A CENTENNIAL CELEBRATION
QRP SSB/CW KIT IN REVIEW
Special to The Sojourner
Covering the entire 3.5 MHz band, the Centennial kit features a roomy 31 square inch board (by FAR circuits), compared, for example, to the 22 square inches of the densely populated Small Wonder Labs White Mountain SSB rig designed by Dave Benson, NN1G. The result with the Centennial is a larger, less trail-friendly rig, but one which is a breeze to build.
The full Centennial kit -- you supply the enclosure, microphone and connectors -- is available from Dan's Small Parts and Kits of Missoula, MT. The company is currently offering a 40-meter version of the Centennial for $79.95 plus $4 shipping.
Each page of the manual includes a complete copy of the silk-screened board, with arrows indicating components to be installed. It is next to impossible to install a component in the wrong place. I give the manual high marks, with only one gripe: the reproduction quality is poor, so that it is difficult, for example, to distinguish between "1 mf" and ".1 mf".
The inexperienced builder may need additional help identifying components, winding toroidal transformers, or determining pin-out on substituted components. An excellent resource for this is Paul Harden's "Electronic Data Book for Homebrewers and QRPers" ($20 plus $2.50 shipping from Quicksilver Printing, P.O. Box 757, Socorro, NM 87801).
Having built the White Mountain for 20 meter SSB and the R2/T2 combo for 40 meter SSB (available from Bill Kelsey at Kanga U.S.), the 80-meter Centennial was the easiest of the three to build and troubleshoot. Like the White Mountain, and in contrast to the T2 phasing rig, the Centennial is designed for easy alignment. Besides peaking receive audio and RF output, the only adjustment is setting gate voltage on the final power FET (an MTP3055E) to obtain linear operation.
However, following the manual and setting the gate voltage at 2 volts may not do the trick. I had some difficulty finding a setting on the small pc-mounted potentiometer. I added a 20 ohm resistor on the input of the gate to suppress oscillation, and found a workable setting. The MTP3055E is a rugged component, available from Mouser for about $1, and will put out up to 20 watts if operated at 24 volts. The Centennial is designed for a 5-8 watts output at 12 volts.
A three crystal filter provides good upper-sideband rejection on transmit, and effective single-signal reception. An NE602 balanced modulator does a fine job of eliminating carrier. No adjustments are required here.
Past ARRL president Harry Dannals, W2HD, graciously agreed to a cross-town QSO to listen to the Centennial on 3.9 MHz, and pronounced it clean, adding that the audio sounded natural. I was using the "power mike" sold by Radio Shack.
The rig is designed to be used with an amplified microphone. The manual includes two simple circuits for building an amplified mike using an inexpensive Radio Shack electret microphone element.
The Centennial has a simple NE602 VFO with a broad tuning range, and it does drift. To minimize drift, be sure to follow the manual's suggestion to glue down and coat the VFO coil with your favorite coil dope (I use clear fingernail polish). A week after applying this treatment, and following a one hour warm-up period, the Centennial displayed a downward drift averaging about 5 Hz per minute. In practice, this drift does not interfere with conversational SSB communication. However, it is not tolerable during CW ragchewing.
During SSB operation, occasional retuning is required, as it is with the White Mountain. It is wise to keep SSB transmissions conversational rather than oratorical, and to restrict CW operation to contest-like exchanges.
The broad tuning range of the Centennial produces a kind of digital effect -- like tuning a synthesized rig with slightly larger than normal steps -- as the wiper in the 10-turn potentiometer traverses windings. It's more noticeable listening to CW than SSB. Builders who cannot tolerate this may decide to replace the 10-turn pot and varactor diode with a variable capacitor and vernier dial.
A good antenna can mean front-end overload for NE602 rigs, and the Centennial is no exception. I hung a dipole at 50' trimmed for use around 3.9 MHz, but found that without an antenna tuner in line, the Centennial suffers from AM breakthrough. Until I add further front-end filtering to the Centennial, the tuner will continue to serve as a preselector. Both the White Mountain rig and the R2 receiver have more rugged passive diode ring mixers in their receive front-ends.
While no other QRP rig I've used (kit or commercial) can match the rich audio of the R2 receiver, the Centennial receive audio quality is good, and there is plenty of output to drive a speaker.
CW operation requires injecting an audio tone into the microphone input, and requires manual TX/RX switching. Plans are provided for a simple two transistor keyed audio oscillator. Some builders might already have a suitable code practice oscillator or keyer with sidetone. The CW transmit offset is determined by the frequency of the audio tone, and an audio gain control here will serve as an RF output control for CW.
Using a Micronta analog multimeter, I measured 30-60 milliampere draw on receive (from a whisper all the way up to ear-splitting speaker volume), and about 500 milliamperes while hollering into the microphone. This makes the rig suitable for use with a small gel cell battery. While not as trail-friendly as the White Mountain rig, the Centennial is certainly a portable radio.
The Centennial includes an output for a frequency counter. I have been listening to the oscillator in my antenna analyzer to find my frequency. This is not as slick as the White Mountain's internal microprocessor which (at the push of a button) sends your frequency in Morse code.
Dan's Small Parts has been prompt in dealing with problems arising with this kit. Six NPO capacitors were missing from the first few kits shipped, but they arrived promptly in the mail. However, I miss the direct e-mail contact with the designer which is available to builders of the White Mountain SSB kits. Daulton directs all inquiries to Dan's Small Parts, and that means a letter or a telephone call.
On the air, SSB fun with the Centennial has included: getting a "10 db over 9" report at night from Galax, VA; visiting the regional QCWA net and being invited to become a regular; and frequent visits to both sessions of the Virginia Fonet.
For the latest information on Centennial availability, prices, details on partial kits, and group discounts, visit Dan's Small Parts web page: www.fix.net/dans.html, or contact the company by phone: (406) 258-2782. The company's mailing address is: Dan's Small Parts, P. O. Box 3634, Missoula, MT, 59806.
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Ralph Irons, N7RI, is a longtime QRPer, avid builder and experimenter living in Charlottesville, VA