for the paddle magnet.) Some other paddles were directly wired. Different bearing systems were also tried. These earlier paddles were made of 2024-T4 aluminum. The whole group is shown below. Here is the test group, Class of 2008. Clockwise, starting at the top, the various other paddles incorporate different features: - (Red paddles) Screw-head pivots, mounted directly onto plastic paddles. Spring tension, internal connections with wires only. Top plate of 3/16 aluminum. - (Red paddles) Brass ball pivots, mounted with through-drilled holes onto aluminum bearing fixtures. Magnetic tension. - (Blue paddles) Brass ball pivots, mounted without through-drilled holes onto aluminum bearing fixtures. Spring tension. - (Green paddles) The final version; all-brass construction, magnetic tension. Internal brass conducting strips in paddles. All of the variations shown worked well. Aluminum has the advantage of light weight, a point to consider if back-packing or contesting when a multiplier is given for
Page 7 the (lack of) weight of the equipment. The use of solid plastic paddles made of stiff material eliminates the paddle flexing that is found in some QRP paddle design. The basic design, in which a center frame supports plates that carry the bearings and also supports the adjusting screw mechanism, would lend itself to other built in applications. At present the key uses a fixed magnetic (or spring) tension, and this does not appear to be a serious drawback. However, there are designs on the drawing board that would allow for adjusting the tension by means of a moveable magnet or an additional magnet partially canceling out the fixed magnet. However, the additional complexity may not be worth the effort. At the present state of development, the paddle is small, portable, and works almost as well as a fullsized one. Like most of my designs, this paddle was built to respond to the challenge of making something both useful and unique. It was also motivated by a desire to further the use of CW and QRP operating modes. Richard A. Meiss, WB9LPU wb9lpu@earthlink.net http://wb9lpu.googlepages.com October 2008