MFJ-976 Legal Limit Fully Balanced Antenna Tuner Instruction Manual
3
Peak Reading SWR/Wattmeter
The cross-needle meter measures the peak or average FORWARD power, REFLECTED power, and SWR, and is
always operating since the tuner is always in line.
The meter's full scale forward and reflected power range is controlled by the POWER switch that selects HI
(1500 watts) or LO (300 watts). If your transmitter or amplifier runs more than 300 watts of output power, set this
switch to the 300 watt HI (in) position. If your transmitter or amplifier has less than 300 watts of output, set this
switch to the 300 watt LO (out) switch position.
Peak envelope power (PEP) is measured when the PEAK or AVG power push button is placed in the PEAK (in)
position. Peak power and average power values are equal with steady un-modulated carriers, FSK, or FM. The
meter reading on these modes will be the same whether the PEAK/AVG button is in or out. On SSB, the PEP
meter reading should be twice the average power with two-tone test modulation.
On SSB, the ratio of PEP to average power varies with voice characteristics. With most voices, the PEP reading is
three to five times higher than the average voice power reading. The most accurate peak envelope power readings
are obtained only with sustained carrier, voice or two tone test modulation. During normal voice modulation the
wattmeter will typically indicate only 70% of the true peak envelope power.
Forward power is displayed on the left-hand FORWARD meter scale. This scale is calibrated from 0 to 300
watts. In the HI position, each picket (scale mark) represents 5 watts below 10 watts, 10 watts between 10 and
100 watts, and 25 watts between 100 and 300 watts. In the LO power position, full-scale meter reading is 30
watts; divide the meter reading by 10 for the correct value.
Reflected power is read on the right-hand REFLECTED meter scale. This scale indicates 60 watts full scale when
the 300W power sensitivity is selected, and 6 watts full scale when the 30W power scale is selected. This scale
has a picket every watt below 10 watts and every 5 watts above 10 watts.
The most accurate power readings occur in the upper half of the meter scales. When trying to measure power
with a less than perfect match, the reflected power should be subtracted from the forward power reading.
The SWR is read directly from eleven red SWR curves that range from 1:1 to infinity. SWR is measured by
observing the point where the forward and reflected power needles cross. The SWR is indicated by the red curve
closest to the needle crossing point. No cumbersome or time consuming SWR sensitivity adjustments are
required with this meter.
The wattmeter has an internal lamp that backlights the meter scale. The lamp circuit requires power from an
external 12 Vdc source, such as the optional MFJ-1312D power supply. The rear panel jack accepts a 2.1 mm
coaxial plug with the center conductor positive (+) and the sleeve negative (-). The negative lead is grounded
inside the tuner. The LAMP ON/OFF switch turns the meter lamp off and on.
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