FM bug

[victor]

But there's also something odd if I increase from 5 to 6 turns my inductance increase but  increasing it further to 7 turn the inductance drops 1 order. Is that a glitch in the calculator ??

Turns
4 = 4.3uH
5 = 6.7uH
6 = 9.7uH
7 = 0.133uH

No, it's not a glitch in the calculator. As was mentioned by guitchess you are misreading the output. When the calculated result says "Inductance = 6.788E-002 microhenries" this means "6.788 x 10^-2 µH" or "0.06788 µH" or "67.88 nH".

You can't ignore the "E-002" bit, it is a really important part of the number.

Your table is:

Turns
4 = 0.043 µH
5 = 0.067 µH
6 = 0.097 µH
7 = 0.133 µH

Thanks, I didn't noticed the scientific notation there. I will make some changes, and try again. I found another transistor and a 4~40pF trimmer cap I will use it instead.

[mikerj]

Such a simple single transistor oscillator isn't very good for FM broadcast. The sound quality will be poor because they're no pre-emphasis. To cut the hiss, the treble part of the audio signal is boosted by the station and all modern FM receivers cut the treble to reconstruct the original signal. As a simple FM bug doesn't have treble boost, the high frequencies will be cut by the receiver, making it sound muffled. Add a pre-amplifier with a treble boost filter; the corner frequency depends on the region you live in - look it up.

Reduce the coupling capacitor value to provide low frequency cut instead

[Zero999]

Such a simple single transistor oscillator isn't very good for FM broadcast. The sound quality will be poor because they're no pre-emphasis. To cut the hiss, the treble part of the audio signal is boosted by the station and all modern FM receivers cut the treble to reconstruct the original signal. As a simple FM bug doesn't have treble boost, the high frequencies will be cut by the receiver, making it sound muffled. Add a pre-amplifier with a treble boost filter; the corner frequency depends on the region you live in - look it up.

Reduce the coupling capacitor value to provide low frequency cut instead

That will make it sound even worse than before.

[janoc]

Extremely low-power FM transmitters in the licensed FM broadcast band are legal in the EU since a few years.

Umm, that made me curious - do you have any pointers?

I was under the impression that anything broadcasting in these broadcast bands required a specific license, period. The power output was irrelevant, there was no exception for these low power devices. I am quite sure this is still the case in Slovakia where I am from. On the other hand, Slovak rules tend to be incredibly backwards compared to the rest of the civilized world ...

Correction - I think I have found a recent change: It seems that the law was indeed changed in 2012 (implementing an EU resolution 2010/368/EU from 30.6. 2010) and if the ERP < 50nW, license is not required in the FM band. However, there are two gotchas - first, wireless microphones are explicitly banned and second, this sort of "bug" can certainly exceed 50nW without special attention ... Also there are some explicit requirements about what the device needs to satisfy, I am pretty sure that rules out any "homebrew" stuff without the right paperwork.

But you can still get into trouble for violating other regulations, like lack of certification, or interfering with aircraft communication. Or, an extremely nasty one, if there is a local law against bugs, listening devices, surveillance devices etc.

I didn't want to get into this, but yes, it is obviously illegal to bug someone or cause interference due to poorly tuned/shielded oscillator (air bands are right next to the FM broadcast band in many places!).

To conclude, if you want to play with radio, the best way is to get the basic HAM radio license and then you can play legally in amateur bands. All that takes is passing a simple test and paying the fee for getting the paperwork issued.

J.

[mikerj]

Such a simple single transistor oscillator isn't very good for FM broadcast. The sound quality will be poor because they're no pre-emphasis. To cut the hiss, the treble part of the audio signal is boosted by the station and all modern FM receivers cut the treble to reconstruct the original signal. As a simple FM bug doesn't have treble boost, the high frequencies will be cut by the receiver, making it sound muffled. Add a pre-amplifier with a treble boost filter; the corner frequency depends on the region you live in - look it up.

Reduce the coupling capacitor value to provide low frequency cut instead

That will make it sound even worse than before.

It's a 1 transistor radio transmitter, why are you so hung up on the audio quality?

[nuno]

I also built a few when I as a kid, none in a PCB. Used "terminal bridges". The cricuit I used to build is more like this one here:

http://www.afiata.com/micro-transmitter-fm/#.VFAtVFcog1I

It uses a high frequency transistor, BF494 and higher supply voltage; apparently it had an open space range of about 100m. The coil was 4 or 5 tuns with 1cm diameter (round them around a pencil or pen). R1 in the schematics is only needed if you have a real eletret mic; I used a piezo so no need for R1.
Once tunned (preferably on the radio ) I couldn't even approach it too much.