Dxers Unlimited's  "HOW TO..."

A series of easy-to-download files devoted to HOW TO....
build your own radio equipment and accesories

by Arnie Coro, radio amateur CO2KK

Arnie Coro's "Easy-to-Build PI Network Antenna Tuner"

The PI network is one of the most flexible arrangements for tuning a wide variety of antenna and transmission line systems. That's why I prefer it over other designs like the popular L and T Match, Z couplers and Pararell tuned circuit plus link that are available from different manufacturers.

From my experience of more than 40 years I strongly recommend the PI Network coupler, to which two more basic building blocks should be added, a simple attenuator circuit, and a protection system for preserving your radio's front end from possible damage due to static electricity accumulations.

The PI Network is nothing more than two capacitors and one coil.....

There is one input capacitor, connected between the antenna input and ground. This capacitor, in the case of a tuner built for general short wave reception as this one, should have a range of from about 20 picoFarads to 300 picoFarads... The output capacitor, placed between your radio's antenna input and ground is also an identical unit.

The coil simply is connected in series between the two capacitors...That's why it is known as a PI Network, as the combination of the two capacitors and the coil have the configuration of the Greek letter PI !

The coil is very easy to build, and when properly designed provides a very wide range of adjustments in combination with the two 20 pF to 300 pF variable capacitors.

The HOW TO of the coil for the PI network antenna tuner

1. Cut a length of plumbers PVC pipe to about 15 centimeters ( 6 inches ) The diameter of the pipe should be 25 mm to 37 mm ( from 1 inch to 1 1/2 inch )

2. Drill two small holes at each end for the wire

3. Drill two bigger holes at each end for the bolts that will support the coil to the base of the antenna tuner

4. Obtain about 10 meters ( 30 feet ) of no.16 or no 18 enamelled wire ( the one used for winding transformers and electric motors )...

5. Pass the wire trough the two holes at one end...

6. Now, wind TWO wires in pararell at the same time, so that when finnished you may then remove one of them leaving you with a winding that is spaced exactly one wire diameter....

7. After you reach the end of the PVC pipe, pass the wire trough the two holes, and do this under tension... you will have a beautiful "factory looking " coil...

8. Don't worry about the number of turns.... just fill the 15 centimeters with wire, spaced at one turn

9. Now, you will have two really do a fancy, and delicate job... every 3 turns, scrap the surface of the enamelled wire so that it is shining bright ( no more enamel ), and very carefully solder a length of no 18 plastic insulated wire, long enough to reach a multi position switch that you are going to install later

10. After you finnish soldering all the no 18 plastic insulated wire lengths, install the coil on the base of the antenna tuner ( which may be just a very simple wooden base, of the base plate of a metal box... in the case of the metal box, the coil must be separated at least one coil diameter from the metal surface )

11. Connect one end of the coil to the fixed ( stator ) plates of the input capacitor, and from there connect a short length of no 18 plastic insulated wire to the center pin of the antenna connector that you are going to use...

12. Do the same at the other end of the coil, that is, connect the end of the coil to the stator plates of the output capacitor, and from there connect another no 18 plastic insulated wire to the output connector of the tuner ( the one that goes to the radio receiver)

13. Now, carefully connect the wires coming every three turns to the 10 position selector switch..

14. The tuner is now complete...

Connect the antenna to the input connector and the radio receiver to the output connector via coaxial cables, then tune your radio to the 6 mHz band and select about 20 turns of the coil...carefully tune the two variable capacitors while listening to the radio... if you are tuned to a weak station, or to the noise coming from an empty channel, you will notice an increase in signal strength, or noise, when the capacitors are tuned to peak resonance.

There are many possible combinations of number of turns and settings of the capacitors that will provide resonance, but you should prefer the ones that provide MAXIMUM C, that is the resonance points at the maximum possible settings of your variable capacitors that is when the movable plates are more into the fixed plates.... You should use the least number of turns in the coil that is possible and still obtain a noticeable peak in signal strength (at resonance).

The diameter of the PVC plumbers pipe used for this project should be 25 to 37 mm (that is 1 inch or 1 1/2 inch) The variation in coil inductance coming from using 25 mm or 37 mm is compensated by selecting the number of turns used, so for all practical purposes you can use either 25 mm or 37 mm.

The selector switch may be replaced by an alligator clip connected to a flexible lead, the lead connects to the output end of the tuner, and the alligator clip selects the number of turns.

You could build a compact portable version of the PI NETWORK antenna tuner, using a POWDERED IRON CORE TOROID ... but that is sometimes somewhat difficult to obtain, so that's why I use the AIR CORE of the PVC pipe which is very easy to find anywhere. In case you can not find PVC, a wooden dowel will make an excellent coil form too...

Any doubts ????

Just e-mail to Arnie Coro

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