MFJ-949E Capacitance and Inductance Settings

Like many amateur radio operators, I use an "Antenna Tuner" to create a good match between my HF transceiver and the feeder and antenna. Really, the best place for a matching device is at the terminals of the antenna, because this ensures a low VSWR on the feeder cable, and not just the output of the transmitter.
However, most of us find it much more convenient to have the tuner alongside the radio, and tolerate the additional loss which having a high VSWR on the feeder causes.

The tuner that I use is a manual one, an MFJ-949E. I have had it for many years and it seems reliable and well made. It has a number of useful features such as a built-in VSWR meter, and a dummy load, but the part which creates the good match between the radio and the antenna system consists of just three components. MFJ provide a circuit diagram in the back of the manual.

The circuit is a "T" filter, with two variable capacitors and a switched inductor. The design has varied a little bit over the years, and I have seen an older version which had a kind of sliding wiper on the inductor to vary its value.

The MFJ circuit diagram shows C1 and C2 are 208pf, but no values are given for the inductor. I thought it would be nice to measure the values of the capacitors and inductors at each position, because this would allow for some analysis of the antenna impedance. The impedance of the matching device should be the "complex conjugate" of the antenna impedance. Which basically means the reactance will be equal and opposite. But the situation is made more complex as the length of the feed line will transform the values. A 1/4 wave line will make the values opposite, and after a half-wave they will be the same again, repeating along the length of the line. If you want to know more about this look up the "Smith Chart" which allows this transforming effect to be calculated easily.

Transmitter / Antenna Control	Capacitance Value (pF)	Inductor Control Position	Inductance Value (μH)
0	221.6	A	31.91
1	202.5	B	15.52
2	180.7	C	9.888
3	163.6	D	6.843
4	141.2	E	4.690
5	119.9	F	2.835
6	101.7	G	2.015
7	80.0	H	1.331
8	60.5	I	0.744
9	39.2	K	0.329
10	26.3	L	0.120

The Transmitter and Antenna capacitors C1 and C2 appear to be identical. The controls are marked 0 to 10, but lining up the dot on the knob with the number is rather approximate.

The table shows that the capacitors have a linear law, and that there is about 20pF between each step from 220 down to 20pF. The inductor is a bit more "logarithmic" in its steps, but the switch makes the setting precise and repeatable.
I find it counter-intuitive that the capacitor numbers and inductor letters get larger as the values get smaller.

I measured these values on a DER EE,  LCR meter DE-5000, with the measurement frequency set to 100kHz.

73
Hugh M0WYE