🚀 go-pugleaf

In sci.electronics, bunky@milton.u.washington.edu (Mike) writes:



> I've been trying to build a notch filter to filter out unwanted interference
> on my tv but I can't seem to make one with a high enough Q.  Does anyone
> have any designs that would work? (frequency over 100 Mhz with a Q about
> 25 or 30)


> thanks       
> Mike

Simply wind a coil on a non magnetic form (such as a low loss plastic tube)
with enameled wire, closely spaced with enough turns to achieve self-resonance
at 100 MHz. (or wherever you want).  The easiest way to tune it is to start
with too many turns, such that the resonant frequency is to low, and glue
the wire to the form with Q-dope.  After the glue hardens, you can
slowly peel off the wire while rechecking the resonant frequency after each
peel and you can get the frequency set very accurately.  It is important to
use glue because it will stabilize the turn-to-turn capacitance, which would
be affected by movement of the wire.  Wire gauge is not critical, anything from
18 to 24 is probably OK.

I made a 56 MHz. notch filter this way to prevent my ham radio from wiping
out TV channel 2 (54 to 60 MHz, video carrier at 56.25 MHz.) when operating
on 28 MHz.  It worked extremely well.

Be careful to avoid lossy plastic forms (like nylon or PVC) or lossy
glue such as epoxy.  You might not get a loaded Q of 30 in that case.


Rick Karlquist
rkarlqu@scd.hp.com