Understanding the Power Handling ability of Common Mode Chokes

The power handling ability of Common Mode Chokes is one of the most difficult parameters to tie down.  Why?  Because the power dissipated in a Common Mode Choke can NOT be determined based only on the applied power and the characteristics of the choke itself.  It is determined by the system in which the choke is used.    

One way to think about this is to realize that everything in an antenna system is a reactive element.  The transmitter, the feedline, the antenna, the matching network, and the counterpoise (or current return) are each reactive elements.  Each element has its own complex impedance.  It is a big tuned circuit.  The choke is also a reactive element.  When we drop a Common Mode Choke into that tuned circuit we change it and the resultant tuned circuit determines the currents seen by the choke.  Those currents will be different that they would be in a purely resistive system and it is those currents that determine how much power is dissipated in the choke.  

A choke maker cannot know all of these system factors in advance and so cannot say with certainty how much transmit power can be used in a particular antenna system that includes the choke.  The most that can be done is to measure the dissipation capacity of the choke under some set of defined constraints.  We can establish a starting point by measuring it with a balanced resistive load.  This provides a useful metric for comparisons between chokes, but it is still inadequate to predict the exact maximum power that the choke can handle in a specific antenna system.  Final determination of this maximum power level requires adjustment based on measurements of the choke's core temperature when installed in the target antenna system.  It is possible that extensive measurements and simulations of a particular antenna system could predict this max power rating.  Perhaps some users could perform this simulation work.  But it is not practical on even a modest production scale. 

What limits the power handling ability of a Common Mode Choke?

The power handling ability of a Common Mode Choke is limited by the temperature of the ferrite core, or the voltage and current limits of the transmission line (coax) used to wind the choke, or the voltage and current limits of the coax connectors.  Of these, the temperature of the ferrite core is by far the most important and the most likely to be dominant. 

The temperature of the ferrite core is determined by the total power dissipated in the choke, which is a combination of Inherent Loss and Common Mode Core Loss.  

Who controls what?

The choke does not control its own destiny.  It is not like a resistor with DC applied to it, wherein the voltage, current, and resistance can all be known deterministically.  When RF is applied to a Common Mode Choke in a system comprised of multiple complex impedances, things are much different.  Predicting the voltage and current at each point in the network is much more difficult and generally can only be determined through extensive measurements and/or simulations.

The choke design itself controls:

• Inherent Loss, i.e. how lossy the coax used to wind the choke is, and
• The size and ventilation of the enclosure, and thus the thermal resistance of the choke
• The complex impedance seen by common mode current.

The system in which the choke is used controls:

• The amount of power that is dissipated in the choke, and
• The amount of power applied to the choke, and
• The magnitude of the differential current (the signal) seen by the choke, and
• The magnitude of common mode current seen by the choke.

Inherent Loss is dissipative loss caused by the differential RF current flowing through the choke to the antenna.  It is the inherent loss of the coax used to wind the choke.  Coax loss increases with frequency, so it is always maximum on 10 meters and minimum on 160 meters.  This loss is very small - less the 0.1 dB - and does not affect signal levels in any discernable way.  But it is a factor in heating of the choke.  

Inherent loss is affected by SWR.  A ratio greater than 1:1 can help or it can hurt.  Any ratio greater than 1:1 causes standing waves, with current peaks and valleys located along the full length of the feedline.  These peaks and valleys move around with frequency.  The choke could be located at a current peak on one band and at a current valley on another band.

The magnitude of current seen by the choke, and therefore the power dissipated in the choke, is determined by the choke's location in this standing wave.  Dissipated power will be increased (relative to a perfect match) when the choke is located at or near a current peak.  Dissipated power will be decreased when the choke is at or near a current valley.   

Common Mode Core Loss is the power dissipated in the choke due to common mode current flowing on the coax shield.  It is a function of the magnitude of the common mode current that the choke has to deal with, which will be different for each particular antenna system.  As a result, we cannot specify in advance the extent to which Common Mode Core Loss will contribute to heating of the choke unless we obtain rather extensive information about the antenna system through measurements or simulations.  To make matters more interesting and even less deterministic, this loss is also affected by SWR.

If your system generates a lot of common mode current, it will be necessary to derate the Power Limit Guideline curves we provide.

The Good, the Bad and the Ugly.  Antennas, that is...

Antennas such as dipoles and yagis generally generate the least common mode current.  These are The Good antennas (from a common mode current perspective).  They often still generate enough common mode current to cause bad RFI.  As a result, such systems benefit greatly from Common Chokes and generally do not cause excessive heating in Common Mode Chokes.  

Unbalanced antennas such as end feds (whether EFHW or EFLW), Off Center Fed (OCF) dipoles, or 1/4λ verticals often generate noticeable amounts of common mode current.  These antennas always need a good Common Mode Choke.  These antennas can be very effective, but they do generate more common mode current.

Antennas that intentionally use the coax shield as part of the antenna, such as Common Mode Radiators and endfeds that use the coax shield as a counterpoise, cause the most common mode current.  These are certainly the Ugly players in ChokeWorld.  We would like to see these antenna designs avoided because they put large currents on the shield which will overheat virtually any Common Mode Choke.

Which loss will dominate?

Common Mode Core Loss will dominate heating of the choke if high common mode current levels are present.  The Power Limit curves we provide must be derated if your system generates a lot of common mode current. 

