What is common-mode? Interfering signals can flow in cables in two ways:Differential-mode currents and common-mode currents. Differential-mode is the mode used by the desired signal on a transmission line. The cure for the differential-mode interference as in a spurious emission is a low-pass filter. Common-mode currents exist just like differential- mode currents. Worse yet, common-mode emission causes the same RFI problems as in differential-mode emission. You cannot tell the cause, whether it is the differential or common- mode offense by looking at the symptoms of RFI. And therefore, common-mode filtering is an essential measure. The cure for the common-mode interference is a common-mode filter; it also prevents a feed-line radiation.

RF Inquiry announced the HI-Q COMMON-MODE Filter Series CF250E and CF5K in 1988. These HI-Q COMMON-MODE Filter Series are in use by many ham radio stations and broadcasting stations and solve many of their RFI problems world wide.

CF250E Filter

Price: $139.95

CF5KV Filter

Price: $209.95


Model # CF250E CF5KV
Application HF/6m transceivers HF/6m transceivers & amps
Operating Frequency (MHz) 1.8-70 1.8-70
Direction of Operation Bi-directional Bi-directional
Impedance (ohms) 50 50
Power Rating (continuous – watts)SWR<1.5 250 5,000
Common mode Attenuation (db) 50-60 (1.5- 250 MHz) 50-60 (1.5- 250 MHz)
Insertion Loss (dB) <0.1 <0.1
Connector SO-239 Teflon SO-239 Teflon
Size (inch) 10 L x 1 diameter 11 3/4 L x 2 1/4 diameter
Weight (lbs) 0.5 1.3

One Year Manufacturer's Warranty

Made in Japan.

Q: Why does the CF series "Common-Mode" Filter have miraculous effects on eliminating RF Interference?

A: RFI is composed of normal mode noise and "Common-Mode" noise. Recently the technology of radio transmitters has improved remarkably and leaks of normal mode noise have decreased substantially. But presently, countermeasures for "Common-Mode" noise are hardly improved.

An electronic circuit made by real world production techniques can not eliminate the generation of "Common-Mode" noise and still be cost effective.

This "Common-Mode" noise causes strong RFI to all electronic devices similar to normal mode noise. A Low Pass Filter reduces normal mode noise but does not have any positive effect on common mode noise, therefore, RF Interference can not be completely prevented by installing only a Low Pass filter, as in a typical installation today.

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Q: What kind of noise is "Common-Mode" ?

A: "Normal mode noise is the energy between a wire of a line, but "Common-Mode" is energy between a transmitter and the ground including everything which makes up the stray capacitance between the ground.

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Q: How come the "Common-Mode" noise is generated ?

A: The RF, without regard to a balanced or unbalanced line, a part of the essential energy is a signal changing its form from generation, conversion and transmission due to the discrepancy of line balance, stray capacitance and stray inductance between the ground or a surrounding object.

This becomes a main component of the "Common-Mode" noise. This value is variable but generally about -20dB. A transmitter can not stop the generation of "Common-Mode" noise since the transmitter does not work  as a perfectly balanced device and is inside of a metallic case, this includes a Low Pass filter for an unbalanced line.

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Q: What kind of RFI does "Common-Mode" noise cause ?

A: Basically "Common-Mode" noise causes the same RFI problems associated with normal mode noise. An example of "Common-Mode" noise is a spark and an electric field, similar to a bolt of lightening and static is generated between the ground and a conductor if the energy is large enough.

Q: Why doesn't a Low Pass filter always work?

A: A Low Pass Filter has no circuit to eliminate the "Common-Mode" noise. It expects to attenuate only normal mode signals inside of a Low Pass filter or the impedance which the transmission line has itself and the stray capacitance between the ground.

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Q: Where can "Common-Mode" noise be radiated ?

A: Coax cable(both sides including the center conductor and the shield), transmitter case, AC line, case of a Low Pass filter and antenna etc. "Common-Mode" noise is radiated by all lines which are connected to the transmitter but the Coax cable(both sides) has the largest amount of radiation.

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Q: What is the standard method to eliminate "Common-Mode" noise ?

A: Increase the impedance of the transmission line against "Common-Mode". For example, wind the coax cable like a coil and wrap it with magnetic material of moderate permeability. Your transmitting energy inside the coax does not decrease because the current flowing on both sides inner and outer of the coax cable is flowing in the opposite direction.

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Q: Can I reduce "Common-Mode" noise by making a device constructed of a ferrite core, like a choke.

A: Yes it is possible but the adjustments are extremely difficult and an expensive analyzer is needed to precisely design it. That's why the number of windings on the core and the magnetic material are closely related with the specific attenuation achieved on a specific frequency. If the number of winding is too high, there will be magnetic saturation. Also winding impedance causes complicated series resonance due to the capacitance between a wire of a line. Above 150MHz these complications become more severely. The CF250E and CE5KV keeps a stable high impedance and have very broad "Common-Mode" characteristics of -50dB to -60dB below 250MHz.

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Q: There is a Line Isolator available which looks similar to a "Common-Mode" Filter in the market. What is the difference ?

A: The Line Isolator is used to prevent stray RF surrounding the transmitter from entering the transmitter. (i.e like a "hot" mike or key) "Common-Mode" Filters like the CF250E and CF5KV are designed for maximum efficiency in eliminating the RFI of TVs, radio, phone and any other electronic devices. Therefore there is a great difference in their design purpose and functionality in eliminating interference.

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Q: Is the CF series "Common-Mode" Filter similar to a Float Balun ?

A: Both of these have same purpose to increase the series impedance of a  transmission line against "Common-Mode". Float Balun is designed to increase a large series impedance at HF/VHF of the transmitting frequency but CF series has this large series impedance at VHF/UHF of the TV broadcasting frequency range. Therefore the CF series "Common-Mode" Filter is effective more than the Float Balun with respect to RFI.

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Q: Is RFI reduced more effectively by a good ground connection?

A: There are many purposes for the ground connection. But great attention is needed when you connect to ground because it can have a harmful influence on RFI. Do you know there are many examples where RFI decreases when the ground is not connected? For example a simple GND wire of 2.5 feet long will become the 1/4ƒÉantenna of 105MHz where is the fifth harmonic of 21MHz. This is a fine antenna for "Common-Mode"  noise to be radiated from a transmitting case and there could be large RFI problems on nearby TVs at 105MHz. When connecting the GND terminal to ground, be sure to make this wire less 1 foot long. If it is not possible, wind the ground wire on a ferritecore to isolate the RF.

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Q: Where does the CF series "Common-Mode" Filter install ?

A: The Filter should be installed as close to the transmitter output as possible. If this distance is long, "Common-Mode" noise will be radiated.

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Q: What if I use both a transmitter and an amplifier ?

A: Install the CF250E after the transmitter and a CF5KV after the amplifier. This installation will eliminate "Common-Mode" noise more effectively.

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Q: Do I need a Low Pass filter if I install the CF series "Common-Mode" Filter ?

A: A Low Pass filter will eliminate the normal mode noise so using both filters together is the most effective method to reduce RFI. If a Low Pass filter is being used, be sure that the CF series "Common-Mode" Filter is installed after the Low Pass filter to reduce balanced/unbalanced line problems.

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Q: What should I do if RFI still effects TVs after installing the CF series "Common-Mode" Filter ?

A: Each TV is designed and manufactured differently and their abilities to defend against RFI is dependent on the TV design. Our best suggestion is to install a choke coil which has large attenuation characteristics at HF/VHF frequencies at the ANTENNA INPUT and around the AC line.

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