Antenna Tuning Circuit for Listening to Shortwave Radio

I felt while listening to shortwave radio that something was still missing for its quality reception, I set about building a tuning circuit for the antenna. The original idea of ​​building a circuit on a varicap diode was abandoned because I found an easier way to achieve a similar result with a much simpler circuit.

- a spread box

- a plastic tube with a diameter of 3 cm and a length of about 4 cm

- a varnished copper wire of 0.8 mm

- a tuning capacitor PVC-22J20T2

- a plastic tube with a diameter of 3 cm

- a screw

- a tuning wheel

- 2x BNC connector for the panel

- a 500k potentiometer or a 340 – 470 ohm resistor

- a ferrite core from a discarded ATX PC power supply (see my instructions for building a balun 1:1)

- about 1 m of twin-wire with a solid conductor (bell wire)

- 2x crocodile clips

- 1x large car battery clamp

- thin wire for the winding handles

- a piece of binding wire for attaching the tuning capacitor and air coil

- old newspapers

- adhesive tape

- aluminum foil

- wooden board

- two electrical tapes

- tin

- rosin

- multimeter

- soldering iron

The basis of my circuit is a PVC-22J20T2 tuning capacitor, which is a high-quality replacement for an older air tuning capacitor. Its purpose is to adapt our dipole antenna made of two binding wires, as I described it in the instructions for building a balun 1:1 , so that it resonates as faithfully as possible at the desired frequency and effectively receives the signal.

A dipole antenna has a certain resonant frequency for a given length, determined by its length and the material used. If the antenna is physically longer or shorter than the ideal length for a given band, its resonant frequency is different. By rotating the capacitor, we change its capacitance and thus change the overall resonant frequency of the antenna system (antenna + capacitor). We are therefore looking for a capacitance value at which the entire circuit resonates exactly at the frequency at which we want to receive the signal. The tuning capacitor therefore functions as a fine adjustment of the antenna resonance, without the need to physically shorten or lengthen its conductors. It ensures better absorption of the energy transmitted by the transmitter, in the form of a carrier wave with a radio signal, by our antenna and its better transmission to the radio receiver.

First, we prepare a ferrite core, around which we make 10 turns of 0.8mm double varnished wire and leave about 2 - 2.5 cm of wire at the ends, from which we scrape off the varnish.

We prepare our spread box and on one side we mount a BNC connector to it, to which we first solder the signal and ground wires. The signal wire does not need to be that long, as we will be soldering it to the winding around the plastic tube (see below), but the ground wire should be a little longer so that it can be soldered to the ground wire of the balun 1:1. The tuning capacitor should be placed so that the ferrite core can fit comfortably next to it, which must not be behind or in front of our air coil (see below).

To tune the dipole to an estimated total length of 30 m (approximately 10.1 MHz) and for a band from about 5.2 to 18 MHz, you will need an inductance in the order of microhenries (µH).

For a single-layer coil, the inductance can be estimated using this formula:

L (µH) ≈ (diameter^2 * turns^2) / (45 * diameter + 100 * length)

But there is no need to worry about it so much. We wind 15 turns of 0.8 mm copper wire with smaller spacing on the coil. When winding them, we scrape off the varnish where the winding will be easily accessible from above, so that the crocodile clip can be attached to them. I tried to feed pieces of wire to the winding to make it easier to attach the crocodile clip, but it also works with scraped varnish of about two centimeters in length. We lift the wire slightly with a screwdriver so that it does not lie on the tube in these places. Of course, we also scrape off both ends of the winding so that the wire from the signal wire of the BNC connector (i.e. the antenna input) can be fed to them. Here it is necessary to make a branch from this wire from a shorter piece of wire, to which we solder a pad or plate so that the crocodile clip can be snapped onto it. At the other end of the coil, we solder a short piece of wire with a pad or plate at the end to the wire. Secure the entire coil using a binding wire into a plastic box, preferably from the bottom and from the side. In my circuit in the photo there are only 12 turns, but the circuit is usable only up to 14 MHz and overall it is most effective around 11 MHz. For example, at 10.5 MHz, the tuning circuit, when I connected the alligator clip to the middle of the twelfth turn, helped me achieve a signal close to FM quality shortly after noon, which would only be possible with a balun 1:1 under absolutely ideal conditions.

Since the PVC-22J20T2 is a double capacitor, for am and fm, with sufficient capacitance (after connecting all stator wires, except for the side ones /see photo/, it should be about 470 pF) for receiving short waves, we will now describe its individual wires and connections to the circuit.

Conductors connected to the tuning axis = ROTOR of the capacitor. This is the moving part of the capacitor (usually a set of plates) that rotates around the axis. By rotating, you change the overlap between the rotor and stator, and thus change the capacitance.

The conductors (without the axis) = STATOR of the capacitor. This is the immovable part of the capacitor (the second set of plates), which is usually firmly connected to the frame.

It is necessary to solder all the stator conductors and both rotor conductors together using wire, leaving 4 cm of wire for further soldering on both. Leave the conductors with additional capacitances and the conductors in the upper part of the capacitor unconnected! It is better to cover the wire that we will use to connect the stator conductors with electrical tape so that nothing is shorted. And it is better to check with a multimeter whether the stator and rotor conductors or any of the conductors from the additional capacitances are connected. This would result in damage to the capacitor.