The only way to know how much common mode current your antenna system generates is to actually measure it.  This can be tricky and we recognize that many people will not have the equipment or inclination to attempt this measurement.  Unless your antenna is grossly unbalanced or intentionally puts a very large amount of common mode current into the choke (such as a Common Mode Radiator design), we recommend that you go ahead and try it.  In most cases common mode current will not be high enough to cause unreasonable heating of the choke.  If it does, you always have 30 days to return the choke.

A real world example:  A system comprised of a 178 foot end fed long wire (EFLW or EFRW), a 9:1 impedance transformer (balun), a 16 ft counterpoise about 7 feet AGL, and 85 ft of LMR400 coax was tested by running continuous FT8 at 1500 watts (at the transmitter).  This type of antenna is highly unbalanced and is reputed to generate significant levels of common mode current.  In fact, severe RFI was noted before installation of one of our Black Beauty "Broad" 160-10 choke at the coax side of the balun.  After such choke installation, RFI in the shack and the household was greatly diminished and no significant heating of the choke was noted.  However, some RFI was still evident in certain equipment, such as the subwoofer of a home theater system.  Installation of a second identical choke at the point where the coax entered the house removed all trace of RFI.  No significant heating of either choke was noted during extended operation.  However, the Balun Designs 9:1 transformer used initially got so hot that its plastic enclosure began to melt and became severely distorted.

Black Beauty choke enclosures are well ventilated and enable the choke to handle about 80% more power than is possible with a sealed enclosure. 

Nevertheless, the choke dissipates some power and experiences a temperature rise that should not be ignored.  

What factors can I control that will influence heating in the Choke?

The use factors that affect power dissipation in the choke are:

1. The average power applied to the choke (PEP transmitter output minus Feedline loss) times the Duty Cycle, 
2. The ambient air temperature outside the choke's enclosure,
3. The frequency of operation.

Duty Cycle is determined by the mode of operation.  Some of the common ones are summarized here:

     Continuous Uncompressed SSB normal speech:  25%

     Continuous Highly Compressed SSB:  50% 

     Conversational Highly Compressed SSB:  25-30%

     Contest "Run Station" Highly Compressed SSB:  30-35%

     Conversational CW:  20-22%

     Continuous CW (English):  40-45%

     High speed CW contest "run" station:  30% 

     Continuous FT8 or FT4:  43%

     Continuous RTTY:  100%

     Conversational RTTY:  50%

     Long winded RTTY:  100%

     Continuous AM:  100%

     Long winded AM:  100%

The baseline power handling ability for several Duty Cycles is shown in the "Power Limit by Band and Mode" charts that we provide.  Use the buttons below to access the charts for the choke of interest.  These charts only take into consideration Inherent Loss and are based on an SWR of 1:1.  They can safely be used directly as long as the SWR is under about 2:1 in systems with modest levels of common mode current.  However, they must be derated for use in systems that generate high levels of common mode current.  Higher SWR levels can substantially increase or decrease the Power Limits shown in these charts.

Use the chart that applies to your intended mode of operation.  Then find the curve for the band of interest and you can readily see how much power you can run at any given ambient temperature. 

Here's an example for FT8 on a 70°F day on 20 meters. The intersection of 0°F and the 20 meter curve indicates a max recommended power limit of about 1350 Watts.  Remember that this is the power incident at the choke.  Given the usual coax loss from the rig to the choke, this probably means that running 1500 Watts output at the transmitter would be fine. 

Remember that this guideline power level may have to be adjusted up or down based on common mode current magnitude, SWR, and the choke's location along the standing waves in the feedline.

If there is no chart for the exact Duty Cycle your are interested in, use the chart for 100% Duty Cycle and uprate it for the that Duty Cycle.  Do this by dividing the power guideline from the chart by the Duty Cycle (expressed as a decimal fraction).

For example: assume you are interested in a Duty Cycle of 31% and you want to use the Black Beauty "Broad" 160-10 choke on a 100°F day on 15 meters.  First, find the intersection of this temperature and band on the 100% Duty Cycle Chart for this particular choke, as indicated by the red lines below:

This indicates that the power limit guideline for that band and temperature is about 430 Watts.  

Now uprate this for the Duty Cycle of 31% Duty Cycle (0.31) and we see that we can run about 1390 Watts at the choke (430/0.31=1390).  This is 0.33 dB down from full legal (U.S.) power of 1500 Watts.  So, as long as the coax between the transmitter and the choke has at least 0.33 dB loss, we can run the full 1500 Watts output at the transmitter.  

These charts provide a useful metric for comparison of one choke to another and provide a baseline for the maximum power we can expect to use while avoiding overheating of the choke.

However, this is just a guideline.  It must be adjusted for SWR and common mode current.  The max power that can be used may end up being higher or lower than this guideline, depending upon the the location of the choke in the standing waves that exist on the feedline.  These are factors that are very difficult to determine in advance and generally the final answer will be found following some experimentation and temperature measurements.

We recommend operating at your best estimate of maximum power while measuring the temperature of the ferrite core inside the choke with an IR thermometer by "looking" up through the vents.  Adjust the power, either up or down, accordingly.  

These plots are conservative and will keep the core temperature below 175F. Under no circumstance should the core temperature ever be allow to reach 230F (12% lower than the Curie Temperature).  This temperature will begin to damage the core and the enclosure.  The margin between these two temperatures is what provides a measure of conservatism.