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Ferrite rod antenna explanation

michael1978

Mar 17, 2012
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Do not use a breadboard for an LM386 because the capacitance between the many rows of contacts and wires will probably cause it to oscillate at a radio frequency and get hot. When it is oscillating then it cannot amplify audio. Solder the LM386 and its capacitors on a compact stripboard or pcb instead.

Will you remove the TL081 opamp from the radio circuit?
i remove tl081, I seperate but is no problem i go to solder lm386, and i with say when is finish
 

michael1978

Mar 17, 2012
388
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Do not use a breadboard for an LM386 because the capacitance between the many rows of contacts and wires will probably cause it to oscillate at a radio frequency and get hot. When it is oscillating then it cannot amplify audio. Solder the LM386 and its capacitors on a compact stripboard or pcb instead.

Will you remove the TL081 opamp from the radio circuit?
is not working man, like my LM386 is not working i think, because the signal is the same with or without LM386
 

Audioguru

Sep 24, 2016
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Test the audio amplifier with an audio input signal.
You did not post the corrected schematic for the "radio" or for the LM386 amplifier so we cannot help you fix them.

Also please post photos of the radio and amplifier you built.
 

michael1978

Mar 17, 2012
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Test the audio amplifier with an audio input signal.
You did not post the corrected schematic for the "radio" or for the LM386 amplifier so we cannot help you fix them.

Also please post photos of the radio and amplifier you built.
thanks ardioguru i make it, but again nothing, i search in google, there are a lot of lm386 configuration

but you know what i did i destroy all,
i make new radio TRF, can you please tell what is the problem, i can hear in 8ohm speaker, i hear in crystalearphone i find the stations, i hear, after i try to to make low 0 volume, you hear nothing, after i start voulme up i hear , when start more volume you hear noise not normal,
here is the circuit
choccy_trf_schematic.jpg
 

Audioguru

Sep 24, 2016
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I saw the video of the Choccy Block radio and it played a lot of noise and distortion. Its audio power amplifier is awful because it has zero negative feedback. The datasheet for the LM386 shows how it should be connected and it has built-in negative feedback for very low distortion.
I have never made a poor quality very simple Mickey Mouse AM radio like this one, instead I made normal super heterodyne AM-FM radios that worked perfectly.
 

michael1978

Mar 17, 2012
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I saw the video of the Choccy Block radio and it played a lot of noise and distortion. Its audio power amplifier is awful because it has zero negative feedback. The datasheet for the LM386 shows how it should be connected and it has built-in negative feedback for very low distortion.
I have never made a poor quality very simple Mickey Mouse AM radio like this one, instead I made normal super heterodyne AM-FM radios that worked perfectly.
but he does not use lm386? in the circuits, i hear some station, when the volume increased i hear only noise,
for me is perfect the radio, do you have and idea how to repair? where to check, can you help me please....
 

Audioguru

Sep 24, 2016
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He made his own amplifier that is awful with no negative feedback. ALL audio amplifiers have negative feedback, but not the simple one he designed.
A simple regenerative circuit makes an extremely poor radio that produces a lot of noise. I think yours begins to oscillate when you turn up the volume control. Increase the value of R14 and decrease the value of R1, then it will have less positive feedback.

A real AM radio is a super heterodyne that has its input tuned, an RF amplifier with automatic gain control, an oscillator, a tuned mixer, an IF amplifier with 3 or 4 tuned transformer stages then a diode detector. It can feed an LM386 power amplifier.
 

michael1978

Mar 17, 2012
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He made his own amplifier that is awful with no negative feedback. ALL audio amplifiers have negative feedback, but not the simple one he designed.
A simple regenerative circuit makes an extremely poor radio that produces a lot of noise. I think yours begins to oscillate when you turn up the volume control. Increase the value of R14 and decrease the value of R1, then it will have less positive feedback.

A real AM radio is a super heterodyne that has its input tuned, an RF amplifier with automatic gain control, an oscillator, a tuned mixer, an IF amplifier with 3 or 4 tuned transformer stages then a diode detector. It can feed an LM386 power amplifier.
Man can just tell me, how much to increase the value of R14 i dont know and to decrase R1, and i will do it, i will tell you how is working?
 

Audioguru

Sep 24, 2016
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Try increasing the value of R14 by 20% to 56k and reducing the value of R1 to 820. But a 10% change to 51k and 910 might work better. Maybe R14 needs to be 68k and R1 be 680.
 

michael1978

Mar 17, 2012
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Try increasing the value of R14 by 20% to 56k and reducing the value of R1 to 820. But a 10% change to 51k and 910 might work better. Maybe R14 needs to be 68k and R1 be 680.
thanks a lot, i go to change so i see one wire was out breedboard, so now is working perfect.....my first radio ;-)

may i ask you something else i make you tired man, with my question without control ;-),
you saw the page, do you understand how to calculate that ferrit rod, i search on the internet, i did not find nothing good, so why primary is main connected to the vc, and the secondary to input antenna, how they calucalte

thank a lot for time
have a nice wekend.
 

