fm trap
 

G'M group,

I recall you could make a fm filter trap using a 300 ohm ribbon cable at some
length and put a cap on the end.

I am looking for something like this but would be more for channel 8/9. Or
would I be just as well to put a parallel L/C ?

What bandwidth and db rejection would I get with one L/C network? Would need
like 10MHz of BW.

de KJ4UO.

I recall you could make a fm filter trap using a 300 ohm ribbon cable at some
length and put a cap on the end.


An electrical half wavelength with a shorted end or an electrical quarter
wavelength with open end are effective traps. Their bandwidth is rather
narrow.
What bandwidth and db rejection would I get with one L/C network? Would need
like 10MHz of BW.

For that type of bandwidth, I would think a single section L/C would not work.
Probably need multiple sections.

73 Gary N4AST

Getting a decent amount of rejection across one or two TV channels takes a
multi-pole filter. You might check out the 5KV series from Microwave Filter
Company http://www.microwavefilter.com/multichannotch.htm . They are well
made and not too expensive.

RF

Visit http://rfry.org for FM broadcast RF system papers.

On 01 May 2004 20:58:32 GMT, (PDRUNEN) wrote:

G'M group,

I recall you could make a fm filter trap using a 300 ohm ribbon cable at some
length and put a cap on the end.

I am looking for something like this but would be more for channel 8/9. Or
would I be just as well to put a parallel L/C ?

What bandwidth and db rejection would I get with one L/C network? Would need
like 10MHz of BW.

Do you mean television channels 8 and 9?

I'm kind of guessing at what you're wanting to do, but the easiest
trap is the 1/4 wave open stub. If you're trying to kill off
TV signals, tune it to the video carrier frequency. The length of
the stub will be a physical 1/4 wave times the velocity factor
of the cable you're using (close to 1 for twin lead). Start
long, and trim off just a bit at a time till you get it right.
(That's hard to do, because you don't really know you've got it
right until you trim it off too short and realize you've gone
past your desired notch frequency). Better is to cut it a bit
short and put a trimmer capacitor across the end of the stub,
then adjust the cap for maximum attenuation.

The notch will be very sharp, so you'll need seperate traps for
each channel.

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