MFJ MFJ-9219 - The Wheatstone Bridge; The MFJ-9219

MFJ-9219 QRP Tuner, Wattmeter, & Dummy Load Instruction & Technical Manual
Figure 3: The MFJ-9219
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-9219.
The circuit used in the MFJ-9218 is known as a Wheatstone bridge and is shown in Figure 4a. 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
variants exist, but the one used in the MFJ-9218 can be simplified to the circuit shown in
Figure 4b. The Wheatstone bridge works by measuring the current flowing between points B and
D (I
BD
). It is considered balanced when I
BD
= 0, or due to Ohm’s Law, V
BD
= 0. In that case,
R
x
=
R
2
R
3
R
1
. If we define R
1
= R
2
= R
3
≡ 50Ω, then the value of R
x
that will balance the bridge is
also 50Ω.
This tells us that in the case of the MFJ-9219 and others like it, any imbalance (i.e. current flowing
between points) B and D will occur only if R
x
6= 50Ω. Assuming that the resistance of the ammeter
-3-