[Moved]: need help with fm transmitter

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MentorEnigma

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Hi,
how can i make a coil of the same value of the inductor in this transmitter circuit and what type of antenna is used ? will it be different if higher frequency ?
 

Re: need help with fm transmitter

is this the right forum to ask this question or is it going to be dead forever ?
 

Re: need help with fm transmitter

Hi,

I´d buy this coil.

In the internet there are many documents, forumulas, tutorials and videos on how to make a coil.
If you are not satisfied with this huge ammount of information, then you should tell us where exactely you see a problem and what your requirements and specifications are.

I doubt you can build it more precise, smaller, cheaper than the ready to buy ones. I wonder why you want to build it on your own.

Klaus
 

Re: need help with fm transmitter

The problem is that i cant find those kinds of inductors in my country, i tried using inductor calc but it would take very small dimensions to make such coil, any alternatives ?
 

Re: need help with fm transmitter

Hi,

but it would take very small dimensions
Click to expand...
Usually you are very flexible in dimensions. You can build a small 22uH coil and you can build a big 22uH coil.
I wonder what input you made .. and what calculator you used: I didn´t find "inductor calc"

It seems to be a resonant circuit. About 8.8MHz if my calculation is correct.
But it is no very special device. It should be available almost all over the world.

***
Please try to give complete and more exact informations. Maybe documents, links, your input values and so on .. in a way that we are able to reproduce your trial.

Klaus
 

Re: need help with fm transmitter

MentorEnigma said:
... i tried using inductor calc but it would take very small dimensions to make such coil, any alternatives ?
Click to expand...

First you decide the frequency; next you calculate the inductor value. It may come only 3-4 turns of 1mm dia Cu wire wound on a plastic former of 4-5mm dia.

I know it is difficult to get an exact value this way. But you can squeeze or stretch the coil little bit and adjust the inductance. It is not difficult to make a 22nH inductor.

I know it is common to suffer from information overload; the idea is that you should also learn some basics on the way- it should not be a soldering job only. That you are here suggests that you want to learn the basics.
 
Re: need help with fm transmitter


Thank you for your reply, I appreciate any info you teach me every bit of knowledge means something to me as i started in this field from not so long ago.
im using this calc for inductance value https://rimstar.org/science_electronics_projects/coil_design_inductance.htm
i've wrote the values you gave me and it gave me 99 nH, i tried to use (Length of coil) instead of (wire diameter), it gave me 10 nH, i don't understand how does the relation of wire diameter work in the inductance equation but will i get the same 10nH value if i use a different wire diameter ?
 

Wire diameter does not matter - the inductance will be same.

We use thicker wire for mechanical strength- the coil is self-supporting and if it moves or shakes or vibrates, the inductance will change- even a little change will cause your transmission to shift frequency.

Usually the diameter of the former is added to the diameter of the wire to get the diameter of the solenoid. In other words, the diameter of the solenoid is measured from the centre of the wire.

Similar considerations are there for the length of the solenoid. Physical length of the coil is taken from the centre of the wire. We usually do not bother much.

Once you get an idea, you make the coil (and measure with a meter) then fix it and squeeze or stretch the length a bit so that the frequency comes in the range.
 

Hi,

Fixing the coil:
I don't recommend to use two compound glue..because it is highly dielectric...and worst of all it changes dielectric value during curing.
This dielectric between the winding will influence overall resonant frequency.

Maybe hot melt solutions are better.

Klaus
 

Like I said on the other forum (I can't remember which one) an ordinary FM transmitter usually operates on the FM broadcast band which is from 88MHz to 108MHz and uses a coil that is usually 100nH made with 5 to 9 turns of wire on a 3mm former. The value of this coil is much lower so the frequency will be much higher. Do you have a radio to receive it?

The value for C4, C6, C7 and C8 are wrong.

This transmitter is so simple that it will have serious problems.
 

alright so i'll have to take measurements of wire diameter and add it to coil's diameter to get the total diameter into the equation

im going to connect components in dead bug style is it ok to use little glue to stick bits of copper clad into the board ?

i want to make a small transmitter with a small range of 10-20 meter for our college lecture hall and use an old radio as a receiver and connect it to an amplifier, it might sound much but i dont have enough capabilities in my country to make something better, i tried looking for other transmitter only few i found that their components are in my country, some others are too complex :S
 

Re: need help with fm transmitter


hey friend sorry i think i skipped a part of your reply, the coil i want to make is 22nH not 22uH, much much smaller value
 

Its radio frequency will change as the battery voltage runs down because it does not have a voltage regulator.
Its radio frequency will change if something moves towards or away from the antenna unless an RF amplifier is used in stage 2 or stage 3.
Its audio will sound muffled on an FM radio because it is missing pre-emphasis.
 

It is OK as the first attempt but you need to proceed systematically. Because you are going to receive the signal on a regular FM radio, you must tune around 100MHz transmission frequency. As mentioned in post 10, commercial receivers can tune from 88-108 MHz and your L and C values are not correct. Yes, your transmitter can be used within a range of 10-20 meters but there will be other problems (but this will work as a proof of concept idea). You need to use a perf board to assemble the circuit.
 

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