why FM tx distance is small?

[o_wsam]

i bult an FM Transmitter and its distance is up to 2meter ,but its suppose to be 100meter, whats the defect?
and i've been told that i can connect aproper class of power amp. ,what it would be? and what is the proper design of that amp. for my FM cct.?
with my deep thankful.

 

[pranam77]

First of all you need to confirm wether you are receiving the principal frequency or its harmonics? Coz when you say just 2 meters, i guess the Tx is transmitting on some other frequency and you are getting the harmonic. Try to find the frequency. Cheers

 

[FvM]

Using an inductance value of 10 uH, as written on the schematic, would result in about 10 MHz operation.
Limiting the range to a few meters can however avoid trouble with authority.

 

[o_wsam]

i tried but nothing changed?
what do you think about the power amp.?
try to find me anther solution if you can
plz

 

[Audioguru]

The coil should be 0.1uH, not 10uH.
My FM transmitter is almost the same but the wires on its coil are tight together with no space.

Mine overloads my Cheap Sony Walkman radio so much that its signal is all over the dial. With it set to local (antenna attenuator) and 4m away then it can be tuned.

Mine has a range of more than 2km to my very sensitive home hi-fi-FM receiver and very sensitive car radio. Its range is 300m to my cheap Sony Walkman radio set to "distant'.
Its range is across the street to a cheap scanning radio from The Dollar store.

 

[EEsj]

have you monitored the spectrum on an analyzer?

Have you swept the output impedance smith chart and antenna impedance?

You're probably not tuned correctly and not delivering enough power to the antenna to complete the link.

 

[Audioguru]

I just noticed that the antenna is 3.8 times shorter than my 80cm antenna. That makes a big difference to its range because 80cm is 1.4 wave-length.

If you built it on a breadboard then it won't work or it will work very poorly. I made mine on compact Veroboard.

 

[o_wsam]

thanks for this fine circuit , but I still have some questions about it:
-is the mic is any ordinary mic, and what about its impedance matching?
-(R5=470)&(R7=220) is it in ohm?
-for the inductance (0.1UH),what type of wire used for winding,and what its diameter?
-what is the (LM2931A 5.0)?
-what type of antena should I use?,would the antena of ordinary radio work?
-do you have an analysis for the work this cct.?
I'll be grateful for your answers.

 

[Audioguru]

-is the mic is any ordinary mic, and what about its impedance matching?

What is "an ordinary mic"? it is not a dynamic coil and magnet mic, it is a common electret mic that was in a broken cell phone.
Electret mics never have their impedance matched because then their output level will be reduced.

-(R5=470)&(R7=220) is it in ohm?

Of course.

-for the inductance (0.1UH),what type of wire used for winding,and what its diameter?

I used 1mm enamelled wire from a speaker's crossover network. See my sketch.

-what is the (LM2931A 5.0)?

It is a "low dropout" 5V regulator that still works when the battery voltage has dropped to 5.5V. All semiconductor manufacturers make 5V low dropout regulators.
An ordinary 7805 regulator needs an input of at least 7V to work and a 9V battery quickly drops to less than 7V.

-what type of antena should I use?,would the antena of ordinary radio work?

It is 1/4 wave and is just a piece of wire 80cm long. If it is shorter then the range is less.

-do you have an analysis for the work this cct.?

It is extremely simple and uses the same circuits as many other FM transmitters.
What do you not understand about it?

 

[o_wsam]

thank you very very much you've been ageat help.
I wish you all the good