SWR greater than 1:1 will affect how much power a Common Mode Choke can handle because it affects the Insertion Loss. In some cases, it actually increases the recommended Power Limits! But in some other cases, it increases Insertion Loss and we must derate our recommended Power Limits accordingly.
There is no derating needed for operation on 20 meters or any frequency that is lower than 14 MHz.
However, for the bands 17m, 15m, 12m, and 10m, a high SWR means that we must lower our recommended Power Limits.
The table below shows how much the Power Limit should be reduced as a function of SWR:
To use this data, refer to the Power Limit Charts for the choke you are using and find the chart for the operating mode of your choice. Then pick the appropriate ambient temperature and band of operation. Whatever Power Limit you then identify from the chart should be reduced by the percentage shown in the table below for your frequency of operation and the SWR that the choke sees.
Example: Suppose are using Black Beauty "Broad" 160-10. Further suppose that you are planning to operate on 15 meters with an SWR of 3 : 1 and the air temperature around the choke is 80 °F. Go to this set of Power Limit Charts (which are for the Black Beauty "Broad" 160-10 choke). Select the chart for FT8. Find the 80°F line along the horizontal axis and follow it up to the 15 meter band line (which is bright pink). You will find that the recommended Power Limit under these conditions is 1150 watts.
Now find the appropriate derating percentage in the table below that corresponds to SWR = 3 : 1 on 15 meters. We see that the derating is 8.1%. So derate the 1150 W recommended Power Limit (which is for a perfect SWR match of 1:1) and knock off 8% of that power. This leaves us with a new recommended Power Limit (as adjusted for SWR) of 1058 W. This is a fairly small adjustment and can probably be safely ignored or treated casually.
However, on the higher bands in the presence of very high SWR, this derating percentage can become quite significant. In the extreme, on 10 meters with an SWR of 10, the derating required exceeds 60%.
Insertion Loss is the loss incurred by the RF signal as it passes through the device - in this case, a Common Mode Choke.
Signals pass through the choke as differential currents on the center conductor and the shield, whether it is the transmitted signal on its way from the rig to the antenna or a received signal on its way from the antenna to the rig. Either way, it experiences loss. We call that loss "Insertion Loss". It is separate and distinct from the choke's effect on common mode current.
Almost all RF Common Mode Chokes used in antenna systems are Transmission Line Transformers. This means they are wound using some sort of transmission line, either parallel wires or coaxial cable. All Watts Engineering Labs chokes are Transmission Line Transformers wound using coaxial cable.
The transmission line is typically wound on a ferrite core of some sort to create an inductor that will act as a Common Mode Choke. Our chokes are wound on ferrite toroids.
The Insertion Loss of a Common Mode Choke is nothing other than the loss in the transmission line that is used to wind the choke. In our chokes, this is normally 3 to 5 feet, depending upon the individual choke, of RG400 low loss coax. The loss in this section of coax would be the same if the coax was stretched out straight rather than wound on a ferrite core. The core does not add any discernable Insertion Loss. All the Insertion Loss is due to loss in the coax itself.
This Insertion Loss is not large enough to have any discernable effect on signals. So, in that regard, Insertion Loss is irrelevant. However, the Insertion Loss is a critical determinant of the power handling ability of the choke, so it is very important in that respect.
We want the Insertion Loss to be as small as possible to minimize heating of the ferrite core. This is why we use very low loss coaxial cable in our chokes. The choke would be just as effective rejecting common mode current if the coax used was not low loss. But the Insertion Loss would be higher, perhaps much higher, which means the choke's power handling ability would be reduced.
The power handling ability of the choke is, in most installations, determined by the temperature of the ferrite core as it is heated by the power dissipated in the coax windings due to the Insertion Loss, which is the coax loss. This is why Insertion Loss is important.
Question: How does SWR influence the Insertion Loss? Answer: In the same way it affects loss in any coax feedline. SWR higher than 1:1 will change the amount of power lost in the feedline coax. Likewise, it will change the loss in the section of coax that is used to wind the choke.
For example: If high SWR were to cause a 10% increase in coax loss, then the Insertion Loss of the choke would also increase by 10%, compared to the Insertion Loss for a perfectly matched load.
In some cases, especially on the lower band (below 20 meters) an increase in SWR actually decreases the loss in the coax and, therefore, in the choke.
There are a number of methods that have been put forward for calculating the increased loss in coax due to SWR. One of the easiest ones to find is in Jerry Sevick's book "Transmission Line Transformers, 2nd Ed." published by ARRL in 1990. It puts forth this formula which is often quoted.
Total Loss (in dB) = −10 * log10( α(1−Γ^2)/(1−αΓ)^2 ). [CAUTION: We do not use this formula. It produces wildly incorrect results.] Many online loss calculators use this formula and, as a result, produce incorrect results.
The ARRL published a very similar formula, which has come to be known as the ARRL's "Additional Loss Due to SWR" formula.
Subsequent work by others found that these are inadequate.
The best and most authoritative treatment of this topic is by Dan Maguire, AC6LA, the author of AutoEZ and TLDetails as well as utility software. That discussion is found on his website on the page titled "Additional Loss Due to SWR".
It is our understanding that the transmission line loss model that Dan developed is incorporated in both TLDetails and SimNEC (simulation software from Ward Harriman, AE6TY). We found that these programs produced identical results.
We use these tools to determine the effect of SWR on coax loss.
Watts Engineering
Dallas, Texas
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