Heathkit SB-Line Inductors

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During the process of re-engineering an SB-101 transceiver a series of measurements was conducted on both the Pi tank and miniature adjustable inductors used on each band in a number of SB-Line units and the results presented below.

With respect to the adjustable inductors, ‘Core in’ is the maximum inductance position and ‘Core out’ is with the outer end of the core level with the top of the former.

All measurements were taken on a Marconi TF-1245A Q-Meter with a Racal 9916 frequency counter and with each coil supported in free air and away from nearby metal.



Core Out

Core In

Inductance (uH)

Winding details


Heath Part No

Freq

MHz

Cap pF

Q

Cap pF

Q

Min

Max

Turns

Length (inches)

Comment

40_529

28

91

110

46

96

0.36

0.7

7

0.33


40_551

10

295

82

138

113

0.86

1.84

13

0.4


40_550

10

119

115

245

94

1.03

2.13

15

0.45


40_551

14

148

92

70

110

0.87

1.85

13

0.4


40_552

14

268

97

148

111

0.48

0.87

9

0.41


40_601

3.5

>500pF

90

280

90

N/A

7.38

26

0.45

40_764 is similar

40_765

7

213

106

89

112

2.43

5.81

21

0.38


40_767

21

188

93

99

112

0.31

0.58

6

0.29


40_768

28

125

108

60

120

0.26

0.54

5

0.25


40_811

14

131

108

57

112

0.99

2.27

13

0.26


40_832

18

187

89

88

91

0.42

0.89

8

0.25


The underscore in the part number should be read as a hyphen  i.e. 40_529 = 40-529.


The capacitance columns are those values required to resonate each inductor at the specified frequency for the respective core positions. Readers can calculate resonant frequencies with other capacity values as required.


All of these coils are wound on formers made of a relatively thin material with an outer diameter of 9/32 (0.28) inch, an adjustable dust iron core and a chassis mounting clip. Considerable care is required in moving the cores as they break very easily - the plastic hexagonal trimming tool must be a good fit in the core and not loose. Single winding coils have a pair of connection tags and those coils with an additional link winding have three connection tags.

SB-101 Pi Tank

Measurements taken without shorting out the unused sections of the inductor

Band MHz

Resonant freq MHz

With test capacitor pF

Measured Q

Calculated inductance uH

3.5

21.8

200

262

9.99

7

17.09

150

220

4.04

14

9.82

150

185

1.75

21

6.46

100

148

0.87

28

3.56

100

140

0.53

Shorting out the unused sections of the inductor as was done in the SB-101 PA circuit resulted in a reduction of about 5% in measured inductance and 8% in unloaded Q.