Welz SP-220 Power Meter
I have owned this
power meter for many years and find it to be accurate over a wide range of
frequencies. Although my SP-200 reads about 10% high on 144 MHz, it's
still a great meter for measuring at QRPP and QRP power levels from 1.8 to
200 MHz.
The meter includes a
facility to measure SWR directly, but I prefer to take forward and reverse
power measurements, and then calculate the SWR using the following formula:
where Pf is forward power,
and Pr is reverse power
This is how ithe
SP-220 was
packaged, and what it looks like out of the box.
Meter illumination is
provided via a 13.8 VDC lead that exits via the rear panel. Because I
don't use the internal lamp, I have
coiled the power lead up, and stowed it within the enclosure.
Meter illumination
Meter illumination, if
required, is provided via a 13.8 VDC lead that exits via the rear panel.
I don't use the illumination feature, I have coiled the DC lead up, and
stowed it within the enclosure.
Calibration
To calibrate this power
meter, I used the combined dummy load and power sensor described
here.
When calibrating the SP-220
power meter, the four adjustment potentiometers are accessed by removing the
lower part of the enclosure.
Balancing the Forward and
Reverse Detectors
The first adjustment to make is to set the balance between forward and
reverse parts of the sensor. This is done by momentarily
applying power to a dummy load via the power meter, and noting the Forward
power. Then swap the TX and ANT connections, and
switch the power meter to read Reverse power. Note that, when
applying power, the meter reading should be the same as previously noted.
If, however, adjustment is required, trim the reverse power potentiometer
(the one nearest the red lead from the sensor) until you obtain the same
power indication as previously noted.
Once you have done this correctly, you will have equalised the sensitivity
of the Forward and Reverse detectors. When done, restore the correct
TX and ANT connections
Main
calibration
There is a separate potentiometer
for adjusting each of the three power measurement ranges. Use a
calibrated power meter to check the calibration on each range, adjusting the
relevant potentiometer, if necessary.
Perform any such adjustments in the following order: 200 watt range, 20 watt
range, and, finally, the 2 watt range.
If your power source (or dummy load) is unable to generate (or dissipate)
200 watts, use the highest power level you can safely apply to the meter.
Calibration at a frequency between 14 and 28 MHz should ensure excellent
accuracy between 1.8 and 200 MHz, with spot checks being carried out at 1.8;
50; and 144 MHz.
SP220 Operation Manual
The Operation Manual for the SP-220 can be downloaded from
here.
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