Crystal radio question

[Dude111]

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[radio nut]

I uploaded them...hope you have better luck than me!

[radio nut]

568991232_1000000212

[radio nut]

[radio nut]

[old_tv_nut]

Here's a page with a diagram of antenna impedance vs. length that assumes a 0.1 wavelength wire antenna that gives an equivalent of 300 pF resonating with the coil.

https://electronics.stackexchange.co...ystal-radio-be

It seems in your case, you have a 15 foot optimum antenna length (about 0.01 wavelength), which would be about 30 pF, if I'm thinking about this right.

Suggested experiment: put a small capacitor (maybe 30 to 60 pf) in series with your antenna wire, and then lengthen the antenna and see if it works better.

[radio nut]

I will did through my tub of stuff and get the cap in question and try this out this weekend..

[old_tv_nut]

Don't be too dissapointed if my idea doeasn't work. It could be that you need a different coil to work with a longer antenna.

[radio nut]

It was kinda interesting...with the cap and a longer ant. the 2 local stations swapped positions. 940 am was on the left side of the coil and 1150 was in the middle. Now 1150 is towards the left and 940 is more in the middle. Funny!
I will try different ant. lengths and a variable cap and see what happens.

[old_tv_nut]

When you say towards the left, do you mean fewer windings in the circuit (left in image xxxxx213 that shows the physical picture)?

I think 1150 (higher frequency) to the left (less resonating coil) is the way it should be, so you are going in the right direction by adding the series cap.

I think having the station positions reversed initially was an indication that the antenna capacitance was loading the coil too much.

Antenna loading can be reduced by using a second, smaller coil for coupling the antenna to the main coil.

https://www.youtube.com/watch?v=mh5KxCpbQ5U

[radio nut]

Yes, fewer windings...I will check out the video you linked.

The local stations are only 2 or 3 miles away, if this makes a difference.

[radio nut]

I watched the video, if I saw it correctly there is not an ant. connection to the main coil.....how does that work?

[old_tv_nut]

Originally Posted by radio nut

I watched the video, if I saw it correctly there is not an ant. connection to the main coil.....how does that work?

The antenna tuning coil is inductively (magnetically) coupled to the main coil. Since the antenna coil is lower inductance, it is affected less by the antenna capacitance. Then the transformer coupling to the higher inductance means that the main coil sees, in effect, a higher impedance (lower capacitance) antenna load, and because of the turns ratio, also outputs a hgher voltage to the diode and earphone.

You could use a lower inductance main coil in the first place to make the antenna capacitance less of a problem, but that would reduce the voltage output to the diode and earphone.

[radio nut]

I am self taught....there is not an iron core in the video, so it is proximity of the 2 coils that causes induction of signal?

[old_tv_nut]

Originally Posted by radio nut

I am self taught....there is not an iron core in the video, so it is proximity of the 2 coils that causes induction of signal?

Yes - air coupled.