# How long should antenna wire be to receive amateur band?

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#### davenn

Moderator
Sep 5, 2009
14,273
..... then a resistor is needed to block off the strong current of all the crap so that only the weak audio signals are heard instead of all the crap.

and that wasnt the explanation I then BobK gave you, instead you went on about the blocking of all the other crap and that isnt correct.
Its sole purpose is the one you were given

Dave

#### dietermoreno

Dec 30, 2012
238
The 100K resistor is there because crystal headphones act like a capacitor and will just charge up to the max voltage seen. The resistor bleeds off this charge and allows them to see the audio signal.

Bob

Oh, I see. That is the primary reason of the resistor. The reason for nothing being audible except hum is that the headphones acting as a capacitor will just charge up and accept no more charge if the charge is not bled off, so then the tuned circuit is destroyed and it is an untuned circuit picking up any random EMI.

I think I can save money and build a 100K resistor at home with a short piece of mechanical pencil graphite. Like in this link using graphite on paper and rolling up the paper:http://www.instructables.com/id/Paper-Resistor/

The site says that home made paper resistor has a value of 1.8 M ohm at the longest separation of the electrodes and a value of 90K ohm at the shortest separation of the electrodes.

So I should use a short piece of mechanical pencil graphite so that the resistance isn't too much and is just right at around 100K.

So that's why the fox hole radio doesn't need the resistor, because the pencil graphite IS the resistor. The actual diode is the blued razor blade. The graphite isn't a diode.

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#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,760
then the tuned circuit is destroyed and it is an untuned circuit
The circuit will still be tuned.

Compare this to a rectifier at the output of a mains transformer. The transformer will spit out a sinusoidal voltage (50/60Hz) contnuously, After the rectifier this becomes a DC with a sinusoidal modulation. If you add a smoothing capacitor (as any decent power supply does), the capacitor will charge up to peak voltage. If you don't drain current from the output, the capacitor's voltage will stay at the peak voltage. No change in voltage, no current -> no power. If you think of the capacitor as the headset, no power mean no sound.
All the while the transformer will happily keep on putting out a sinusoidal signal with mains frequency.
The same happens to the radio setup.

#### dietermoreno

Dec 30, 2012
238
The circuit will still be tuned.

Compare this to a rectifier at the output of a mains transformer. The transformer will spit out a sinusoidal voltage (50/60Hz) contnuously, After the rectifier this becomes a DC with a sinusoidal modulation. If you add a smoothing capacitor (as any decent power supply does), the capacitor will charge up to peak voltage. If you don't drain current from the output, the capacitor's voltage will stay at the peak voltage. No change in voltage, no current -> no power. If you think of the capacitor as the headset, no power mean no sound.
All the while the transformer will happily keep on putting out a sinusoidal signal with mains frequency.
The same happens to the radio setup.

So if the headset acts as a capacitor that stores charge and does not discharge it, then there is no sound heard except the luck (misfortune) of random EMI hum that is a strong enough current to discharge the headset.

Just a curiosity: does it matter if the resistor (pencil lead) is placed before or after the detector?

If a mains power rectifier had the capacitors before the diodes, then would smoothing out the wave peaks result in no current ever reaching the diodes?

This is not a problem. It is easy to fix. I could use my OP-Amp audio mixer as the detector and then the audio output of the detector is connected to the pencil lead before being connected to my guitar amp. Just this would require me to sacrifice (cut open to expose the wire and shield) two more cheap 1/4 inch cables and buy another male-to-male 1/4 inch adapter since the vari-cap is already before the detector.

Or I could save money and use a blued razor blade instead of the fuss with the OP-Amp, but I think a transistor will have better results than a blued razor blade. And if I use the OP-Amp audio mixer as the detector then the crystal receiver can easily be modded into a regenerative receiver by having the feedback loop tickled by wires wound around the input cables and output cables and those wires are all wound in coils next to each other on a prescription bottle to tickle each other by inductance and then the vari-cap is connected to the tickling coil instead of connected to the inductor.

Here is my schematic that I drew in Paint:

#### BobK

Jan 5, 2010
7,682
I think I can save money and build a 100K resistor at home with a short piece of mechanical pencil graphite.
Personally, I would spend the 5 cents.

