Do I need an antenna tuner?

qrpis4me,

When you feed a random length of wire at the end it is being voltage fed versus current fed and you will need an Impedance Matching Network (IMN) regardless of the amount of RF power transmitted, because the impedance of a random wire connected to your transmitters output will not provide an appropriate 50-Ohm load as required by the transmitter output circuitry which is universally 50-Ohm for communications equipment. To provide a 50-Ohm load for your Transmitter, Transceiver Transmitter stage or any external in-line RF Power Amplifier (either a Linear Class-'A'/'AB'/'B' or Non-Linear Class-'C' type), the use of an IMN to provide an appropriate conjugate match is necessary. This requirement is the same whether 50-Ohm or 75-Ohm coaxial cable RF feedline or 300-Ohm, 450-Ohm, 600-Ohm open wire RF feedline or random wire used as an active load (antenna) is connected directly to the IMN output connector.

An IMN is commonly called a “Transmatch”, “Antenna Coupler”, “Antenna System Tuner”, “Antenna Tuner” or just a “Tuner”. First of all, an IMN is normally located inside a building at the Amateur Radio Station operating position with the Active Antennas RF Feedline hooked to its output, so the name “Antenna Tuner” is inaccurate because it does not tune just the Antenna feedpoint. If it were hooked directly to the Active Antennas feedpoint it would be an “Antenna Tuner”. This is one piece of equipment that usually finds its way at one time or another into almost every Amateur Radio Station, which is operated on the Medium Frequency [MF] (160 Meters) and High Frequency [HF] (80 Meters - 10 Meters) Amateur Radio Service Bands.

An IMN is an Electrical Transformer that contains electrical components that can be adjusted to transform mismatches of impedances between the various different types of Active Antenna feedpoint impedances and the stations’ connecting RF Feedline characteristic impedance. These necessary adjustments will provide the stations’ transmitting and receiving equipment with a proper circuit impedance of 50-Ohms. Therefore it provides a proper conjugate match for the RF Feedline input impedance to a required circuit impedance of 50-Ohms.

Why use an IMN? Many Amateur Radio Stations have limited antenna choices, with many various feedpoint impedances and the need to operate on frequencies other than where the antenna is resonant, or use a non-resonant multiband antenna, all of which necessitate an IMN. The IMN is a necessary piece of equipment to correct any impedance mismatches between an Active Antennas feedpoint Impedance (Z) and the connecting RF Feedlines Characteristic Impedance (Zo). The impedance mismatches are caused when the Antenna’s feedpoint Impedance and the connecting RF Feedlines Characteristic Impedance are not of an equal value and do not provide a circuit impedance of 50Ohm. Some but not all modern transceivers provide as a standard or optional feature, an internal automatic IMN. These internal automatic IMNs can usually transform an impedance mismatch on a RF Feedline that has a Voltage Standing Wave Ratio (VSWR) of up to approximately a ratio of 3.0:1, but beyond that point either an external automatic or manual IMN will be needed. If you are using an active load (antenna) connected by a coaxial cable RF feedline and it has a good impedance match between the antennas feedpoint impedance and the RF feedlines characteristic impedance at the single resonant frequency and a reasonable impedance match within a bandwidth that maintains a 2.0:1 SWR on the feedline, than using a IMN is not necessary.

Best Regards Larry K4RFE (Retired RF Engineer)