Measuring
the MFJ949B Antenna Tuner
Determining the Capacitance and Inductance Ranges of this TNetwork
Antenna Tuning Unit
by Dr. Carol F. Milazzo, KP4MD (posted 18 February 2011)
Email: [email protected]
The MFJ949 Deluxe Versa Tuner II is a popular Tnetwork 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 noninductive 50
ohm dummy load, a standing wave ratio bridge, an internal 4:1 voltage balun,
and can select between two SO239 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 Tnetwork composed of two Oren Elliott 32APL208DE 15.9208 pF variable capacitors^{1} and an air core
inductor with 12 switchselectable taps (Figs. 1 and 2).


Fig. 1. MFJ949B Schematic Diagram (1979)

Fig. 2. MFJ949E Schematic Diagram (2011)

I have been recently been evaluating antenna system parameters and wished
to use tools such as Schmidt's "TNetwork
Tuner Simulator"^{2} and Duffy's "T
match efficiency estimator,"^{3} For this I needed to
determine the actual values of inductance and capacitance that the tuner dial
settings represented. To accomplish this I connected an MFJ202B
noise bridge to the tuner's "Coax 1" connector with a 2 foot RG58/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) = 20829.25*Dial and Dial = (208C(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 MFJ949E antenna tuning
unit. Note that similar to Duffy's finding, the 33 pF mean minimum capacitance
is about twice the 15.9 pF minimum stated in Oren Elliott's specifications for
the 32APL208DE variable capacitor, likely due to significant stray capacitance in
the tuner construction. In the MFJ949E, the inductor switch is labeled in an order
reverse to that of the MFJ949B, 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 MFJ949B


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
MFJ949E by
Duffy, VK1OD

The accuracy of the measurements made on the MFJ949B 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

"Oren Elliott
32APL208DE Capacitor Specifications"

"TNetwork Tuner
Simulator," Schmidt, K, W9CF

"T match efficiency estimator,"
Duffy, O, VK1OD
Return to KP4MD Home Page