Hello, how can I replace a variable capacitor with a varicap

[Merlin3189]

This is his circuit

The two coils are coupled - ie. close together on the same axis. I suppose you'd call the 25 turn coil the primary and the 90 turn coil the secondary, since that is the direction of the signal path and power flow, but it does not have any significance IMO.
The 90 turn coil is the tuning coil equivalent to the 200 μH inductance in your circuit. The 25 t coil is to couple the signal from the antenna into the tuned circuit.

I'm not sure what you are asking here, "what doe mean, when they say primary coil 200micro, and what about secondary? they don't say nothing about secondary?" As far as I can see, your first diagram makes no reference at all to primary nor secondary and it is IMO immaterial anyway. In the video of the crystal set, he talks about the primary and secondary, but does not mention any inductance value - he is just working by guesswork (probably well-informed guesswork).

I think it is unhelpful to worry about primary and secondary here. You are not designing a transformer and probably don't have enough information to do so.
The large coil - the 200 μH in the first circuit, the 90 turns in the second circuit - is the important one. It needs to have the correct inductance to match the tuning capacitor for the frequency you want to receive. 200 μH with 500 pF resonates about 500 kHz, the low frequency end of the medium wave band. As the capacitance is reduced, the resonant frequency increases to about 1.6 MHz at the high frequency end of MW band when the capacitance is down to 50 pF.

The smaller coil could possibly be calculated, but it depends on the aerial used and the construction of the main coil and position of the small coil. The 4-5 turns of the first circuit and the 25 turns of the crystal set are probably educated guesses or trial and error values. The first circuit appears to have a much higher turns ratio than the crystal set, which will reduce the aerial loading on the tuned circuit and give a higher Q to sharpen the tuning. I'm not sure why the crystal set has a lower ratio,

Aside from your question, I would very much agree with Baluncore in #33. Get the straightforward circuit working, before you start the modifications.
Stay with making your own coil: it will be much better than the inductor designed for blocking or damping RF and it is really quite easy to do. If you don't get exactly the right inductance, that won't matter much: you'll just get a different tuning range. (And you can then have another go at winding a coil, using the same wire and tube knowing proportionally how many turns more or less you need. Or if the coil is air cored, you may simply be able to add a core.)

 

[michael1978]

This is his circuit
View attachment 221964
The two coils are coupled - ie. close together on the same axis. I suppose you'd call the 25 turn coil the primary and the 90 turn coil the secondary, since that is the direction of the signal path and power flow, but it does not have any significance IMO.
The 90 turn coil is the tuning coil equivalent to the 200 μH inductance in your circuit. The 25 t coil is to couple the signal from the antenna into the tuned circuit.

I'm not sure what you are asking here, "what doe mean, when they say primary coil 200micro, and what about secondary? they don't say nothing about secondary?" As far as I can see, your first diagram makes no reference at all to primary nor secondary and it is IMO immaterial anyway. In the video of the crystal set, he talks about the primary and secondary, but does not mention any inductance value - he is just working by guesswork (probably well-informed guesswork).

I think it is unhelpful to worry about primary and secondary here. You are not designing a transformer and probably don't have enough information to do so.
The large coil - the 200 μH in the first circuit, the 90 turns in the second circuit - is the important one. It needs to have the correct inductance to match the tuning capacitor for the frequency you want to receive. 200 μH with 500 pF resonates about 500 kHz, the low frequency end of the medium wave band. As the capacitance is reduced, the resonant frequency increases to about 1.6 MHz at the high frequency end of MW band when the capacitance is down to 50 pF.

The smaller coil could possibly be calculated, but it depends on the aerial used and the construction of the main coil and position of the small coil. The 4-5 turns of the first circuit and the 25 turns of the crystal set are probably educated guesses or trial and error values. The first circuit appears to have a much higher turns ratio than the crystal set, which will reduce the aerial loading on the tuned circuit and give a higher Q to sharpen the tuning. I'm not sure why the crystal set has a lower ratio,

Aside from your question, I would very much agree with Baluncore in #33. Get the straightforward circuit working, before you start the modifications.
Stay with making your own coil: it will be much better than the inductor designed for blocking or damping RF and it is really quite easy to do. If you don't get exactly the right inductance, that won't matter much: you'll just get a different tuning range. (And you can then have another go at winding a coil, using the same wire and tube knowing proportionally how many turns more or less you need. Or if the coil is air cored, you may simply be able to add a core.)

