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This circuit is a part of a field strength meter circuit in an old magazine, the value of inductor J1 is unknown (due to a print error in the magazine.)

The circuit works like this:

With S1 open and S2A connecting the TR1 base, we take signal from the A2 antenna and through the DG1 we pass signal to TR1 base only on negative semi cycles. R1 has the function to "load" DG1, I think to bring the diode to the edge of breakdown voltage (more sensitive.) I think J1 provides a return path for current that is passed from R1 through DG1, and that J1 is needed to "return" only DC to the positive power rail blocking the RF.

Considering that this instrument works on the CB band (around 28MHz,) would it make sense that J1 has a value that represents a high impedance around this frequency?

If I'm interpreting the circuit in a wrong way please point me to the right direction.

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  • \$\begingroup\$ due to a print error in the magazine ... what is printed? ... are you certain that it is an error? \$\endgroup\$ Commented Sep 26, 2021 at 19:09
  • \$\begingroup\$ yes it is, because in the bill of component J1 is not listed but is present in schematic and in the board, in the circuit description of the magazine the J1 is not even mentioned. \$\endgroup\$ Commented Sep 26, 2021 at 19:11
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    \$\begingroup\$ label J1 implies a jumper ... maybe it is a long piece of wire that is somehow coupled to one of the other inductors \$\endgroup\$ Commented Sep 26, 2021 at 19:18
  • \$\begingroup\$ J1 have the universal symbol to designate an inductor.... like the coupling coil L2 on its left and the leftmost L1 main tuning coil. \$\endgroup\$ Commented Sep 26, 2021 at 19:20
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    \$\begingroup\$ J1 appears to be an RFC to allow DC path and block signal. So, yes, high impedance at the frequency seems good. \$\endgroup\$ Commented Sep 26, 2021 at 23:24

1 Answer 1

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The value of inductor J1 was identified by tests on a circuit prototype.

I have used values that I have available at the time considering the center of the freuency span that the receiver tank circuit have for this project (7-28MHz.)

With 690 uH, the instrument has no reaction to an incoming CH14 (27.125MHz) transmission.

With 1mH the intrument works as expected.

I will try to use some 3mH value for J1 to see if the instrument increases sensitivity.

The value of J1 must be at high Z at the freqeuncies of CB band. Russel McMahon gave the correct answer in the comments.

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