Common Mode Noise Filter – Coax

Coax Common Mode Noise Filter

Common mode current induced by radiated sources (plasma TV, routers, computers, transmitters, etc) can be picked up by the outside of the coax braid from the antenna feed point back to the receiver. This portion of the coax braid acts like a receiving antenna, picking up common mode noise signals that override weak signals making them difficult or impossible to hear!

The solution? A common mode noise filter with high choking impedance at the receiver end of the coax.

Application Note: For coax lines with receive and transmit signals, we suggest the CMNF-1 or CMNF-2 with SO-239 connectors in and out which are connected in series with the coax feed line to the receiver/transceiver effectively reducing common mode noise existing on the coax.  For best results, and to reduce co-interference between choked and unchoked feed lines with multiple antennas, ALL antenna feed lines entering the radio room need to have a feed line choke particularly if they all feed a common coax switch.  Choking only 1 line if you have 5 antenna coax feeds will do little good – you need to choke all five – use the feed line chokes  for the individual feed lines and the CMNF between the antenna coax switch and the receiver.  All CMNF models are SO-239 input and output.  Adapters are available for converting to “F”, or “N” connectors.

The CMNF-1 is meant to be placed in the coax line entering the transceiver or receiver (between the transceiver and amplifier or antenna tuner/coax switch if used).  The CMNF-1 has a high choking impedance from 1-60 MHz with higher choking on the lower frequencies where noise is usually a larger problem then on higher frequencies.  Convenient SO-239 input and output connectors all easy insert in series in your coax feed line.  Use a short jumper from the CMNF-1 to your radio and plug the feed line into the other SO-239 CMNF-1 input.  Either connector can be input or output.  Rated at 500 watts PEP – if you have a higher powered transceiver, consider the CMNF-2 or CMNF-3.  Note that the CMNF’s work best closest to the receiver.

Installation Note: Putting a CMNF at the output of your antenna tuner lessens the choking effectiveness since the impedance may be much higher or lower than the 50 ohm design impedance of the noise filter.   The best place to install the CMNF is between the antenna tuner/linear (high power CMNF-2 or CMNF-3) or closest to the receiver/transceiver (CMNF-1).  See diagram below.  Use sleeve or CUBE™ feed line chokes at the antenna feed point and radio end of the coax to reduce RFI and common mode noise.



Typical user comment:

The filter was quite effective. Was surprised that it actually reduced the noise level
more than the signal so S+N/N improved by 1.9 S units.