Merlin is very good you learn also to make a variable capacitor en inductor, thank you a lot, and for antenna primary coil and they don't tell about secondary this site am receiver, normal when you have a time http://electronics-diy.com/electronic_schematic.php?id=979

Parts List for Varactor Crystal Detector Radio:

L1 - Primary, Antenna coil 1 - 100 uH <---------
L2 - Primary, Antenna coil 2 - 100 uH <----------
C1 - Antenna Capacitor - 500 pF, 200 VDC
C2 - Capacitor - 0.1 uF 50 VDC
C3 - Capacitor - 0.01 uF, Disc, 10 VDC
C4 - 1 nF, Disc, 10 VDC
C5 - 250pF, Band Extender Capacitor, Mica, 50 VDC
D1 - Diode - Germanium, point contact
R1 - Potentiometer - 10 kOhm
R2 - Resistor - 1 MOhm, 0.25 Watt, CC
V - Varactor Diode - Motorola MV1662, 250 pF
B - Battery 9 Volt - Transistor carbon zinc is fine
SW1 - SPST Switch, Band Extender
Magnetic headphone, or high impedance speaker

 

[Merlin3189]

The first crystal set is interesting - eg. I haven't seen that construction for a variable capacitor before. But I'm bot sure the result will be very good.
If I wanted a variable inductor, I think I'd either use an adjustable core or use a variometer design. The variable capacitor design I might consider, if I used copper foil and soldered the wires. I'd like to use a thinner dielectric, perhaps a rolled up sheet of plastic.

and for antenna primary coil and they don't tell about secondary this site am receiver, normal when you have a time http://electronics-diy.com/electronic_schematic.php?id=979

As I said, I don't like the transformer analogy for these circuits. But here it would be an autotransformer. The primary is the two coils in series and the secondary is the single coil where the crystal is attached. The primary here is the tuning inductor and the secondary is to provide a lower impedance output for the crystal.
Again, an alternative way of attaching the aerial would be another coil wound over the top of either section.

This looks an interesting circuit. And you could add a transistor stage rather like your first circuit.
I used a crystal set for several years as a boy, but was very impressed by the improvement when I added a transistor stage. Not so much the increase in volume, but the great improvement in tuning selectivity, because it reduced the loading on the tuned circuit.

 

[Baluncore]

In post #36 the 90 turn tuned secondary winding is shown with a ground connection and a tuning bar. If a tuning bar is used, the ground connection should be removed. That is because the turns between the ground end and the tuning bar, will form a short circuited section that will waste energy.

That is true of all inductors and transformers. Shorted turns are bad news.

 

[michael1978]

In post #36 the 90 turn tuned secondary winding is shown with a ground connection and a tuning bar. If a tuning bar is used, the ground connection should be removed. That is because the turns between the ground end and the tuning bar, will form a short circuited section that will waste energy.

That is true of all inductors and transformers. Shorted turns are bad news.

thanks, and where do you have to connect the secondary right side? where to connect?

have a nice wekend.

 

[michael1978]

nice wekend to everybody here.

 

[Averagesupernova]

In post #36 the 90 turn tuned secondary winding is shown with a ground connection and a tuning bar. If a tuning bar is used, the ground connection should be removed. That is because the turns between the ground end and the tuning bar, will form a short circuited section that will waste energy.

That is true of all inductors and transformers. Shorted turns are bad news.

How often though do we see shorted turns in coils similar to what you describe? A quick google turned up what I recall seeing in many antenna tuners/transmatches. I've always questioned it but usually just left stuff alone when fiddling with this equipment.
https://www.google.com/search?q=mfj...gB&biw=1280&bih=623#imgrc=_&spf=1521223166418
Linear amplifiers with tubes will do the same thing on the output matching network.

 

[michael1978]

How often though do we see shorted turns in coils similar to what you describe? A quick google turned up what I recall seeing in many antenna tuners/transmatches. I've always questioned it but usually just left stuff alone when fiddling with this equipment.
https://www.google.com/search?q=mfj...gB&biw=1280&bih=623#imgrc=_&spf=1521223166418
Linear amplifiers with tubes will do the same thing on the output matching network.

i don't understand, that video which show merlin, why is not good, when in the end the crystal set work, i watch all the video

 

[michael1978]

This is his circuit
View attachment 221964
The two coils are coupled - ie. close together on the same axis. I suppose you'd call the 25 turn coil the primary and the 90 turn coil the secondary, since that is the direction of the signal path and power flow, but it does not have any significance IMO.
The 90 turn coil is the tuning coil equivalent to the 200 μH inductance in your circuit. The 25 t coil is to couple the signal from the antenna into the tuned circuit.

I'm not sure what you are asking here, "what doe mean, when they say primary coil 200micro, and what about secondary? they don't say nothing about secondary?" As far as I can see, your first diagram makes no reference at all to primary nor secondary and it is IMO immaterial anyway. In the video of the crystal set, he talks about the primary and secondary, but does not mention any inductance value - he is just working by guesswork (probably well-informed guesswork).

I think it is unhelpful to worry about primary and secondary here. You are not designing a transformer and probably don't have enough information to do so.
The large coil - the 200 μH in the first circuit, the 90 turns in the second circuit - is the important one. It needs to have the correct inductance to match the tuning capacitor for the frequency you want to receive. 200 μH with 500 pF resonates about 500 kHz, the low frequency end of the medium wave band. As the capacitance is reduced, the resonant frequency increases to about 1.6 MHz at the high frequency end of MW band when the capacitance is down to 50 pF.

