Hey all!

Last time i wanted to make a 3V microphone FM transmitter circuit that transmits other than 300m. I have selected all required electronic components for this circuit. This circuit runs on a LC tank without a trimmer! The circuit will be in attachments! There is one problem (If there is any information required to help me to answer on my own question, please comment): A coil that i should to use is a single layer air core copper wire, of 0,61mm wire diameter. The coil have 3mm diameter with N=5 turns, and it will work on 87-108MHz. The last required information is, how to determine the length of a coil, that is required so the coil will work properly on this range of frequencies? Is there any formula to calculate the length of this coil, taking into consideration the frequency/cies on which it will work? Yes i know the FM frequencies is radio frequencies, and is too risky to use this frequencies, but i cannot generate 150 and above MHz, because i haven't enough knowledge to do that! So i start from something easy!

Any tip is highly appreciated

P.S. A question that haven't anything with this post! The frequency modifier is installed on receiver or on transmitter?

 

A question that haven't anything with this post! The frequency modifier is installed on receiver or on transmitter?

As your schematic shows, the audio signal from the microphone is amplified and used to alter the frequency of the transmitter. The receiver must be able to tune to the carrier frequency and generate a voltage in proportion to the received signal's deviance from that carrier frequency. That varying 'error' voltage re-creates the original audio signal. It's more complicated than this, but that's the general idea of FM.

 

L1 , C4 form a tuned circuit which will resonate at one frequency, the microphone will alter its central frequency +/- (Deviation) depending on the amount of volume coming from it in Khz, ideally +/-10Khz is the norm for FM mono.

The only way to tune the transmitter without a trimmer is to alter the gaps between the coil which alters it's inductance. This is a very crude method.

The length of your coil will be Approx 3mm.

I would use a variable inductor or preset capacitor trimmer.

For 150Mhz you need a 34nH inductor, like these from your country.

http://www.mouser.co.uk/Search/m_Pr...craft/164-02A06L/&qs=ZYnrCdKdyeeGbWnVnEPmWQ==

 

The length of your coil will be Approx 3mm.

So, if i understood well, the coil which is a single layer air core copper wire with wire diameter d=0.61mm, coil diameter D=3mm and number of turns N=5, will have length l=3mm? If i fail to understand this quottetion, please correct me! So, the diameter of the single layer air core coil, is equal to its length, or these values is inversly proportional values? Is there any formula that could help me to find it out?

 

5 turns of 0.61mm wire = 3.05 mm

 

5 turns of 0.61mm wire = 3.05 mm

yes, but this equation is not the same for every frequency! For example if i had the same coil, with the same wire dia, same coil dia, same turns but for frequency of 85MHz or lower, there would be a different length of a coil for example 4mm, 5mm, maybe 6mm, but if it will be 108MHz and above there we need coil of 1,5mm length, im just saying nonsense things, but i want to give an example of what i mean! Am i right, or the coil doesn't work with this way? Is there any formula that could help me to find the correct length of the coil for a specific frequency? The equation that you just gave me, really helps me a lot, but then i will need to use a frequency-meter or how its called to find the frequency ! Let's say, we use the same coil that will work on 90MHz. We know number of turns (N=5), we also know the wire diameter and the coil diameter (d=0.61mm, D=3mm). The coil works on frequency f=90MHz! How much is its length l=? ?

 

The length is determined by the wire thickness, and number of turns, so your wire will be 0.61mm X 5 = 3.05mm, this is a tight wound coil, you then have to alter the gaps between the coil to tune it,

https://www.fmsystems-inc.com/air-coil-inductor-winding-easy-way/

http://www.radio-circuits.com/coils.htm


Why do you need to do it the hard way?

 

The length is determined by the wire thickness, and number of turns, so your wire will be 0.61mm X 5 = 3.05mm, this is a tight wound coil, you then have to alter the gaps between the coil to tune it,

https://www.fmsystems-inc.com/air-coil-inductor-winding-easy-way/

http://www.radio-circuits.com/coils.htm


Why do you need to do it the hard way?

Oooo i understood what you trying to say me about! Sorry it is my fault! I dont know that much english so i cant understand everything! So i translated your answer to my language (Greek) and understood all that things you trying to say me! The length of a normal (by default) coil is the multiplication of the turns x the wire thikness, so in total we have the default length of the coil, and for the frequency that we need, we should to alter the gaps (gaps = κενο in my language), of the coil so we success the right frequency! Now i undestood how all that works! Thank you very much guys! And in adition i found a inductor calculator that helps me out a lot! Good job guys! This site have everything

 

Oooo i understood what you trying to say me about! Sorry it is my fault! I dont know that much english so i cant understand everything! So i translated your answer to my language (Greek) and understood all that things you trying to say me! The length of a normal (by default) coil is the multiplication of the turns x the wire thikness, so in total we have the default length of the coil, and for the frequency that we need, we should to alter the gaps (gaps = κενο in my language), of the coil so we success the right frequency! Now i undestood how all that works! Thank you very much guys! And in adition i found a inductor calculator that helps me out a lot! Good job guys! This site have everything

As said in post #7 English..

Γεια σου, ναι για να συντονιστείτε το πηνίο που χρειάζεστε για να αλλάξετε τα κενά, είναι καλύτερο να χρησιμοποιήσετε ένα μεταβλητό πηνίο, 34 μικρο Henries..

http://www.circuitstoday.com/how-to-make-an-air-core-inductor

 

ωωω ευχαριστώ πολύ! τώρα τα κατάλαβα όλα! Σε ευχαριστώ πολύ για ό,τι κάνατε για μένα!

Translation:
Thank you very much! I understood everything now! Thank you very much for everything you did for me!