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FM Micromitter only transmitting 8M

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Hi all,

I'm new here.
I have just assembled a FM Micromitter (Miniature FM transmitter).


Construction went well with very few re soldered points (put a couple of things in the wrong way round, but hey I'm learning).

When I went to do the testing and adjustment it says that I should be reading 5v between TP GND and pin 8 of IC1 but I only get a reading of 2.5v.

Can anyone tell me why this might be and what the consequences of this might be.

It DOES transmitt (I am currently transmitting on 88.9) but I loose the signal after approx 8M from the antenna.  Is this related to my drop it voltage measured against what it should be?

Should I trim my antenna to a specific length?

Any comments or suggestions would be much appreciated.


David ???

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Pin 8 of the IC should be 5.0V which is the output of the 78L05 voltage regulator or the 6V battery voltage. If you use the voltage regulator and D1 in series with it, then the minimum input voltage is 8.2V. Don't use a 9V battery because its voltage quickly falls lower.

The micromitter has an attenuator of 3 resistors at its output to reduce its range. Remove the attenuator for much greater range.

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Thanks for the INFO,

I am using a 9v DC Power Supply that actually measures 13.3v.  I don't want to use a battery as I need this to be broadcasting for 9 days.

I will take some further Measurements to try to determine why my voltage loss is occuring.

As far as the attenuator is concerned, if I remove the resistors one at a time will this gradually increase my broadcast range? eg remove resistor, gain 10M transmission distance.

I also found that by feeding my input through my mixer/amp I was able to increase my transmission distance out to about 12M.

As far as my antenna is concerned should I just use a single strand of wire approx 84.5cm long (I beleive this is the right length for 88.7Mhz) or should I use a different type of antenna? eg dipole

If I should use a dipole how should I attach it to the board? (which connection points should I use?)

Thanks for your help thus far.


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Hi David,
Maybe your 78L05 voltage regulator is installed backwards or is damaged.

Remove all 3 resistors of the attenuator for best range.

Your antenna's length is correct and if it is vertical then it radiates all around, then the receiver also must also have its antenna vertical.
A dipole antenna adds range and directionality to the broadside directions. There will be nulls in the directions the wires point. A balun is needed to connect to a dipole.

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Thanks again, the 78L05 could only be installed one way, according to the circuit diagram.

I will purchase another one and install it to see if it is damaged.

I wont bother changing the antenna till I have removed the attenuator and tested again.

Have you come across a kit that allows PC control of a FM Transmitter? (Frequency and attenuation adjustment?)


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