Crystal Radio Magic

[jake]

Im Back!

So before I was freaked out earlier. I was tring to figure out the standing issue I have.
@ mad, I like it when you lazy.

So what I really wanted to ask was why is there only a partial wave along the coil at resonance? With a 1 turn primary I get 1/4 of a wave along the entire coil. Meaning the the can has highest "potential and the potential along the coductor goes down to nothing as you approach ground.

However a few days ago I had 1/2 wave at resonance when I was using my primary instead of a 1 turn copper strip. Meaning highest potential on terminal, the 5 turns on in the middle of the coil had nothing and voltage went back up(actually -) as you approached the ground.

I had just got my primary back on when. I pooped my pants.. Spent the rest of the day at FCC.. and wiki.

What did I learn there: 2mHz-3.5mHz is wierd and is allocated to maritime, ship to shore, and other , but the coast guard uses it as well and I can't forget 2.1735MHz mobile distress and calling frequency.. I was all over that one today. With the right termination/salad bowl. I learned I can keep it under 2MHz and still play as long as I keep it turned down all the way..

I used a FWBR made from SF35s and SF37s diodes. Across the DC of the FWBR I used a 100pF cap. Powering the function generator at 1.8MHz the max I could get was 90mA straight across a DMM out from the rectifier. How do I go from that to "milliwatts of DC input power to the final RF stage"?


time to do the pots.

[xee2]

and voltage went back up(actually -) as you approached the ground.

You must have had a bad ground connection. Ground should never have a voltage on it (it is the reference for all other voltages).

[xee2]

I used a FWBR made from SF35s and SF37s diodes. Across the DC of the FWBR I used a 100pF cap. Powering the function generator at 1.8MHz the max I could get was 90mA straight across a DMM out from the rectifier. How do I go from that to "milliwatts of DC input power to the final RF stage"?

You need to measure imput impedance of antenna to determine how much power is going into it. But it will look like like a small capacitor. To get power into the antenna you need to make a matching network to convert generator impedance to complex conjugate of antenna impedance. As is, 90% of power appllied to your wire antenna is being reflected back to generator (less than 10% is going into antenna). So the amount of power your antenna is readiating is very small.

[jake]

You must have had a bad ground connection. Ground should never have a voltage on it (it is the reference for all other voltages).

Awesome.. Thats why I went outside and soldered directly to decent "ground". Did not stay out long enough to prove it to myself.

[jake]

You need to measure imput impedance of antenna to determine how much power is going into it. But it will look like like a small capacitor. To get power into the antenna you need to make a matching network to convert generator impedance to complex conjugate of antenna impedance. As is, 90% of power appllied to your wire antenna is being reflected back to generator (less than 10% is going into antenna). So the amount of power your antenna is readiating is very small.

I was hoping for more answers like this...

BUT what the #?*! did you just say..

I never wanted to transmit.. never planned on it.. still dont want to. I'm more of a receiver.

Is "very small" enough to make a radio 15' away scream like a banchee. By scream like a banchee I mean, completely drown out the local station that was coming in loud and clear? And boost the volume?

I have absolutely no reference of radiated RF and its capabilities. So I appreciate the replies.

jake

[xee2]

I was hoping for more answers like this...

BUT what the #?*! did you just say..

I never wanted to transmit.. never planned on it.. still dont want to. I'm more of a receiver.

Is "very small" enough to make a radio 15' away scream like a banchee. By scream like a banchee I mean, completely drown out the local station that was coming in loud and clear? And boost the volume?

I have absolutely no reference of radiated RF and its capabilities. So I appreciate the replies.

jake

At 15 feet you are coupling to the receiver directly not picking up the RF radiation. A wavelength at 1 MHz is 1000 feet so 15 feet is only a few percent of a wavelength away. This is called near field coupling and is very strong.

[jake]

Really 90%

You need to measure imput impedance of antenna to determine how much power is going into it. But it will look like like a small capacitor. To get power into the antenna you need to make a matching network to convert generator impedance to complex conjugate of antenna impedance. As is, 90% of power appllied to your wire antenna is being reflected back to generator (less than 10% is going into antenna). So the amount of power your antenna is readiating is very small.

In the bottom diagram 90% is being reflected to the generator???