Bob

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
And I would remove the guitar.

#### BobK

Jan 5, 2010
7,682
What? You can make a radio without a guitar?

Bob

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
What? You can make a radio without a guitar?

Yes.

It is well known that the first radio receiver was invented by Jimmi Hendrix, 17 months before the invention of the radio transmitter.

What is less well known is that electric guitars were inside every radio receiver made up until the early 1970's.

It was the rapid rise in miniaturization by the Japanese in the late 60's which allowed radios to become small enough to carry.

Some suspect that the presence of tiny guitars in radios was the inspiration for the Sony Walkman.

#### BobK

Jan 5, 2010
7,682
No wonder they are called Stratocasters.

Bob

#### dietermoreno

Dec 30, 2012
238
typical regens use changes in the operating point of the transistor to demodulate the RF
http://www.techlib.com/electronics/crystal.html

So is this what is happening that is resulting in any transistor that I connect to my guitar (guitar amp or audio mixer) will demodulate RF into audio?

but then how would transistors used in RF amps work if all the transistor did was demodulate the RF into audio?

Here is a schematic from that website that shows a crystal receiver with a transistor instead of a diode! (I think it is better than a diode because it amplifies the RF simultaneously as it demodulates it resulting in the RF tuned to having greater voltage to drive the tank circuit so it has greater frequency selectability):

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
http://www.techlib.com/electronics/crystal.htmlbut then how would transistors used in RF amps work if all the transistor did was demodulate the RF into audio?

They work because the circuit is designed to amplify that frequency.

Circuits that are not designed to handle a particular type of signal may behave strangely when that signal appears on an input. If the signal enters via some other method power supply for example) then unusual things might happen even if the circuit is designed for that signal, because the signal is not arriving at the input.

The signal could be too high or low in frequency, it may be too high an amplitude, it may have an incorrect DC offset. In some cases the amplitude might even be too low.

The strange thing it does may include demodulating the signal -- or the effect that you notice is a demodulation of the signal.

These things are often not entirely predictable and often not interesting to explain except insomuch as it takes to eliminate the cause.

It's a bit like having a flat tyre and noticing that the car steers left. One is typically not interested in determining exactly how much reduction in pressure it takes to turn the car left a certain amount. One would not really be interested in reducing the pressure in your tyres to turn a corner. You just want to get the tyre repaired.

#### dietermoreno

Dec 30, 2012
238
Oh okay, so the regen detector only works to demodulate because it has a difference of gain of RF signals to compare to so the high gain RF signal when combined with the low gain RF signal, in accordance with the principle of linear supposition, the resulting output of the mixer has the wave peaks of modulation index of increased amplitude enough that the audio is loud enough to drive an audio pre amp transistor (or tube) and the RF remaining is ignored by the audio gain section since the audio gain section can not handle RF (and also a filter can be added to remove any remaining RF to prevent RF from being demodulated again by the audio gain section from either accidental tickling of signals by induction interference or from the unpredictable behavior of an audio transistor at RF).

So a transistor or tube has unpredicatable behavior when it receives a signal at a higher frequency than it can respond to (such as if the transistor or tube is designed for audio), and one of these unpredicatable behaviors is demodulation.

So in summary, a transistor or tube designed for an audio application will most likely demodulate RF while a transistor or tube designed to handle RF will not demodulate RF.

Well so I think an audio transistor or audio tube would be better than a diode in a crystal set since a diode has too much loss since its a passive component, correct? At least that's what that website that I showed you seems to think.

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
It's more the circuit than the actual device. For example the transistors in an audio amplifier may be capable of working up to (say) 100MHz, but the circuit they're in isn't designed for those frequencies.

A tube diode will have vastly more losses than a semiconductor diode.

An amplifier only amplifies because there's power going in tot he amplifier from a power supply. The crystal radio doesn't have external power.

Look at biased diodes if you're interested in a way of minimising loss in the diode (with additional external power though)

I might be sorry, but here is another way using a device that has a very low turn on voltage.

#### dietermoreno

Dec 30, 2012
238
Oh okay.

biasing the diode with a battery to offset the losses sounds like a good idea. So then it makes a diode into an active component!

