Signed in as:
filler@godaddy.com
Signed in as:
filler@godaddy.com
SWR greater than 1:1 will affect how much power a Common Mode Choke can handle because it creates current peaks and valleys along the feedline.
The position of the choke relative to those peaks and valleys affects loss in the choke, and therefore heating of the ferrite - which is the primary limitation on power handling ability.
SWR greater than 1:1 can increase or decrease loss in the choke. There will be more loss in the choke when it is positioned near standing wave current peaks and less loss when it is near standing wave current valleys.
As long as the SWR is less than 3:1, there is no need to derate the Power Guideline curves we provide. There is no need for derating of these curves for operation on 20 the meter band or any frequency lower than 14 MHz.
However, for the bands 17m, 15m, 12m, and 10m, a high SWR means that we may need to 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.
This data is conservative because it assumes the choke is positioned at a standing wave current peak. This is rarely the case.
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.
Note also that this applies to the amount of power incident at the choke - after the feedline loss, which can easily be 10 - 20%.
Having said all that, it is just as likely that the choke is positioned near a standing wave current valley. So the Power Limit chart values can then be UPRATED by the same amount! Instead of a the 1080 W mentioned in the example above, the limit would then become 1250 W.
Insertion Loss is the loss incurred by the RF signal as it passes through the device 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.
Insertion Loss is comprised of dissipative Inherent Loss as well as non-dissipative mismatch loss. You deal with mismatch loss by providing a better match to the antenna so that more of the power is absorbed by the antenna.
Inherent Loss is power that is dissipated in the choke.
Almost all RF Common Mode Chokes used in today's 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 Inherent 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 6 feet, depending upon the individual choke, of RG400 low loss silver plated copper and PTFE 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 to Inherent Loss.
This Inherent Loss in our chokes is not large enough to have any discernable effect on signals - not even close. So, in that regard, Inherent Loss is irrelevant. However, it does contribute to heating of the ferrite core and, therefore, to the power handling ability of the choke. In that respect, it can be important.
We want the Inherent 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 Inherent Loss would be higher, which means the choke's power handling ability would be reduced.
The power handling ability of the choke is, in almost all situations, determined by the temperature of the ferrite core. That temperature is determined by the power dissipated in the coax windings due to the Inherent Loss plus the core loss created by common mode current. This is the only reason that Inherent Loss is important.
Question: How does SWR influence Inherent Loss? Answer: In much the same way it affects loss in any coax feedline, with a very important difference. Average loss over a long (relative to a wavelength) length of coax will be increased by SWR greater than 1:1. However, that loss is not distributed evenly along the coax. The standing waves created by an SWR higher than 1:1 have peaks and valleys. Loss will be greatest near standing wave current peaks and minimum near standing wave current valleys. This means loss is highest near standing wave current peaks.
Since the length of coax used to wind a choke is small compared to a wavelength, the loss in that specific length of coax can be higher or lower than it would be in a perfectly matched situation. SWR greater than 1:1 can increase or decrease Inherent Loss in the choke. This is a function of each specific installation.
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 other 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 produce identical results.
We use these tools to determine the effect of SWR on coax loss.
Watts Engineering Labs
Dallas, Texas
We use cookies to analyze website traffic and optimize your website experience. By accepting our use of cookies, your data will be aggregated with all other user data.