How to Make FM transmitter from FM radio

ok here is how to convert your FM radio receiver to a FM transmitter.

i did it 10 years ago and with a long 1.5 meter antenna, i got range may be 200 meter.

although its vary with the radio receiver modal.

that time i was little and didn’t new how it works. but now i know.
every radio receiver has a oscillator itself. so it mix signal with inverting phase to cancel out carrier signal and extract out remaining audio signal.

lets put all theory aside( its so long) and make it out.

we will take two ordinary Radio receivers. one of them we will call “Rx” (the Receiver)
and another is “Tx” (the Transmitter).

first we haven’t modified any of them.
just tune R at 100 MHz. or near where no radio station is available and you hear only noise.
now tune Tx from 88 to 108 MHz.

while tuning, you will hear sudden silence in Rx. yes, Tx will eliminate all noise of Rx.

at that frequency of Rx, the Tx is transmitting.

its just the oscillator. what you listening on Rx now is just FM carrier frequency. without sound.

a ideal FM receiver will be completly silent at this moment. becoz what Tx is transmitting is just carrier frequency without any sound. and in receiver, the carrier frequency is cancelled out by mixing same frequency with inverting phase. while there is no sound mixed by Tx, you shouldn’t hear any sound.

but nothing in world is ideal.

ok let again put the theory aside and add a audio to it.

i am not going to tell u how to add audio coming from music player to it. instead, i will only tell how to add mic to it.
bcoz, i admit, i never tried it.

but i tried adding mic to it and got huge success.
it gives you very crystal clear sound.

only problem is, the sensetivity of mic is decrease with weakness of signals received by Rx.

i make you remember at this point, we haven’t modified any of receiver yet and any of them can be used as Rx and other is as Tx.

both have a oscillator inside them which can silence out other receiver.

i cant remember (10 years, you know!) that Tx should be tuned at whether lower or higher frequency than Rx.

but i remember one thing. a fix few MHz frequency gap between them. always.

if you haven’t got any silence then try this-

tune Rx at 108 MHz and try tuning Tx from 88 and to 108 MHz.

if you don’t get any silence in Rx, then do it reverse. i mean-
tune Rx at 88 MHz and then tune Tx starting from 88 to the 108 MHz.

i bet , at one point, you 100% got a silence in Rx.

ok now add a mic to transmitter.
we will add a mic to TANK circuit of Tx, in parallel.

it should be a condenser mic.
as you know, capacitance of condenser mic changes with audio. and while we are adding it in parallel to the tank circuit, the changes in capacitance will cause change in frequency. so, here, carrier frequency is changing according to the audio. which is called frequency modulation of audio or the FM.

this changes in capacitance is very low and thus, changes in carrier frequency is also very slight, say under 100kHz. so its a perfect frequency modulation.

so open up Tx, and look at gang tuning capacitor from behind. behind mean you should see soldring.

image

figure: where and how to connect mic

first we will find out which pole is for FM tuning. to do this, while silencing out the Rx, touch all poles of Tx one by one.

on the pole, by touching, you get noise again in Rx, is pole for FM tuning.

or if you dont get that hisssssss type noise again, at least you would get some/any kind of sound. like if you put a screwdriver on that pole, you should get “thak thak” or “fat fat” or “tan tan” sound.

so after recognizing that pole, we just going to put a condenser mic between that pole and ground.
ground mean, the negetive of battery.

in the picture, you can see.

here are some , based on my experience.

1. i can’t remember my all experiments but adding mic to that tunning circuit will change the transmitting frequency. it will increase it or decrease it i don’t remember. but i clearly remember it will change its frequency and you have to re tune the Tx.
by changing frequency i also mean it will make that Gap increased.
by theory, what i know, see the footnotes.

2. as the Gap will increase, its possible that you tune Rx and 108 MHz and dont get silenced even tuning Tx at 88 MHz or vice versa.
in that case, the increased gap will be more than 20 MHz.
so to get rid of this situation,
we will have to recognize the FM tunning coil first.

you will see many coils on the PCB. one of them is for FM.
just touch them one by one.
while Rx silencing out.
touching mean touch one end of them with screwdriver at the soldering side of PCB.
the coil, by touching it you get maximam sound/distortion/noise in Rx, is used for FM.

i make you remember again, our Rx is as it is. we just examining out Tx. dont do any experiment with Rx accept tuning it.
all experiments are being done on Tx.

2. condenser mic should not have long leads. keep it as short as you can. i experienced that by connecting long leads of condenser mic, frequency gap between Rx and Tx is increased. so keep them short.

3. i also experianced that if we add normal connecting copper wires as leads, that gap(frequency shift) is maximam.
so use a coil copper wire as condenser mic leads.

so after adding mic, you will see that Rx is not silenced any more. and you going to re tune Tx again.

becoz the transmitter frequency is changed.

if in the case you cant silence out Rx again with all scale tuning of Tx, just expend that coil that we recognized earlier.

and then try to re tune Tx again.

all of worst case is when you dont get Rx silenced out even when you expended the coil as long as you can, is that you used very long condenser mic leads or uncommon condenser mic.
in that case the frequency shifted very far below.

if you keep mic leads short and made of strong single copper wire, it will work perfectly. in that case you can either enjoy its crystal clear sound or come back here at my blog to post your thank in comments.
enjoy!!

foot notes.
1. i cant remember but Rx should set 108MHz and Tx will transmit on this frequency when tuned on some megahertz below it. say 9 MHz below(99 MHz).
so Rx tuned at 108 and Tx at 99 MHz and you will hear no sound at all from Rx.
here the GAP i am talking about is 9 MHz.

then you add comdencer mic and Rx is no more silenced.

in this situation, the condeser mic’s capacitance is in parallel with tank circuits capacitor. so it is directly added to it. and frequency is decreased according to well known fromula
f = 1/ [2*pi* root(LC)].

so now that Gap should be higher. say its now 19 MHz.
so while tunning Rx at 108 MHz, Tx have to be tuned 19 MHz below it (at 89 MHz).

in the case you done anything wrong with condenser mic, the gap is very High. say it is 25 MHz.

in this situation even if you tune Tx at minimam 88 MHz, it will be transmitting at 88+25 = 113 MHz. so you cant silenced out Rx anywhere between 88-108 MHz.
in this case you have to stretch FM coil. by stretching tank inductor coil you decreasing its inductance. and frequency is increasing according to that well known frequency formula. thus decreasing the Gap. and you might be able to make transmitter transmitting near 108 MHz, so that you can tune Rx at this frequency.

2. if your mobile phone has FM radio then it can happily be used as Rx.

3. this transmitter is perfect stable. means its transmitting frequency does not change with touch or battery voltage.

4. its range depend on length of antenna. for transmitting all power generated by transmitter, antenna should be equel to wavelength of transmitting frequency.
and half for half.
that is said by antenna theory.

5. who already made it and looking for further-
the point to attach antenna for transmitting should be defer than receiving. and according to me, it must be that pole where you attached positive lead of condenser mic. but by attaching antenna, its frequency will change.

6. even after you converted the Tx into a transmitter, you will see its still a receiver at tuned frequency!

thanks for reading my soooo long lecture.

i write blog of what is unique. all this is done by me. i don’t write for showing you ad and earning money. i just want my credit.
so copy cats, even if you make this. dont forget to credit me.

my Name is ManojBhakarPCM.

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