Coax Noise Filters
The Problem:
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 second receiving antenna, picking up common mode noise signals that combine with weak signals (traveling down the center conductor and inside of the coax braid) making it difficult or impossible to hear weak signals! See diagram below.
The Solution:
A coax common mode noise filter with high choking impedance at the receiver/transceiver end of the coax effectively reduces common mode noise present on the coax braid while passing desired signals present on the coax center conductor. For best results, and to reduce co-interference between choked and un-choked feed lines with multiple antennas, EACH antenna feed line entering the radio room needs to have a coax noise filter even if it feeds a common coax switch since most antenna switches ONLY switch the center conductor of the coax line, yet all the coax braids are ALWAYS connected together in parallel.So when an antenna is NOT SELECTED, its coax sheath still contributes to the noise level of the connected antenna through the common connector ground!
The solution? A common mode noise filter with high choking impedance at the receiver end of the coax.
Do you have common mode noise on your coax?
Make this simple test to find out.
- Remove the coax connector and measure the noise level.
- Now insert the coax connector CENTER CONDUCTOR ONLY into the SO-239 antenna input and measure the noise level (it should be higher and include possible signals)
- Now connect the OUTER SHELL of the coax connector to the antenna input and measure the noise level.If it is higher you have common mode noise and the common mode noise filter will help suppress this noise which is carried on the outside of the coax braid (acting as a second receive antenna).
Common mode noise suppression with the Palomar CMNF series of filters is typically 25-36 dB which is equivalent to 4-6 “S” units on radios with 6 dB/”S” unit or may be more on radios with 3dB/”S” unit.
Note for antenna switch users, perform the above test on each antenna coax lead separately to determine if that particular antenna braid is contributing noise to the receiver – use a coax noise filter on the antennas that have common mode noise.
Coax Common Mode Noise Test (pdf)
Coax Noise Filter ManualRefine by
Coax Noise Filters
The Problem:
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 second receiving antenna, picking up common mode noise signals that combine with weak signals (traveling down the center conductor and inside of the coax braid) making it difficult or impossible to hear weak signals! See diagram below.
The Solution:
A coax common mode noise filter with high choking impedance at the receiver/transceiver end of the coax effectively reduces common mode noise present on the coax braid while passing desired signals present on the coax center conductor. For best results, and to reduce co-interference between choked and un-choked feed lines with multiple antennas, EACH antenna feed line entering the radio room needs to have a coax noise filter even if it feeds a common coax switch since most antenna switches ONLY switch the center conductor of the coax line, yet all the coax braids are ALWAYS connected together in parallel.So when an antenna is NOT SELECTED, its coax sheath still contributes to the noise level of the connected antenna through the common connector ground!
The solution? A common mode noise filter with high choking impedance at the receiver end of the coax.
Do you have common mode noise on your coax?
Make this simple test to find out.
- Remove the coax connector and measure the noise level.
- Now insert the coax connector CENTER CONDUCTOR ONLY into the SO-239 antenna input and measure the noise level (it should be higher and include possible signals)
- Now connect the OUTER SHELL of the coax connector to the antenna input and measure the noise level.If it is higher you have common mode noise and the common mode noise filter will help suppress this noise which is carried on the outside of the coax braid (acting as a second receive antenna).
Common mode noise suppression with the Palomar CMNF series of filters is typically 25-36 dB which is equivalent to 4-6 “S” units on radios with 6 dB/”S” unit or may be more on radios with 3dB/”S” unit.
Note for antenna switch users, perform the above test on each antenna coax lead separately to determine if that particular antenna braid is contributing noise to the receiver – use a coax noise filter on the antennas that have common mode noise.
Coax Common Mode Noise Test (pdf)
TECH NOTE: We also stock RFI kits to protect effected devices such as garage door openers, computers (laptops, desktops), dsl/cable routers, ethernet hubs and many more devices. If you run a high powered RF amplifier in your ham station also consider a linear amp RFI kit which will cut down RFI transmitted by your station. Remember there is always a “transmitter” and a “receiver” of RFI and the quest is to find and choke the “path” the RFI has selected – you need to add ferrites to both the transmitting side and the receiving side to eliminate RFI problems. The RFI-HTS kit only helps suppress RFI on the receiving side.
Typical RFI Problem/Solution:
Hello, I am Larry, a ham operator and I am having interference issues with my Direct TV system. My station is a FT450D through an Ameritron ALS-600. From the amp through a coax switch to choose between an OCF dipole or an Hustler 5BTV vertical. Any power over 100 watts and my DTV goes off the air and must completly reboot/format. Coax is RG-8. The DTV dish is pointed away from the dipole and separated by about 25'. DTV was totally useless! They suggested I not talk on the radio while she is watching TV! Please help me with a reasonable, cost effective soultion. (The wife gets really pissed when I shutdown the tv!) Thank you, Larry
Hi Larry,
Your RFI problem is quite common and is usually a sign of either a radiating coax feed line or reception of your radiated signal (source) by the Direct TV (the victim) through either the AC power lines acting as “antennas” for your ham signal or the coax braid of the satellite antenna acting as an antenna for your signal.
The solutions are straight forward:
1. To make sure your feed line is not radiating you need a feed line choke at each antenna feed point (also makes your antenna work better since it is not using the coax as part of the antenna). You can use our simple Kit 110 with 5 snap on chokes on each antenna for this purpose. Part # Kit 110 at $27.50 each – one for each antenna. These chokes will keep your feed line from radiating and coupling into your ac power lines. INFO HERE
2. The radiation from your antennas themselves may also be getting into the ac power lines and coax braid of the Direct TV antenna so you need to protect the victim receiver. Depending upon the sophistication of your direct TV setup (e.g. home theater, dvd, tape player, sub woofer speakers, etc), the path of the RFI may be coming thru the AC power lines of any connected device to the video including the video monitor itself! To help solve this problem we have a home theater RFI kit for up to 5 devices (you need a minimum of two – one for the Direct TV receiver(AC and coax input) and another for the video monitor AC power lines and HDMI or video input cable. Our kit part # is RFI-HTS. INFO HERE
These items should help reduce the RFI and let you operate on the air while your wife watches TV.
Bob Brehm, AK6R
Chief Engineer