Audioguru

Sep 24, 2016
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I am glad you fixed the wiring problem on the breadboard. Breadboards cause many problems.
A normal radio has at least 5 tuned LC circuits to tune in the station you want but reject interference from all the other AM radio stations. This simple radio has only a single LC tuned circuit so it must have a very narrow bandpass. It has a narrow bandpass because it has more turns of wire for its tuned coil producing a high impedance than the coil that is loaded with the transistor.

A good real AM radio has automatic gain control so that a weak distant stations sounds as loud as a strong local station. This simple radio does not have AGC.
 

michael1978

Mar 17, 2012
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I am glad you fixed the wiring problem on the breadboard. Breadboards cause many problems.
A normal radio has at least 5 tuned LC circuits to tune in the station you want but reject interference from all the other AM radio stations. This simple radio has only a single LC tuned circuit so it must have a very narrow bandpass. It has a narrow bandpass because it has more turns of wire for its tuned coil producing a high impedance than the coil that is loaded with the transistor.

A good real AM radio has automatic gain control so that a weak distant stations sounds as loud as a strong local station. This simple radio does not have AGC.
thank, man i make it in the end.... now i have to study good this am radio. just for more experience.
 

michael1978

Mar 17, 2012
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I have never seen a ferrite rod 90mm diameter, they are more likely to be 9mm diameter and 100 to 150mm long.

The schematic looks to be a medium wave receiver. Do you have strong signals in yout area.

Make a wooden rod the same diameter as the ferrite rod.
Wind some paper around it and glue with PVA to make a tube.
Put a dint in each end of the wooden rod so that it can be rotated on two spikes or put it into a hand drill, my lttle drill has a 4:1 gear ratio.
Wind on L1 with the turns touching. Glue the ends down perhaps with hot melt glue Slide off the rod.
Wind on L2 and L3 side by side in a similar way.
Put the two assemblies on the ferrite rod spaced at a few mm.

The ferrite rod is very brittle so should be handled with care.
Some PTFE plumbers tape wound on the wooden rod may make the coils easier to extract.
The position of the coils on the rod can be fixed with traditional beeswax but I tend to favour hot melt glue as it can be removed cleanly.

If you break the rod, it can be glued together with epoxy with little degradation in performance.

A scrap transistorised radio may well have a ferrite rod complete with L1 and Cv

Hi, how are you doing, this circuits wich i post it, i make the ferrit rod, but can you explain me how you calculate the ferrit rod, what is the formula of ferrit rod....how do you calculate,
 

duke37

Jan 9, 2011
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The coils need to have a certain inductance to resonate at the wanted frequency.
The number of turns on the ferrite rod controls the inductance and the permeability of the ferrite will ehhance the inductance. So put on a few turns and measure the inductance and increase to give the required amount. The inductance is proportional to the square of the number of turns and will be highest when the coil is at the centre of the rod.

P.S You cannot make a ferrite rod. This requires complex powder metallurgy and a closely controlled furnace.
 

michael1978

Mar 17, 2012
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The coils need to have a certain inductance to resonate at the wanted frequency.
The number of turns on the ferrite rod controls the inductance and the permeability of the ferrite will ehhance the inductance. So put on a few turns and measure the inductance and increase to give the required amount. The inductance is proportional to the square of the number of turns and will be highest when the coil is at the centre of the rod.

P.S You cannot make a ferrite rod. This requires complex powder metallurgy and a closely controlled furnace.
Hi Duke i dont want to make a ferrit rod, but i dont know how to select the inductane of L2 and L3, for example the circuit wich i post here, why how he calculate L2 10 turn L3 5 turns, why excactly 10 turn and 5 turn?
thnx man
 

michael1978

Mar 17, 2012
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because those are the number of turns with a specific wire gauge required to give the needed inductance
but is there a formula or if i want to make one ferrit rod anntena for myself, how, the L1 i know to connected with vc for desired frequency, but for L2 or L3 how can i know how much turn i need to make it? where can i get info for L2 or L3
 

duke37

Jan 9, 2011
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The formula for resonant frequency is F = 1/(2*Pi*Sqrt(L*C))
If you want to pick up a transmitter on 640kHz with a 200pF capacitor, then you can calculate the required inductance by re-arranging the equation.
A tuning capacitor will have a ratio of about 10:1 maximum to minimum. Because of the square root in the equation, this will give a frequency ratio of about 3:1.

The turns connected to the transistor will depend on the impedances. A light coupling will give a high Q and good selectively, a tight coupling will give lower losses. I think that the turns of L2 and L3 will be determined by experiment.
 

michael1978

Mar 17, 2012
388
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The formula for resonant frequency is F = 1/(2*Pi*Sqrt(L*C))
If you want to pick up a transmitter on 640kHz with a 200pF capacitor, then you can calculate the required inductance by re-arranging the equation.
A tuning capacitor will have a ratio of about 10:1 maximum to minimum. Because of the square root in the equation, this will give a frequency ratio of about 3:1.

The turns connected to the transistor will depend on the impedances. A light coupling will give a high Q and good selectively, a tight coupling will give lower losses. I think that the turns of L2 and L3 will be determined by experiment.
you mean L2 or L3 they need to have low impedance to connect to base of transistor, can you please show just one example for L2 and L3, so after i know how to experiments, because i build the trf radio is working, only the output of ferrit rod i dont know, is there a formula to give

thanks man.....
 
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