Simple Basic Theory around RF Chokes

Q&A on RF-Choke Theory

QUESTION:  Why do we need RF Chokes in our Antenna Systems?


ANSWER:  To prevent "Common-Mode-Current" (CMC) from flowing on the outside of our coaxial transmission lines or along our Balanced-Feedlines.

QUESTION:  What is Common-Mode-Current (CMC)?


SIMPLE ANSWER:  CMC is RF Current flowing where we don't want it to flow (i.e., on the outside of our coax).  When it does that, it can cause a multitude of problems when transmitting as well as when receiving.



QUESTION:  What kind of problems does Common-Mode-Current (CMC) cause?


ANSWER (RX):  CMC causes an increase in the noise level heard in the receiver.  This makes it difficult or even impossible to hear weak DX signals.  It may also cause fatigue when listening to strong signals.


ANSWER (TX):  We'll need a list for this:

  • CMC on the transmission line means that the transmission line is not only transferring power to the antenna, it is also radiating some of the power; thus it becomes part of the antenna. 
  • When the transmission line radiates, it can cause several problems, and some of them can be quite nasty:
  • Burning of the operator's lips on the microphone
  • Burning of the operator's fingers on the code key, metal skirts of knobs, equipment chassis, etc.
  • Eroneous (bogus) readings of instruments (i.e., Watt Meter, SWR Bridge, Antenna Analyzer).
  • In some cases it can cause equipment to fault (i.e., shuts down with fault lamp burning).
  • Radio Frequency Interference to consummer products, (i.e., Telephone, Stereo, Television, Computer, etc.).


QUESTION:  How does an RF-Choke help prevent Common-Mode-Current from flowing on the transmission line?


ANSWER:  Just as a Resistor reduces or impedes the flow of Direct Current (D.C.), a Choke reduces or impedes the flow of Alternating Current (A.C.) as well as RF Current. 

  • Radio Frequency (RF) is nothing more than high-frequency A.C. 
  • The amount of RF impeding the choke has, depends on its impedance, measured in Ohms. We call this impedance "Common Mode Impedance" (CMI)*
  • Though similar to a resistor in a D.C. circuit, the choke's impedance also depends on the frequency of the RF
  • Therefore, a given choke will have a given impedance at one particular frequency, but it will be different at other frequencies. 
  • When comparing chokes, we tend to look at them by ham band (i.e. 80m, 40m, 20m, etc.), not by frequency.
  • Thus, a Choke impedes or even blocks the flow of RF current through it. 
  • When the Choke is over the outside of the coax, it impedes or, when its CMI is strong enough, even blocks the flow of RF current on the outside of the coax.  This prevents the coax from becoming part of the antenna.

*Note: In U.S.A., BALUN and RF-Choke manufactures often state the impeding ability of chokes in decibel (dB), rather than in CMI.   Here in Europe, the CMI value is more often stated.     (So don't shoot me, I'm only the messenger.)







to be continued . . .





Remember:  Rome wasn't destroyed in a single day!