MFJ-998RT Automatic Antenna Tuner

The MFJ-998RT is an automatic remote antenna tuner rated at 1.5 kW. It is powered by 12VDC on the RF coaxial line. It is the remote version of the MFJ-998. Impedance conversion is done with an L network consisting of a series L and a shunt C. The shunt C can be placed before or after the inductor as required. The inductor consists of a series of binary weighted inductors that are shorted out by relays when not needed. The capacitor similarly consists of parallel connected binary weighted capacitors that are switched in and out of the circuit as needed (the ground side is switched by relays). A sampler combines a sample of the input RF voltage and current to determine the forward and reflected power at the input of the tuner. These samples go to analog inputs of the microcontroller which then adjusts the LC relays as required to minimize the SWR at the tuner input. In addition, an RF sample drives a microcontroller timer input to count the transmit frequency. If this frequency had been previously used, the LC settings previously used are set. If the input SWR is low with these settings, they are used. If the SWR is not low with these memorized settings, the tuner adjusts to find the optimum LC values.

The MFJ-998RT is mounted on a metal panel with a fiberglass weatherproof cover. Below is a photograph of the unit with the fiberglass cover removed.



Note that there is an internal display and switches similar to those on the
MFJ-998. These are not used in normal operation. In addition, there is a loudspeaker that sends QRP or QRO if the input power is too high or low for the tuner to change its settings. While the tuner can handle 1.5 kW, the relays cannot "hot switch" this power. Instead, the tuner adjusts at low power. When the user sees the SWR on the transmission line is low as indicated on a meter in the shack, the power can be increased. The photograph below shows a closeup of the display.

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