Somewhere on the side of the box, where we have to leave room for the balun 1:1, we measure the place for the axis of the tuning capacitor and make a hole for it there. We place the capacitor there. We attach the capacitor to the bottom of the box with a binding wire, or possibly to the side of the box.

Now, to prevent interference, we wrap the box from the outside with aluminium foil, with the appropriate holes for the connectors and the axis of the tuning capacitor. It is necessary to remember that here we have to cut out a piece of foil in the place where the axis of the tuning capacitor will be, which must not touch any layer of foil, unlike the grounding parts of the BNC connector, where it is okay, so we can solder the first layer of foil to the connector in this place. We stick pieces of newspaper on the foil, which we cover with another layer of foil. It is also necessary to pay attention that the BNC connector remains easily accessible and that the antenna can be easily connected to and disconnected from it. The second layer of foil should therefore have a hole for the connector and the connector should not touch it. We press the layers together properly. Then, we tightly tape the entire box with electrical tape. We check that the axis of the tuning capacitor does not touch any of the layers of foil and mount the tuning wheel on it. We then screw the box to a piece of wood to better ground it. (If we want to have an output via a BNC connector, we place it on the opposite side of the box similarly to the input one, where we place a two-pole toggle switch next to it, to whose outer conductor we solder the signal wire of this BNC connector, to the other outer conductor we solder an insulated wire with a solid wire /from a bell-shaped pair/, with an alligator clip at the end of it, for connection to the telescopic antenna of the radio. To the center conductor we solder the wire from the signal wire from the balun 1:1. We solder the ground of the BNC connector to the ground wire of the balun 1:1.)

When connecting the antenna circuit, we solder a wire with an alligator clip to the stator of the capacitor, which we will connect to the individual turns of the winding of our tube according to the tuned frequency. We solder the stator of the capacitor to the signal wire of the balun 1:1, we solder the ground of the balun to the wire leading from the ground of the BNC connector. (We can also solder a resistor with a value of 340k - 470k between these two balun wires, and think about it in advance, to compress the sound of the tuned station. We do not solder it there directly, however, because we must first test the entire finished circuit. Instead of this resistor, we can also use a 500k potentiometer, one of whose outer conductors we solder to the capacitor rotor, its middle conductor to the balun signal wire, and the other outer conductor to the balun GND. This way, the harshness of the sound can be easily adjusted. The potentiometer can also be mounted next to the GND output and the signal wire.) We separate the previously prepared bell-pair wires from each other so as not to damage their insulation, pull them into the box and solder them to the balun output 1:1. We check with a multimeter which wire is which. We solder a large clamp to the car battery and to the signal alligator clip to the ground wire, which we will connect to the telescopic antenna of the radio.

Since the tuning circuit is not an isolated block, it is interconnected by the internal circuits of the radio. So when we tune this circuit, we change the impedances and couplings in this entire assembly. This also affects how wideband or narrowband the entire assembly receives the radio signal. The tuning capacitor in the circuit also acts to adjust for changes in certain musical styles in the station's broadcast. It sometimes even responds better than the bass setting and treble on the radio itself. Strong shortwave stations can "bleed" into neighboring frequencies. This bleed often contains unwanted bass or treble components. When you "push" the sharpness of the resonance with your tuning, you suppress these interference components, which cleans up the midrange and makes the music sound "cleaner". Every tuning circuit has a phase characteristic that changes with frequency. By slightly changing the capacitance, you also change the phase shifts of the various frequency components of the signal. Human hearing is sensitive to the phase relationships between tones, although we are not aware of it. This change in phase can be perceived as a change in the "color" or "spatiality" of the sound. Since the tuning circuit is not an isolated block, it is interconnected with the internal circuits of the radio. When we tune this circuit, we change the impedances and couplings in this entire assembly. This also affects how wideband or narrowband the entire set receives the radio signal.

The tuning capacitor in the circuit also affects the tuning when changing some musical styles in the station's broadcast. It sometimes responds better than the bass and treble settings on the radio itself. Strong shortwave stations can "overflow" into neighboring frequencies (so-called "splatter"). This overflow often contains unwanted bass or treble components. When you "push" the sharpness of the resonance with your tuning, you suppress these interference components, which cleans up the midrange and the music sounds "cleaner". Each tuning circuit has a phase characteristic that changes with frequency. By slightly changing the capacitance, you also change the phase shifts of the various frequency components of the signal. Human hearing is sensitive to the phase relationships between tones, although we are not aware of it. This change in phase can be perceived as a change in the "color" or "spatiality" of the sound.

Tone correction (bass/treble) in a radio works with the audio signal after demodulation. Capacitor tuning works with the undemodulated high-frequency signal. You are influencing the very "raw material" that the receiver receives for processing. It's like changing the lens of a camera before you take a picture. This way you can remove defects that cannot be corrected in the demodulated signal.

Similar circuits were used in Czechoslovakia during the Cold War to listen to Radio Luxemburg and other Western stations.

The open source application deepseek helped me put together the design of the tuning circuit.