The merit of measuring VSWR on an open wire line

(Danzer 2012) describes a modern take on an old VSWR indicator for open wire line.

 In the opening paragraph he states [d]espite the fact that losses in transmission line go up with SWR, the low matched SWR of window line keeps the total losses low, usually lower than matched coax.

Well, no it is not a fact that losses in transmission line go up with SWR. Losses may go up with increased VSWR, but not necessarily. For example, using TLLC to calculate the loss in 1m of Wireman 551 at 7MHz with a load of 4000 ohms (eg a fullwave dipole), we find that loss is 0.0035dB whereas the matched line loss is 0.0046dB. These might seems small and insignificant, but it demonstrates that the quote is not true in general. For further discussion see (Duffy 2010).

The next part of the statement looks like an error not caught during review (do QST review submissions before publication?), it is probably meant to say the low matched line loss of window line keeps the total losses low. But does it keep the total losses low, usually lower than matched coax?

Taking an example of the very popular G5RV on 80m (assume feed point Z of 10-j340Ω) with 10m of WIreman 551 feedline, line loss is 1.5dB, much higher than the matched line loss of any practical coax (eg 10m of RG58 has a matched line loss of 0.25dB). So, no, in this case it is not lower than matched line loss of coax of the same length. It is a bit of an impractical comparison, and is likely to suggest that you 'never' need to run the numbers, but there are some common scenarios where it is worth quantifying the losses, they might be more than suggested.

So, lets say you built Danzer's instrument and applied it to the G5RV example mentioned above. TLLC suggests that the VSWR at the source end is 50:1, and ρ=0.96, to the instrument applied at the source end of the line would read Vr equal to 96% of Vf. It is difficult to read a practical meter to an accuracy of better than a few percent, but lets assume that you did read it accurately, and calculated VSWR to be 50:1, what you you estimate the loss to be under this mismatch?

The formula LossRatio=(1+S^2)/(2*S) might spring to mind, and solving it for VSWR=50, you would obtain LossRatio=25.01 and given that the matched line loss is 0.0318dB, you would calculate loss under mismatch to be 0.795dB. But, as TLLC more correctly calculates using the give load impedance, it is 1.532dB, almost twice that of using the so called "additional loss due to VSWR" formula.

Note that the formula LossRatio=(1+S^2)/(2*S) has a set of underlying assumptions not usually stated, and as mostly used is invalid, as in this example, the reason for the error noted.

So, at the extreme VSWR often experience in multi band antennas on the lower bands, it may be difficult to measure VSWR accurately, and then, the figure is not an accurate way of calculating line loss for the mismatch scenario.

BTW, if you do want to construct Danzer's meter, 1N34A germanium diodes are not difficult to find. A search of eBay showed more than a dozen sellers in the AU market, prices down to below A$0.20 each in lots of 50... always good stock to have on hand. Germanium diodes will result in a more linear scale shape than silicon diodes as recommended.

A footnote on TLLC's figures for Wireman 551. They are an extrapolation by model of measurements at much higher frequency by N7WS, and the model assumes homogenous conductor (as noted in the documentation). Wireman 551 is specified as a copper clad steel conductor and may have insufficient copper cladding for the homogenous conductor assumption to apply and loss may be worse than estimated. Window lines with multi stranded copper clad steel conductors are probably worse again. This doesn't seem to stop hams buying and using these lines with no specifications for performance on the low end of HF.

Links / References


Version Date Description
1.01 02/05/2012 Initial.