Damping Higher Harmonics on a parallel LC-Oscillator? (1 port Low-,High-, Bandpasses?)

[kb]

Hi,
I built a small FM Radio as shown on
http://members.aol.com/torkraemer/ukwsender/ukwsender.html.
As it is it works pretty well, allthough I operate it without any
adition antenna.

Of course there are pretty high (unwanted) peaks in the frequency-band
at multiples of the operating frequency.
Because I don't want to use an additional antenna, I can't filter the
signal on its way from L1 to the antenna with a common "2-port
Bandpass".
Putting such a Bandpass (or Lowpass) between the DC-Source and the
LC-Circuit did not seem to work well.

So how can I damp those harmonics on L1 Coil? I thought of some kind
of frequency dependent impedance in parallel with the Coil.

I tried this with another LC-prallel Circuit which has an impedance of
approx. 0 Ohms at 200 Mhz. This works as expected but seems a bit
unsophisticated and the Impedance falls too slow as the frequency
approaches the 200Mhz.

Unfortunately I have no Idea how to design a "higher order
1-port-Bandpass", or even more likeably a Highpass.

What would you recommend?
What shoud I read?
Any suggestions are welcome.

Thanks to all, and sorry for the lenght of the message.

Diego S.

 

[Skeptic]

How high are the harmonics relative to the carrier?

Your layout looks pretty spread out. All the connections in the RF
portions of the circuit should be as short as possible. It's impossible
to tell by the photo whether you are using a ground plane. If not, you
should be. The power connections should be close to the RF section and
bypassed with a capacitor that has a very low reactance at 100 MHz. It
won't hurt to try bypassing the power supply at other places too,
particularly around TR2. Another thing you might try is moving the
connection of C1 from TR1-R1 to the other side of R1 (ground). The use
of a resonant antenna may also reduce harmonics.

jgreimer