MFJ-9218
SWR Meter, Wattmeter, & Dummy Load
Instruction & Technical Manual
wave transmitted by a 2-meter VHF station has a frequency of about 145 MHz. Dividing the speed
of light (
∼
3
×
10
8
m/s) by the frequency in Hz, we find that the wavelength in atmosphere of the
signal from our station is 2.06 m. Since wavelength and frequency are reciprocal, an increase in
wavelength corresponds to a decrease in frequency, hence, the 160 m band has a rather low
frequency of 1.8 MHz.
The electromagnetic spectrum (Figure 2) includes all of the various energies of electromagnetic
radiation ranging from extremely low frequency (ELF) ranges (with very long wavelengths) to all
the way up to x rays and
γ
rays, which have very high frequencies and correspondingly short
wavelengths. In between these extremes lie radio waves, microwaves, infrared radiation, visible
light, ultraviolet radiation, and the entirety of the FCC spectrum allocation chart. The RF part of
the electromagnetic spectrum is generally defined as that part of the spectrum from about 3 kHz
to 300 GHz.
Figure 2: The Electromagnetic Spectrum
2
THE MFJ-9218
2.1
INTRODUCTION
The MFJ-9218 is a simple resistive QRP SWR Meter, Wattmeter, and Dummy Load. It is
designed to be used with QRP transmitters (
<
5W) but can handle up to 20W for short periods.
The MFJ-9218 uses 100Ω 2W surface-mount resistors in the network will work well up to 54MHz
with a very low insertion SWR. The power measurement ranges are set by the METER SET pot
and the reference marks on the front panel. These ranges are 0.5W, 1W, 5W, and 20W. Note these
marks are only for reference and the power readings are only approximate.
2.2
THE WHEATSTONE BRIDGE
This section is solely for the enjoyment of those who wish to learn some of the theory behind
amateur radio. Knowledge or understanding of it is not required to operate and enjoy the
MFJ-9218.
The circuit used in the MFJ-9218 is known as a Wheatstone bridge and is shown in Figure 3a. The
Wheatstone bridge is a simple circuit that was originally designed to determine an unknown
resistance in a DC circuit but also works to determine unknown impedances in RF circuits. Several
-2-
Содержание MFJ-9218
Страница 10: ...USER NOTES...