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Correcting SWR measurement made through a long coax cable
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Question
WA3LTJ
Problem
My portable antenna is a very lightweight inverted V suspended by a 22 ft fiberglass pole. It uses RG174 coax because anything heavier would bend the top of the pole. The cable is 40 ft long. I can measure the SWR at the end of the cable, but that introduces an error associated with the losses in the cable. How does one correct the SWR measurement?
SWR meter  40 ft coax cable  Antenna
Proposed solution
Convert VSWR meter reading to return loss.
Subtract additional losses of coax cable
Convert return loss (at the antenna) to VSWR
Equations
This will convert measured VSWR into return loss.
RLt total return loss
VSWRm SWR measured by the meter (assume ratio, e.g., 2.5:1 expressed as 2.5)
RLt = 20 log10 ( (VSWRm1)/(VSWRm+1) ) (1)
Estimated loss of the coax cable based on manufacturer’s data
RLc Cable loss estimate
RLc = [loss per 100 ft at test frequency] * [length of cable in feet] / 100 (2)
We can now express all the losses. Note the signal traverses the cable twice, first the forward power, then the reflected power.
RLm match loss (a result of the true SWR of the antenna)
RLt = RLm  2 RLc (3)
Rearrange (3) to get the match loss of the antenna
RLm = RLt – 2 RLc (4)
Calculate the VSWR based on return loss (reverse of equation (1))
1 + 10 ^ (RLm/20)
VSWRa =  (5)
1 – 10 ^ (RLm/20)
Algorithm: Given SWR reading, coax loss per 100 ft, coax length
Calculate RLt with (1)
Calculate RLc with (2)
Calculate RLm with (4)
Calculate VSWRa with (5)
I have uploaded a simple Matlab function that implements the algorithm. Note, the .txt should be changed to .m if you want to run it with Matlab.
Note: I submitted this question to Ask Dave in QST. It was answered in the February issue. Dave said I missed "the additional loss in the coax cable due to an SWR that is higher than 1:1". I am not certain he is correct. The mismatch loss is referred to the antenna. The SWR meter is correctly matched to the coax line. But, maybe there is another term I am missing.
Antenna_SWR.txt
Edited by K3MRIEdited title for brevity
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