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Keithinatlanta
Advanced Member
Username: Keithinatlanta
Post Number: 959
Registered: 3-2003
| | Posted on Saturday, January 19, 2013 - 7:16 pm: | 
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What is the purpose of a low pass filter? Copper sent me an e mail about one on sale. Thanks.
Keith in Atlanta
CEF 150 |
Tech237
Moderator
Username: Tech237
Post Number: 1644
Registered: 4-2004
| | Posted on Sunday, January 20, 2013 - 5:39 pm: |
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Low pass filters do exactly what the name says - stops or reduces all frequencies ABOVE a predetermined (by design)frequency. IN Ham and CB they usualt cut-off around 30 MHz.
The idea is to reduce (hopefully stop completely) the any unwanted signal above the cut-off in order to reduce the risk of TVI from a harmonic.
They need to be close to the radio (I used a back to back male connector), and if you have an SWR mete and/or amp another between that and the antenna is sometimes a good idea.
Hope this helps, not only Keith, but others who may not have beensure.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Tech833
Moderator
Username: Tech833
Post Number: 2237
Registered: 8-2002
| | Posted on Tuesday, January 22, 2013 - 12:46 pm: |
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Type accepted CB transmitters have some harmonic suppression built in. However, they are not always as effective as one needs them to be.
Adding an additional lowpass filter to the CB transmitter will help reduce harmonics and spurs above cutoff freq. In addition, a lowpass filter works both ways (there is no specific INPUT and OUTPUT port) so a lowpass filter can also help reduce noise in your CB receiver from external sources.
For example, there is an FM broadcast transmitter very close to my home. The SW and amateur receivers pick up noise from the nearby FM band transmitter without a lowpass filter installed. With the filter installed, it is as if the FM transmitter doesn't even exist.
Like Simon mentioned, the lowpass filter should be installed as close to the radio as possible with nothing between them except a short piece of coax cable or connector as Simon uses. If using an amplifier (as in ham installations), the lowpass filter should go immediately behind the amplifier.
I did a review of the filter that is currently on sale. See the review for much more detail and tips on its use.
Your radio 'Mythbuster' since 1998
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