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VSWR Frequently asked questions ***DRAFT***

Does VSWR vary with line length?

VSWR measurements made with an instrument calibrated for Zo real (as is usually the case) will show a smooth decrease in VSWR from load to source if inserted in the line at different points. This is caused by line loss, and can be predicted with knowledge of line loss.

If VSWR changes with a change in line length by more or less than accounted for by the line loss, something else is happening, most commonly feedline common mode currents are affecting the impedance at the load end of the transmission line.

Can the magnitude of the reflection coefficient (ρ) be greater than 1?

Yes, under conditions of quite reactive Zo (which will usually be a negative reactance) and a highly inductive load, ρ may be greater than 1. This is unlikely to happen with practical lines at HF and above.

Note that an instrument calibrated for Zo real should never show ρ > 1, it is a sign of mismatched detectors in such an instrument. Reverse the instrument and see if it still happens.

Can VSWR be negative?

Considering the definition of VSWR stated above, no.

The common formula for predicting VSWR based on observation of conditions at a point is VSWR=(1+ρ)/(1- ρ) which will yield negative VSWR for ρ>1 (which is a rather unusual case). VSWR is a dimensionless positive ratio so a negative value does not seem sensible, a better formula is VSWR=(1+ρ)/|1- ρ|.

Is there a the minimum length of transmission line required on either side of the VSWR meter for valid readings?

The VSWR meter described above samples a very small region of the transmission line within the meter, sufficiently so that it closely approximates a point source measurement for al practical purpose in well designed instruments. The instruments measurement of conditions at that point are independent of things external to the sampling section.

The measurements made at that point apply at that point. With additional knowledge, the measurements may be extended to another place. For example, with additional knowledge of the loss in a length transmission line of the same Zo as the instrument calibration, the VSWR at the other end of that line can be estimated. 

Is the characteristic impedance of the sampler line section important?

The sampler line section behaves like any transmission line, and may transform impedance. Quality instruments will usually have a sampler line section of exactly the same Zo as the VSWR calibration base so that impedance transformation caused by the sampler line is minimised.

However, the detectors may be calibrated (nulled) at some Z that is different to the sampler section line Zo. If the sampler line section is very short, the impedance transformation will be very small. For example, consider a HF sampler line section of length 20mm and Zo=75+j0 calibrated for reflected null at 50+j0Ω, the insertion VSWR in a 50Ω system is around 1.02 which is comparable to the error using a true 50+j0Ω sampler section in RG58C/U nominal 50Ω cable at low HF (ignoring cable tolerance errors).

What is an acceptable VSWR?

Whilst there are lots of Rules of Thumb (ROT) proposed, the answer lies in determining the loss and power handling of the transmission line in the intended scenario, and making a decision about whether they are acceptable.

In general,

  • coaxial transmission lines (by virtue of the lower Zo and small conductor sizes) are relatively higher loss, and high VSWR and length together result in higher loss overall; and
  • open wire transmission lines (by virtue of the higher Zo and larger conductor sizes) are relatively lower loss and high VSWR and length increases loss, but often lower loss than coaxial lines for the same scenario.

To model the effect of VSWR on transmission line, try the RF Transmission Line Loss Calculator .

Equipment specifications is another potential source of guidance on acceptable VSWR from a load point of view.

 

V1.01 20 February 2009 09:42:12 -0700

 


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