Low Pass Filter circuit?

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TrumpyNZ
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Joined: Thursday 30th Sep 2004, 9:34
First Name: Mike
Location: Ashburton, New Zealand

Low Pass Filter circuit?

Post by TrumpyNZ » Friday 29th Jan 2010, 11:31

Hi guys,
I'm looking to build a LP filter, to prevent my CB signal from splattering the neighbours, when I fire my set up this weekend.
Now, I realise these filters are essentially Pi-filters, but I've been all over the Internet and can't find a circuit diagram or any sort of formulae for calculating the components of the Pi-network. :cry:

At one time, you used to be able to BUY these filters, however with "electronics" stores the way they are these days, you're lucky if you can find PL-259 connectors. :x

Can anyone please help?

Thanks,
Mike. :)
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Cheers,
Mike. :)

lbcomms
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Joined: Monday 27th Feb 2006, 22:57
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Re: Low Pass Filter circuit?

Post by lbcomms » Thursday 4th Feb 2010, 21:54

A simple pi-filter (also known as a band pass filter) is unlikely to help you - unless your own gear is so "dirty" that you shouldn't be putting it on air in the first place! In 99% of cases of interference to TV, radio, audio systems, telephones, etc it needs to be tackled at the device being interfered with, and NOT the source of the RF.

It could be as simple as a coaxial stub filter or a high pass filter in the case of TV. With the other things mentioned above, a ferrite rod sometimes helps, but it often means adding components inside the gear by someone who knows exactly what they are doing. The cost can often exceed the new price of the item, and invalidate the warranties...

I did two for the neighbor of a a local amateur operator a few weeks ago - a "5.1" computer speaker set (cost $89 new, and an extra $100 for the mods) and a home theater surround system (cost $300 new, plus $250 more for mods). The amateur paid for the interference eliminating mods and had to agree to cover the cost of repairs if the items failed under warranty.

Luckily for him, the neighbors got along together, in most cases it's not that easy.

Hope this helps,
Sue

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