For simple receiving of radio signals, the electrical characteristics are only marginally important. This is why a modern car’s FM antenna can be smaller than ¼ wavelength vertical, instead appearing as a mere nub on the car’s roof, and still function. The end user probably notices no major inefficiencies even if the antenna is not at a resonant frequency of the incoming signal.

However, antennas that must carry transmitted signals must be designed near a resonant frequency. If they are not, this is known as an impedance mismatch, and some of the transmitted signal is reflected back into the transmitter. Not only is this inefficient, it can actually damage the transmitter.

## Impedance mismatch

In order for there to be efficient transfer of power through a transmission system (transmitter, feed line and antenna), each component of the system must have a matching impedance. Impedance is basically the frequency-dependent version of resistance, where the impedance takes into account actual resistance and the reactance generated through capacitive and inductive effects.

If the impedance does not match, some of the signal is reflected away at the connection point. Instead of this power being used to transmit the intended signal, it is lost, and may even damage the transmitter. Most modern transmitters will detect this incoming signal and scale back the power to reduce the reflection and prevent damage, but this should not be relied upon. In either case, the signal is still being lost instead of transmitted.

Perfect matches in impedance are difficult to achieve naturally. Antenna geometries have different characteristic impedances, and most transmitters and feed lines have fixed impedance. Therefore, there needs to be a way to match the impedance of the antenna to the rest of the transmitter system. This is the main purpose of an antenna tuner, which adds capacitive and inductive elements to adjust the reactance, changing the impedance of the system.

## Antenna tuners

Amateur radio operators and other multiband or multi-frequency radio operators often need a way to adjust the resonant frequency of their antenna so that they can transmit from a range of frequencies. This is particularly true when the radio operator may only have one antenna that is expected to cover a wide range of frequencies.

An antenna tuner is a device that adds inductance and capacitance to the feedline to make the antenna look electrically different. More specifically, the tuner reduces the reactive impedance, making the antenna look like a purely resistive load. This changes the resonant frequency of the antenna and the feedline, which reduces the reflected signals back into the transmitter, protecting it and increasing the overall efficiency of the system.

This is not a magic box; some energy is lost in the antenna tuner. The tuner does not actually tune anything, it adds components to the system, increasing its complexity and the amount of time spent troubleshooting, if there are transmitting issues.

## Example: A teardown of an MFJ-941C antenna tuner

The MFJ-941C Antenna Tuner was designed to match antennas for amateur radio operators. In particular, it is for the 10 meter, 15 meter, 20 meter, 40 meter and 80 meter (28 MHz, 21 MHz, 14 MHz, 7 MHz, and 3.5 MHz, respectively) amateur radio bands.

The user has a number of controls to adjust to tune the antenna. There are two knobs at opposite ends of the tuner labeled “Transmitter” and “Antenna,” as well as a knob to the right of the meter labeled “Inductance.” These are the primary controls for tuning the antenna. There is a knob to the left of the meter labeled “Sens/Power,” where the user can adjust the sensitivity of the meter to correspond to how much output power is being transmitted. This meter will handle up to 300 W.

The meter in the center can be used to show the output power through the tuner or used to show the reflected power back into the tuner, depending on the position of the button labeled “FWD REF,” standing for “forward” or “reflected” power.

The front panel of the MFJ-941C. Source: Seth Price

The back panel has multiple connection points for different antenna feedlines. This particular model can be used with two coaxial cables, a balanced line or one unbalanced wire. While it cannot handle tuning them all at the same time, the user can turn a large knob to select which antenna they want to tune. There is also a “bypass” option for when the antenna is a good electrical match and the tuner is unnecessary.

The back panel of the MFJ-941C. Source: Seth Price

Under the cover, the components are quite simple, though they are large. There are two large air capacitors, a large air inductor and a bird’s nest of wires connecting the antenna switch and the different feedlines to the rest of the tuner.

Attached to the “Transmitter” knob and the “Antenna” knob are large air capacitors. As the knob turns, a series of plates overlap. When calculating capacitance, one of the variables is the amount of surface area that overlaps, so as the knob moves the plates, the capacitance changes.

The large air capacitor is shown on the left, with the plates overlapping nearly completely. Source: Seth Price

Connected to the “Inductance” knob is a large open-air inductor. Whereas the “Transmitter” knob and the “Antenna” knob are continuous, the “Inductance” knob is discrete, with multiple positions available. With the cover off, it is clear that each of those positions corresponds to a different tap on the large inductor.

A closeup of the large inductor. Notice how each position on the back of the knob corresponds to a different contact point on the inductor. Source: Seth Price

## Final thoughts

Antenna tuners represent a compromise; they help make an antenna work across a wider bandwidth of frequencies while trading off some efficiency and complexity. For most radio operators, this is a welcome trade, allowing them to operate across a wider selection of frequencies without the need to build and place additional antennas.