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Acoustic feedback on FM transmitter

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New Member
I have just recently modified a small FM transmitter which I have built to hold an external microphone, the system does work, but when I hold the external mic away from the transmitter, sometimes, the reciever starts emitting a high frequency feedback like sound, but if I touch the mic, or even bring my hand near, or hold it up to the transmitter it goes away, the internal mick(Half in the baox I.E) does not have any problem, presumably because it is near the transmitter circuit, also sometims holding the mick away from the transmitter a little farther stops it, neither mics have any shielding, can anyone help me with this?

Thanks, SlicRic
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It's good practice to bypass the mic and it's cable for RF. You might use a ferrite bead or choke in series with the mic and a .001 capacitor to ground at the xmitter side of the choke.
I hope your transmitter is in a shielded metal enclosure. Put these components at the mike input socket - not at the mike itself.

Since your transmitter is very low power this might not be the problem. Your audio circuits should also have proper bypassing for audio and RF.

Good luck.


New Member
Thanks Flat 5 I will try a ferrite bead, but, what was the problem in the first place, wht can I do to try and avoid it, also why is it that when I bring the Mic close to the transmitter, touch it or even hold it farther away the feedback seems to disappear?


slic, please don't duplicate posts.
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