Unstable LC oscillator, RF.

Discussion in 'Wireless & RF Design' started by KarlE, Feb 13, 2012.

  1. KarlE

    Thread Starter New Member

    Jan 23, 2012

    Im working on a small tracking transmitter for my model airplanes, they often land in high gras and are hard to be found, and sometimes they just fly away and are extremely hard to find.

    I found a schematic for a small FM transmitter on the internet and went ahead and built it.


    It worked really nice, reasonably stable and rather good sound quality(considering). But this transmitter is in the comercial band, 90-107Mhz and i dont want to disturb people listening to radio. Also my only handheld radio is a Uniden Scanner that scanns 30-80Mhz.

    I rebuilt the oscillator connected it to an antenna and tried it out with an extra turn on the coil, and used a 47pf cap. I got the frequency down to 77Mhz. For modulataing the signal intend to use an AVR to get a nice "beep"

    But somehow the circuit is now wildely unstable, if i come anywhere nere the battery(4inches from it) it changes freq. downwards by 0.4Mhz. It was much more stable at the original 94Mhz, and touching the battery didnt do anything at all with the frequency. I use the same Decoupling cap of 22nf over the battery. Does someone know why the circuit is behaving this way?

    Last edited: Feb 13, 2012