RADIO HABANA CUBA SIMPLEST ANTENNA-TUNER
Dxers Unlimited 17 August, 1999
with Arnie Coro, radio amateur CO2KK
Arnie's Simplest-to-Build Antenna Tuner
Parts List: Here is the list of materials: one ferrite rod, of the ones that are cylindrical in shape, and of a length of about 4 inches or more. You will also need two identical variable capacitors. I prefer using air spaced variables taken from defunct old vaccuum tube radios, the ones you can still find at junkyards and garage sales. If you see vaccuum tubes inside, buy them, as each radio has a very nice air spaced variable capacitor. You will also need about 1 meter (3 feet) of standard, plastic-insulated wire.
Assembly: The tuner is made in the following way: First, you cover the ferrite rod with one layer of vinyl plastic electrician's tape. Then, you wind 10 turns of the plastic insulated speaker wire, spaced one turn diameter from each other. Connect the stator plates of the variable capacitors -- those are the fixed ones -- to each end of the wire, and connect the two capacitors' rotor plates -- the movables ones -- to each other and to your radio's ground connection. Now connect the antenna to one side of the ferrite rod coil where it is joins one of the capacitors' fixed plates. The other identical side goes to the radio's antenna input. And... you have a very nice, easy-to-build PI network antenna tuner! Tune your receiver to the 49 meter or 6 megaHertz band, look for a weak station, and then slowly turn first one capacitor, and then the other for maximum signal. That's it!
Variations: You may want to experiment -- remember that the radio hobby is an experimental one -- and wind more turns, say about 20 tightly wound, and see if you get improved reception at lower frequencies. With a standard 4-inch rod and 15 turns, using two 365 picofarad surplus variable capacitors, my low-cost PI antenna tuner will provide a good match between about 5.5 and 29 megahertz. Remember that the lower the frequency, the more capacitance you will need; in other words, the tuner will reach resonance with the capacitors' plates fully meshed into the fixed plates. Higher frequencies will tune with the capacitor plates out, with the maximum frequency reached when the two capacitors' plates are fully out.
I added a little five-position, single-pole switch and made taps to the coil in a later model. So, if you can get hold of a similar 4, 5 or 6 or even 10-position switch, you can add this feature to make a still-more-flexible antenna tuner. This simple tuner helps reduce image frequencies that are a problem with less expensive radios, and it also stops some intermodulation problems that make reception difficult in those receivers, too... I have used my prototype with a little amateur radio QRP 10-watt single-sideband phone rig with excellent results, as well.
You can learn more about PI network antenna tuners by visiting your library and looking at the antennas chapters of any radio handbook.
Questions? Send e-mail to Arnie Coro
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