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It is fairly well understood that due to transmission line losses, the SWR as measured at the transmitter will be less than the SWR at the antenna.

Some of the new antenna analyzers and some vector network analyzers can 'zero out' or compensate for the losses in the transmission line. This allows them to display the SWR at the antenna while taking the SWR measurement at the transmitter end.

Can something similar be done using a much less expensive, conventional SWR meter or dual needle SWR meter?

Glenn W9IQ
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2 Answers2

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Given the matched loss of the feedline and the SWR at the transmitter, we can calculate the SWR at the antenna in three simple steps.

First convert the SWR at the transmitter to the corresponding magnitude of the reflection coefficient (Gamma), or MRC for short within the context of this answer. The MRC is the magnitude of the complex ratio of the reflected voltage wave to that of the incident voltage wave. We can calculate the MRC using this formula:

$$|\Gamma|=\frac {\text{SWR}-1}{\text{SWR}+1} \tag 1$$

where SWR is the SWR (without the :1 notation) at the point of measurement.

For example, if the SWR at the transmitter is 3:1, the corresponding MRC ($|\Gamma|$) at the transmitter is 0.5.

Now we convert this MRC at the transmitter to its corresponding MRC at the antenna by reversing the effect of the losses in the feedline:

$$|\Gamma_{\text{ant}}|=\frac{|\Gamma_{\text{xmtr}}|}{e^{(-2*L_{\text{dB}}/8.6858)}} \tag 2$$

where LdB is the matched loss of the feedline in dB. Note that the division by 8.6858 converts dB loss to Nepers - a more convenient unit for transmission line calculations. The effect of this loss on SWR has been described in Does Coax Limit the Maximum SWR a Transmitter Sees.

Using formula 2 with a feedline that has a matched loss of 1.0 dB along with the previously calculated 0.5 MRC at the transmitter, we find that we have an MRC of 0.63 at the antenna. We can now convert this to the SWR at the antenna using the formula:

$$\text{SWR}=\frac{1+|\Gamma|}{1-|\Gamma|} \tag 3$$

So we find using formula 3, that the SWR that measured 3:1 at the transmitter is 4.4:1 at the antenna.

These formulas are not too difficult to do on a hand calculator, but by placing them into a spreadsheet, it is possible to construct a nice chart depicting the transmitter SWR versus antenna SWR for a given amateur band:

enter image description here

This idea could be extended to show a differently colored series plot on the chart for each band of interest using the respective matched feedline loss for the band.

Note: It should be pointed out that the antenna SWR as calculated here is technically the minimum antenna SWR. This is due to the coax limiting the maximum possible SWR at the transmitter as presented in Does Coax Limit the Maximum SWR a Transmitter Sees.

Glenn W9IQ
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That's simply not true. All the power gets radiated at the antenna. If you want proof take a look at this video: https://www.youtube.com/watch?v=6P6UJuMATBY

K3ZD
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  • Ah, YouTube, the idiot's bible. Like most hams, he has no clue what he is talking about. Stop and ask yourself a simple question. Why would radio manufacturers go to all the expense and trouble of designing and implementing VSWR detectors to roll the eTX power back when high VSWR is detected? Many commercial radios will shut down to protect the finals and send messages and telemetry alerting maintenance personnel. Try telling that to professional engineers who are designing the circuitry.

    Besides, the thread is 6 years old.

     – Dereck Mar 23 '24 at 16:09
  • It's very surprising to hear from someone who seems to have a ham license that all power gets radiated by an antenna. That is not the case; if that was the case, we wouldn't need to properly design antennas (because no matter how badly you designed an antenna, it would radiate all the power, that would be awesome!). You can find a youtube video that states the fallacy for most fallacies on this earth, so I'm afraid what you found is simply wrong. – Marcus Müller Mar 24 '24 at 18:07
  • If by "power" the video author means that the antenna and feedline can warm the air quite inefficiently if the system is mismatched, then absolutely it radiates "all the power". Just maybe not the way we'd like. – clvrmnky Mar 26 '24 at 13:07