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.
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. 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 Filters
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$79.95
$79.95
Iron powder cores are more stable than ferrites and do not saturate as easily so they are best for tuned circuits, filters, and high power inductors. They come in different “Mixes” for use at different frequencies. For best “Q” use the Mix specified for your frequency.To find the number of turns to give the desired inductance for your coil use the formula below:
Turns = 100 x 
Toroid Dimensions
To order specify core size and mix (e.g. T200A-2). Available in sizes that have µH per 100 turns shown in table.
All iron powder cores are special orders. Please allow one week for shipment after order received.
The iron powder cores shown in the above tables are primarily used for either high power feed line chokes for common mode current reduction or for 4:1 and 9;1 very high power transmission line transformers.
For high power feed line chokes, choose the inside diameter of the iron powder core closest to the transmission line outside diameter and use 6-20 cores at the junction with the antenna feed.
For 4:1 (50/200 or 75/300 ohm) transmission line transformers in the range of 1.8-30 Mhz, select the size appropriate for the power level as follows:
T200A x 2 = 1.5KW
T300A x 2 = 2.5KW
T400A x 2 = 5KW
Use Mix 2 for all the above transmission line transformers. Fully assembled and enclosed units are available for each of the power levels. Click HERE for information.
4:1 transmission line transformers using iron powder cores are typically used for ladder line to 50 ohm coax connections
For .1 – 2 Mhz wide band transformers use Mix 26 which has a higher permeability and requires less turns to achieve a given inductance. Mix 2 has almost constant permeability from 1-30 Mhz. Mix 26 has an initial permeability of 75 at .1 Mhz and gradually decreases to about 10 at 30 Mhz. For high power AM broadcast range (.5-1.7 Mhz) transformer, use Mix 26 to reduce the turn count.
Minimum order for iron powder cores is $50
Shipping on larger cores in quantity may be more than $10 – email/call for quote.