fter I got the QRP spy transmitter going (see From the Shack, Spy vs Spy in April 2000 issue of antenneX) and found it had an objectionable harmonic output on 20 and 15 meters, I decided to build an antenna tuner. The one and only circuit I had considered was the venerable Pi-L, which is an excellent attenuator of harmonics and is capable of loading up almost anything.
Now I know there are other circuits around, but my own experimenting has shown me that the capacitive T network is not as efficient as others. It does have fewer parts, but harmonics like capacitive coupling like mice like cookies. Also such networks are limited as to the range of antenna impedances that they can match well. And, I wanted a tuner that is cheap to produce.
I might add that the Pi-L is also used in many of the high-power linear amplifiers such as the Ameritron linear amplifiers. My elderly Johnson Thunderbolt has a Pi network, so both of these circuits have been around for a long time
Figure 1 shows the diagram of the PI-L network. Now if you only want to build the simple Pi network, all you have to do is drop L2. The range of impedances the network can match is a little less, but still does a good job on random lengths of wire. You must use a good ground or radial system though.
In the diagram, C1 is a 400 pF variable and L1 consists of 22 turns of #16 wire. C2 is a 1,100 pF 3-section variable that came out of an old broadcast receiver. L3 is 11 turns of #16 wire and both coils are wound on a 1-1/2inch form, (3.81 cm). L1 is on a 2-inch (5.08cm) long and L3 is on a 1-3/4 inch (4.45 cm) form. The turns are spaced wire diameter. These values are those I had on hand as far as the capacitors are concerned and the coils were wound to fit the switch I intended to use. I had found a double section, 11 position ceramic switch that I chose to use, so I tapped L1 every other turn and L2 every turn.
Ideally the two coils should be roller inductors, but finding a matching set would be extremely hard to do, since roller inductors are hard to find anywhere at a reasonable price and I still wanted cheap!
After this version was built, it was tested by tuning up my 40-meter dipole. On 160 meters, C2 was not large enough and will need more capacitance in parallel with it before it will tune correctly. So I will add a switch and a fixed capacitor to accomplish this. 75 meters was a little more difficult to tune and VSWR could not be reduced any lower than 2:1. There is some indication that the length of the ground wire influenced the tuning on 75 only. On 40 meters the tuner reduced the VSWR to 1.2:1 with no difficulty.
Tuning 30 meters could be done, but the VSWR could not be lowered to less than 2:1 as C2 now had too much capacitance. Since this capacitor is made up of 3 sections with 365 pF per section, I installed a pair of switches to let me switch in and out the sections as needed. 20 meters and the higher bands can now be tuned easily. I also added the switch to add capacity for 160 meters, as well as an additional connection between the Pi and L sections so that the antenna could be connected to the Pi section and bypass the L section when it is not needed. After these modifications, tuning 160 meters should be tuned easily too.
This network will tune short wires and/or whips very well, such as you might have to use during an emergency. So it would be a handy thing to have around, in addition to general ham use. It is suggested the power be limited to less than 100 watts with components similar in size to what I use here. Of course, larger size capacitors and inductors will allow the use of much higher power.
You will need a decent SWR bridge or field strength meter for accurate tuning. Set the capacitors to maximum and the inductance to minimum. Apply low power and set the SWR bridge for maximum forward power, then start tuning C1 for minimum SWR, then if no dip is noted, set C1 at midpoint and shut off the transmitter. Increase the inductance and reapply power and tune both capacitors for minimum SWR. Follow this procedure until you get minimum SWR and you are tuned up. A field strength meter makes tuning up much easier.
This coupler was built from scrap parts, even the chassis. You could build your own box out of a Tupperware container and line the inside with thin sheets of brass or copper that can be purchased at a hobby or craft shops. The capacitor used for C1 was found in one of my junk boxes and C2 could come out of a table model broadcast receiver.
Photo 1 shows the front of the tuner before labels are installed. The small knob to the left is for C1 and the center knob is the inductor switch. The two vertical switches operate the sections of C2 in and out as needed. Above them and to the right is the switch for the added capacitor for 160 meters and the small knob is for C3.
Photo 2 is of the back panel. The red connectors on the left are for the antenna. The red connector directly above the black connector is for the L connection and the red one to the right is for the connection to the Pi section. The input coax connector is to the right.
Photo 3 below shows the switches and the general layout of the tuner. The switches can be seen in the upper left of the photo, next to C3.
The main Photo at the top of this article is a top view of the tuner. C3 is to the left of center and the small disk capacitor that is switched in for 160 meters can be seen over C3 and to the left of the top switch. The coil for the L section is mounted almost perpendicular to the front panel and at right angles to the Pi section. C1 is to the right of center.
If you decide to build one of these tuners, use what you have at hand or can scrounge up. Either way, it will add to your Shack. Portable operation with almost any kind of antenna will be easier with this tuner. -30-
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Last modified: December 31, 2010