MFJ-949B Antenna TunerMeasuring the MFJ-949B Antenna Tuner

Determining the Capacitance and Inductance Ranges of this T-Network Antenna Tuning Unit

by Dr. Carol F. Milazzo, KP4MD (posted 18 February 2011)
E-mail: kp4md@arrl.net

The MFJ-949 Deluxe Versa Tuner II is a popular T-network antenna tuning unit used by many amateur radio operators.  It is rated for 300 watt operation over the range 1.8 to 30 MHz and includes a non-inductive 50 ohm dummy load, a standing wave ratio bridge, an internal 4:1 voltage balun, and can select between two SO-239 coaxial outputs and a balanced line or wire antenna.  Despite several revisions to the associated circuitry that have occurred since 1979, the tuner's core functional design has remained a T-network composed of two 208 pF variable capacitors and an air core inductor with 12 switch-selectable taps (Figs. 1 and 2).

MFJ-949 Schematic Diagram
MFJ-949E Schematic Diagram
Fig. 1.  MFJ-949B Schematic Diagram (1979)
Fig. 2.  MFJ-949E Schematic Diagram (2011)

I have been recently been evaluating antenna system parameters and wished to use tools such as Schmidt's "T-Network Tuner Simulator"1 and Duffy's "T match efficiency estimator,"2  For this I needed to determine the values of inductance and capacitance that the tuner dial settings represented.  To accomplish this I connected an MFJ-202B noise bridge to the tuner's "Coax 1" connector with a 2 foot RG-58/U jumper cable.  With the plates of the "Transmitter" capacitor fully opened to minimize the loading effect of the circuitry on the transmitter side of the network coil, Table 1 lists the measured resonant frequencies with each setting of the Inductor switch and with the "Antenna" capacitor set at "0" on the dial (maximum capacitance or nominally 208 pF), at "3" on the dial (plates meshed half way), and at "6" on the dial (minimum capacitance).  The inductance value for each setting of the inductor switch was calculated from its resonant frequency with the capacitor set to 208 pF.  For each of these inductance values, the capacitance values with the Antenna capacitor at the minimum and ½ meshed settings were calculated from the measured resonant frequencies.  The mean of these values was then calculated for each capacitor setting.  All these figures are listed in Table 2.

Inductor
Dial
Resonant Frequencies kHz
Capacitor
Dial "0"
208 pF
Capacitor
Dial "3"
½ meshed
Capacitor
Dial "6"
min. pF
A
11480
15350
27250
B
10370
14000
27100
C
8950
12130
24100
D
7500
10370
20880
E
6680
8950
17750
F
5860
7750
15270
G
4700
6240
11840
H
3930
5200
9700
I
3410
4490
8275
J
3020
3980
7250
K
2730
3580
6490
L
1930
2510
4460
Table 1.  Measured resonant frequencies
 
Inductor Capacitor
Dial "0"
Capacitor
Dial "3"
Capacitor
Dial "6"
Dial Calc.
µH
Res. Freq.
kHz
pF Res. Freq.
kHz
Calc.
pF
Res. Freq.
kHz
Calc.
pF
A
0.92
11480
208
15350
116.3
27250
36.9
B
1.13
10370
208
14000
114.1
27100
30.5
C
1.52
8950
208
12130
113.2
24100
28.7
D
2.16
7500
208
10370
108.8
20880
26.8
E
2.73
6680
208
8950
115.9
17750
29.5
F
3.55
5860
208
7750
118.9
15270
30.6
G
5.51
4700
208
6240
118.0
11840
32.8
H
7.88
3930
208
5200
118.8
9700
34.1
I
10.47
3410
208
4490
120.0
8275
35.3
J
13.35
3020
208
3980
119.8
7250
36.1
K
16.34
2730
208
3580
121.0
6490
36.8
L
32.69
1930
208
2510
123.0
4460
39.0
Mean Calc.
Cap. values
Mean
Dial "0" pF
208 Mean 
Dial "3" pF
117.3 Mean
Dial "6" pF
33.1
Table 2.  Calculated inductance and capacitance values

Linear regression of the mean calculated capacitance values produced the following formulas relating capacitor dial settings and their equivalent capacitances:

C(pF) = 208-29.25*Dial  and   Dial = (208-C(pF))/29.25

These values were used to produce Table 3 for dial settings of the inductor and capacitors with their equivalent values of inductance and capacitance.  For comparison, Table 4 lists Duffy's calculations for the equivalent values of inductance and the input capacitance for the MFJ-949E antenna tuning unit.  In the MFJ-949E, the inductor switch is labeled in an order reverse to that of the MFJ-949B, and the capacitor dial range is marked from 0 to 10 instead of from 0 to 6.  Also, the current study measured the output or "Antenna" capacitor while Duffy measured the input capacitor.

Inductor  Capacitor 
Dial µH Dial pF
A
0.9
0
208
B
1.1
0.5
193
C
1.5
1
179
D
2.2
1.5
164
E
2.7
2
150
F
3.5
2.5
135
G
5.5
3
121
H
7.9
3.5
106
I
10.5
4
91
J
13.4
4.5
77
K
16.3
5
62
L
32.7
5.5
48
 
6
33
Table 3. Dial settings and equivalent values
for MFJ-949B
 
Inductor Input
Capacitor
Dial
µH
Dial
pF
A
26
0
228
B
17
1
208
C
14
2
188
D
11
3
168
E
8.3
4
148
F
6.0
5
128
G
3.9
6
108
H
3.0
7
88
I
2.1
8
68
J
1.4
9
48
K
0.86
10
28
L
0.47
Table 4. Dial settings and equivalent values for MFJ-949E by
Duffy, VK1OD
The accuracy of the measurements made on the MFJ-949B antenna tuner is dependent on the assumption that the maximum capacitance is 208 pF as stated by the manufacturer, and the assumption that when set to minimum, the "Transmitter" capacitor and other stray capacitance do not add significant shunting capacitance across the inductor.


REFERENCES

  1. "T-Network Tuner Simulator," Schmidt, K, W9CF
  2. "T match efficiency estimator," Duffy, O, VK1OD
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