I'm not sure if that link you gave to the receiver with the transistor with no power supply is a very good idea. I think if its already using a transistor why is it so hard to have a power supply for it and if you have a power supply then it will amplify much better and the antenna doesn't have to be as long and you don't have to be as close to the stations.

Well so what is going on here in this audio file I uploaded to YouTube that I recorded today with a Fast Track M-Audio interface connected to my guitar amp and my guitar amp connected to my guitar with the gain and volume turned all the way up on everything? My guitar amp is being driven to oscillation? It sounds like the oscillatory frequency has a beat every half second. If I disconnect the antenna, the beat still occurs at the same volume! So the beat is not RF and it rather is being generated by my guitar amp.

It seems like when I turn up the volume to a level hoping to make the voices intelligible, it makes it even worse because now at the wave peaks of the voices my guitar amp is beeping on and off when it catches some voices.

How can this problem be solved since I want to hear voices, not voice amplitude beeping?

And how can the problem be solved to remove the annoying beat that the guitar amp is apparently generating when it is at full gain and full volume when my guitar is at full volume?

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
biasing the diode with a battery to offset the losses sounds like a good idea. So then it makes a diode into an active component!

Yeah, and if my grandmother had wheels she'd be a wagon.

I'm not sure if that link you gave to the receiver with the transistor with no power supply is a very good idea. I think if its already using a transistor why is it so hard to have a power supply for it and if you have a power supply then it will amplify much better and the antenna doesn't have to be as long and you don't have to be as close to the stations.
I knew it was a bad idea. You have failed to grasp what it does.

The rest of your message is complete rubbish again and I'm ignoring it.

edit: other than to say "Any sufficiently poor wiring can become an antenna -- even a guitar". You're picking up hum (I'm tempted to add "you idiot", but I'm not quite that frustrated yet -- almost though)

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#### dietermoreno

Dec 30, 2012
238
Oh okay, so what is being picked up is simple mains hum, and the mains hum tickles the guitar amp into oscillation to generate a beat frequency when the guitar amp is over driven.

So I thought maybe a longer antenna would solve the hum problem since I was only using a 20 foot long 1/4 inch instrument cable coaxial cable and I have read on the internet that a long antenna is the most important part of RF gain on a crystal receiver (I obviously have no problem with audio gain, RF gain is the problem) so I pick up anything useful instead of simple mains hum since the crystal receiver does not have an RF gain stage.

So I bought about 70 feet of telephone wire for \$5.50 before tax at the local second hand store that I will use to create the long wire antenna. Also I think telephone wire will work better as an antenna than a 1/4 inch instrument cable because the instrument cable has the wires twisted to cancel out any RF picked up, defeating the purpose of an antenna. Just I have to make sure that the telephone wire isn't twisted and if it is twisted I have to take the individual wires out of the plastic sheath.

#### davenn

Moderator
Sep 5, 2009
14,273
The longer the antenna, the more hum you will pick up

Dieter

How about finally forgetting about the guitar and using for something its NOT SUPPOSED to be used for
Use it to play music and only music!!
Build a basic radio receiver and learn about its operations and as you learn, then improve its performance etc

If you mention radio receiving and using a guitar and amplifier in this thread again, I WILL lock the thread
If you want to learn about radio receiving ... great then do it properly and we all will help
BUT STOP wasting all our time involving guitars and their audio amplifiers in the process OK ??!!

Dave

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#### dietermoreno

Dec 30, 2012
238
So now I see how silly using guitar pickups as inductor is, and I will wind the coils around a toliet paper roll using some of the telephone wire and then connect the inductor to the guitar amp by cutting open an instrument cable to expose the wire inside and expose the shield which the shield will be the ground of the inductor.

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#### davenn

Moderator
Sep 5, 2009
14,273
Oh okay. I think what your trying to say is that the reason that it isn't working is exactly because its a guitar. There is no tuning really so all that is heard is mains hum. So the solution is to remove the guitar.

and the guitar amplifier .... forget about it ... stop trying to use it as a receiver ... it wasnt designed for that purpose

Dave

#### BobK

Jan 5, 2010
7,682
Though you can use it as an amplifier after the detector.

Bob

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