This article is very old, from around 2009. It was originally published on my old website, flylab.ovh.org, now defunct. It’s unlikely to have much educational value. I’ve republished it mainly for sentimental reasons.
I had the luck and opportunity to borrow a Kenwood shortwave transceiver to experiment with amateur radio. I had a random wire antenna installed beforehand; to get on the air, I needed a tuner to match it to the transceiver. Buying a factory-made tuner was out of the question, so I quickly built a device known in Poland as the SP6SYV antenna tuner. In English literature, this is commonly referred to as the Z-match antenna tuner. Its main advantage is the lack of tunable or tapped coils, which eliminates the need for a high-quality, multi-position switch. A widely available two-position switch to select the band is sufficient. I also built in an SWR meter using a piece of coaxial cable. This is also a low-cost solution, as it doesn’t require toroidal cores.
Antenna tuner is a device that matches the impedance of the antenna to the impedance of the transmitter, which is usually 50 Ω. It is a very useful addition to many amateur radio setups and is almost necessary when broadband antennas are used. Apart from matching the impedance, it also performs signal filtering during transmission and reception. When transmitting, it cuts off higher harmonics. When receiving, it filters out signals outside the band, reducing possible intermodulation and mixer overdrive. These advantages, in most cases, outweigh the inconvenience of additional knobs to turn and the slight power loss in the tuner’s components.
Building the device
The entire device was assembled using parts from my junk box or donated by local radio amateurs. The general appearance of the tuner is shown in the photos:
The building instructions and the schematic were taken from SP8GSC’s website.
The design is quite simple.
It consists of two tuning capacitors and two high-frequency transformers.
The switch selects the band.
Position A is normally used below 10 MHz, and B is used above.
However, during tuning, the main goal is to achieve the lowest SWR and best sensitivity, so sometimes the other position may work better.
I made a cabinet for the tuner from a 2 mm aluminum sheet with dimensions of 28x10x24 cm.
The front panel features two tuning knobs, a band switch, and SWR meter controls: a REF/FWD switch and a potentiometer.
The back panel includes two UC1 sockets and a grounding terminal.
The coils
The coils were wound on a plastic coil former, as shown in the diagram below (drawn by SP4ABR). The dimensions are in millimeters. The coil former can be made of fiberglass laminate without copper; I simply used a random piece of plastic.
The coils should be wound with wire having a diameter of 1.2–2.0 mm. I used 1.7 mm wire from a 2.5 mm² electrical installation cable, stripping the plastic insulation beforehand. After winding, I applied a layer of paint to the coils to prevent corrosion. Using proper enamel-coated wire is recommended. Some may opt for silver-plated copper wire, but for frequencies below 30 MHz, it hardly matters. The number of turns and diameters of the coils are shown in the table below:
| Part no. | Number of turns | Diameter |
|---|---|---|
L1 |
10 | 38 mm |
L2 |
7 | 38 mm |
L3 |
8 | 54 mm |
L4 |
6 | 54 mm |
The photos show the process of winding the coils. Not pretty, but they get the job done.
Tuning capacitors
SP2IJ was kind enough to give me two variable capacitors for my build.
C1 is the main tuning capacitor from a Pionier tube receiver.
Every second plate was removed beforehand, resulting in a variable capacitor with a range of 20–500 pF.
C2 was a 2x 20–540 pF capacitor from an unknown source.
C1 had to be insulated from the ground, so I mounted it on a piece of Novotext sheet (the material is called Tekstolit in Poland).
C2 was fixed directly to the aluminum chassis.
Each capacitor was supplemented with a planetary gear knob, colloquially called настройка in Russian or nastrojka in Polish.
These were supposedly obtained from an unknown Soviet military transceiver.
The knobs can be turned in two ways: quickly, with a 1:1 ratio, and precisely, with a ratio of about 1:40.
Precision tuning is required for C2, while C1 can be tuned adequately without it.
The nastrojkas were mechanically coupled with the capacitors using a piece of universal PCB with no copper (C1) or a piece of copper sheet (C2).
The shafts were slot-cut to accommodate the couplers, which were fixed by M3 screws.
This setup allows the couplers to tolerate some misalignment between the shafts and the knobs.
SWR meter
An SWR meter is necessary to tune the antenna tuner properly. I used a clever design of an SWR meter made from a piece of RG213 coaxial cable. I discovered it in a Polish amateur radio book and scanned it, but I can’t remember the title anymore.
You are supposed to place two wires just under the outer shield of the coaxial cable, as shown in the diagrams. The length of the measuring line is determined by the lowest band you want to use. In my case, it is 22 centimeters, which is enough for the 160 m band. The diodes used are germanium diodes, and the resistors are 180 Ohms.
I don’t have any more information, as I lost the title of the book. I might fill in this section in the future.
Usage and tuning
The tuning process is as follows:
- Select the band using the band switch.
- Set
C1to maximum capacitance, then tuneC2to the maximum noise heard in the receiver. - Start transmitting a carrier and tune
C2to achieve the maximum transmitted power and minimum reflected power. - If the SWR is not satisfactory, decrease the capacitance of
C1slightly and retuneC2. - Repeat until the SWR is acceptable (1.0–1.3).
Testing with various antennas
They say that a good tuner will match any antenna, even a random wire. However, that says nothing about the efficiency…
I tested my tuner with a G5RV multi-band antenna (situated 12 meters above the ground) and a random wire antenna near my house, which is 25 m long and 5 m above the ground. The central heating piping served as a counterpoise/ground. I tuned both antennas with the assistance of the factory-made SWR meter. A bit of patience and fine-tuning are necessary to reduce the SWR to a negligible value, but it is indeed possible. The table below shows the results of the moderately precise tuning:
| Band | Random wire | G5RV |
|---|---|---|
| 80 m | 1.3 | 1.2 |
| 40 m | 1.2 | 1.2 |
| 20 m | 1.1 | 1.3 |
| 15 m | 1.5 | 1.0 |
| 10 m | 1.4 | 1.2 |