The smaller coil could possibly be calculated, but it depends on the aerial used and the construction of the main coil and position of the small coil. The 4-5 turns of the first circuit and the 25 turns of the crystal set are probably educated guesses or trial and error values. The first circuit appears to have a much higher turns ratio than the crystal set, which will reduce the aerial loading on the tuned circuit and give a higher Q to sharpen the tuning. I'm not sure why the crystal set has a lower ratio,

Aside from your question, I would very much agree with Baluncore in #33. Get the straightforward circuit working, before you start the modifications.
Stay with making your own coil: it will be much better than the inductor designed for blocking or damping RF and it is really quite easy to do. If you don't get exactly the right inductance, that won't matter much: you'll just get a different tuning range. (And you can then have another go at winding a coil, using the same wire and tube knowing proportionally how many turns more or less you need. Or if the coil is air cored, you may simply be able to add a core.)

sorry Merlin, what's wrong with that transformer?, how do i have to connect, can you tell me please

 

[Baluncore]

thanks, and where do you have to connect the secondary right side?

The end of the coil is simply left open circuit. The inductance is determined by the square of the number of turns that carry current. Those turns that are not connected have RF voltage but no RF current.

How often though do we see shorted turns in coils similar to what you describe? A quick google turned up what I recall seeing in many antenna tuners/transmatches. I've always questioned it but usually just left stuff alone when fiddling with this equipment.

One problem with switched multi-band tuners is that with RF, everything couples to everything else. It is standard practice to short circuit the tuned circuits for all unused bands. That prevents harmonic resonance of tuned circuits that might radiate outside the selected band, or upset the SWR reflectometer metering circuits.

 

[michael1978]

The end of the coil is simply left open circuit. The inductance is determined by the square of the number of turns that carry current. Those turns that are not connected have RF voltage but no RF current.One problem with switched multi-band tuners is that with RF, everything couples to everything else. It is standard practice to short circuit the tuned circuits for all unused bands. That prevents harmonic resonance of tuned circuits that might radiate outside the selected band, or upset the SWR reflectometer metering circuits.

Ah now i understand the lefte side of secondary you connect antenna and the right side you juse left the wire without connection, thanks.

 

[Merlin3189]

sorry Merlin, what's wrong with that transformer?, how do i have to connect, can you tell me please

Sorry Michael, I don't understand what you are asking here.

In all the circuits you need a coil with the right inductance (number of turns & size) to resonate with the capacitor at the frequency you want.

Any other coils - which are indeed wrapped round the same core, like a transformer - are there either to couple the antenna signal into the tuned circuit, or to couple the signal out of the tuned circuit into a detector or amplifier. To choose them or design them, we are thinking of impedance matching, with the dual aims (sometimes contradictory) of getting good signal transfer and avoiding loading the tuned circuit too much (or loading it just enough.)

Since the aerial impedance and the amplifier/detector impedance are usually complex (reactive) and often unknown, choosing the number of turns for these windings is often a matter of guesswork, experience and trial and error. (Well it was, and still is for me: some people use simulation software to work it out these days.)

 

[michael1978]

Sorry Michael, I don't understand what you are asking here.

In all the circuits you need a coil with the right inductance (number of turns & size) to resonate with the capacitor at the frequency you want.

Any other coils - which are indeed wrapped round the same core, like a transformer - are there either to couple the antenna signal into the tuned circuit, or to couple the signal out of the tuned circuit into a detector or amplifier. To choose them or design them, we are thinking of impedance matching, with the dual aims (sometimes contradictory) of getting good signal transfer and avoiding loading the tuned circuit too much (or loading it just enough.)

Since the aerial impedance and the amplifier/detector impedance are usually complex (reactive) and often unknown, choosing the number of turns for these windings is often a matter of guesswork, experience and trial and error. (Well it was, and still is for me: some people use simulation software to work it out these days.)

in post 36, you show me a picture how to connect a coil, so everything was good, but the right side of secondary was connected to the ground, so
mr Baluncore said (
In post #36 the 90 turn tuned secondary winding is shown with a ground connection and a tuning bar. If a tuning bar is used, the ground connection should be removed. That is because the turns between the ground end and the tuning bar, will form a short circuited section that will waste energy.

That is true of all inductors and transformers. Shorted turns are bad news.)

so that mean the right wire of secondary you don't have to connect to nothing, is that correct?

 

[Merlin3189]

Yes. I would agree with Baluncore: shorted turns seems a bad idea to me too. I just hadn't noticed it.
But you may like to look at this explanation why shorted turns do not matter in RF coils by AB2EZ. I may have to rethink my position! And Averagesupernova's link shows many examples where professional designs do short turns.

Yes, I think the right end of the coil can be left open. You can try it both ways and discover any effect. Let us know.

 

[michael1978]

Yes. I would agree with Baluncore: shorted turns seems a bad idea to me too. I just hadn't noticed it.
But you may like to look at this explanation why shorted turns do not matter in RF coils by AB2EZ. I may have to rethink my position! And Averagesupernova's link shows many examples where professional designs do short turns.

Yes, I think the right end of the coil can be left open. You can try it both ways and discover any effect. Let us know.

Thanks...