[xee2]

In the bottom diagram 90% is being reflected to the generator???

It is hard to tell from diagram. I was assuming you were just connecting a wire to signal generator. In this case there is a whole circuit attached. But I suspect that most of the energy is being reflected back into the generator. Getting an electrically small antenna (length only a small fraction of a wavelength) to radiate all of the power being fed to it requires much work. When the length gets to be 1/4 wavelength the antenna impedance will be close enough to 50 ohms to radiate most of power. But less than than 1/4 wavelength will reflect most of the power fed to it from a 50 ohm source.

If you are tuning with the set up in diagram, be aware that the frequency is going to be effected by the loop attached to the generator. I think it would be better to use a short wire well away from the receiver resonator to transmit with when tuning the resonator.

[dR-Green]

I don't think I've seen an extra coil on your setup, or have I?

No you haven't, not yet. I suppose I could make an extra coil first and test it on the spiral

[xee2]

I think it would be better to use a short wire well away from the receiver resonator to transmit with when tuning the resonator.

Being close to the resonator will also effect tuning. I recommend sweeping the frequency to see where it resonates, adjust, move away, sweep frequency to see where it is resonant, .... etc.

[jake]

It is hard to tell from diagram. I was assuming you were just connecting a wire to signal generator. In this case there is a whole circuit attached. But I suspect that most of the energy is being reflected back into the generator. Getting an electrically small antenna (length only a small fraction of a wavelength) to radiate all of the power being fed to it requires much work. When the length gets to be 1/4 wavelength the antenna impedance will be close enough to 50 ohms to radiate most of power. But less than than 1/4 wavelength will reflect most of the power fed to it from a 50 ohm source.

If you are tuning with the set up in diagram, be aware that the frequency is going to be effected by the loop attached to the generator. I think it would be better to use a short wire well away from the receiver resonator to transmit with when tuning the resonator.

-1 turn primary flat 1/2". in series across the Function generator
-20 turn secondary 29.5-30.0 meters. about 2" above the secondary, same diameter
-Coil was most active at 2000kcycles. It lit an av plug the brightest at the greatest distance.

Considering these three observations, and considering the 20 turn secondary as the antenna;

Can I conclude the coil is acting like a (1/4 lamda) 150m antenna? (i.e. I have electrically lengthened the 30m wire I used in the secondary)

[xee2]

Can I conclude the coil is acting like a (1/4 lamda) 150m antenna? (i.e. I have electrically lengthened the 30m wire I used in the secondary)

It is possible but there is no way for me to tell if that is happening. But the resonant frequency is going to change when you disconnect the generator if the generator is tightly coupled to the resonator. I think your objective is to get the receiver tuned to the correct frequency, not to make a transmitter tuned to the correct frequency.

[jake]

I think your objective is to get the receiver tuned to the correct frequency, not to make a transmitter tuned to the correct frequency.

You are correct

and on the way there I'm disecting each component.

I do not know what frequency the secondary should resonate as it is shown in the diagram. What part the secondary plays in the TRT I do not know. I think Eric may be the only one. But this is not a TRT, just yet.

So I am looking at the coil from all angles. How it was a radiator in this case study, not exactly what I was expecting but Ill take what I can get.

When excited @ 2000kcycles it radiates quite readily but at 1620 not so well . Same goes for receiving I thought...

'CAN' or Beer can/ dr pepper can, is the standard end termination for the Crystal Radio Initative. Dollard recommended it and I think those building are also using it in testing.

I need a lot of 'CAN' to bring it down to 1620. But how much 'CAN' will be provided by the extra coil, I do not know. The two split rings in the final design of the TRT will allow for some adjustment. Of 'what' is still to be determined. So much I do not know.


As far as the 150m question I was just trying to figure out why it ended up that way since I was only using 30m of wire. But I think I got that part figured out.

[jake]

Trt

The final product.

by EPD


I do not know whos site this picture is hosted on but, I hope you don't mind that I used it here. If you do please let me know.

[xee2]

@ jake

Wow, that looks complicated.

To lower your frequency you need to add parallel capacitance to the coil. Probably about 5 to 10 pF. You can make an adjustable capacitor with two sheets of copper overlapped with insulator in between and held together with a clothes pin.