Baselining an antenna system with an analyser

I often receive emails from folk trying to validate continued performance of an installed antenna system using their analyser.

With foresight they have swept the antenna system from the tx end and saved the data to serve as a baseline.

The following are example sweeps from one of my own antennas, a Diamond X50N with 10m of LDF4-50A feed line.

Now I have plotted Return Loss rather than VSWR for several reasons:

  • Return Loss is more sensitive to the problems that we might want to identify;
  • Rigexpert in this case decided that the Antscope user could not be interested in plotting VSWR>5 (Return Loss<3.5dB).

Now a hazard in working with Return Loss is that many authors of articles and software don’t use the industry standard meaning.

Return Loss

Lets just remind ourselves of the meaning of the term Return Loss. (IEEE 1988) defines Return Loss as:

(1) (data transmission) (A) At a discontinuity in a transmission system the difference between the power incident upon the discontinuity. (B) The ratio in decibels of the power incident upon the discontinuity to the power reflected from the discontinuity. Note: This ratio is also the square of the reciprocal to the magnitude of the reflection coefficient. (C) More broadly, the return loss is a measure of the dissimilarity between two impedances, being equal to the number of decibels that corresponds to the scalar value of the reciprocal of the reflection coefficient, and hence being expressed by the following formula:

20*log10|(Z1+Z2)/(Z1-Z2)| decibel

where Z1 and Z2 = the two impedances.

(2) (or gain) (waveguide). The ratio of incident to reflected power at a reference plane of a network.

Return Loss expressed in dB will ALWAYS be a positive number in passive networks.

The relationship between ReturnLoss in dB and VSWR is given by the equations:

  • ReturnLoss=-20*log((VSWR-1)/(VSWR+1))
  • VSWR=(1+10^(-ReturnLoss/20))/(1-10^(-ReturnLoss/20))

Diamond X50N on 2m

So now that we are on the same page about Return Loss, lets look at my 2m plot.

The X50N does not have VSWR or Return Loss specs, but we might expect that at the antenna itself, VSWR<1.5 which implies Return Loss>25dB. Measuring into feed line, you can add twice the matched line loss to the Return Loss target (see why Return Loss is a better measure).

So, this looks good.

Diamond X50N on 70cm

In this case Return Loss does not make the target range over the whole band, but it does in the narrow region of local FM repeater operation.


These plots are sensitive to things that might degrade the maximum Return Loss, faults like loose connectors, failed internal capacitors (an issue with the X50N), but they are not sensitive to small increases in coax attenuation due to for example, water  ingress (a common problem). Sure, an extra 1dB of coax attenuation improves the maximum Return Loss (which could be mistaken for goodness), but not by much in terms of measurement uncertainty.

Wide Return Loss

Lets look at a wide sweep of Return Loss.

Above a wide Return Loss sweep. What I am most interested in capturing here is some region of quite low Return Loss because it is more revealing of a change in coax loss.

In this case, I am really interested in the Return Loss at 215MHz where it is 1.7dB. If the Return Loss through-life increases by say 1dB at this frequency, it will be measured with less uncertainty than an increase in-band where Return Loss>25dB.

A small increase in Return Loss in a region of very low Return Loss might well indicate increased coax loss (for some reason) and warrant further investigation (eg measurement of the coax disconnected from the antenna).

This test should be performed BEFORE looking at in-band Return Loss (or VSWR) as failure of this test means the in-band Return Loss (or VSWR) may be tainted by increased line loss and not a good indicator of antenna health.

VSWR based

If you have an analyser that cannot display Return Loss, but can reliably / consistently measure and display high VSWR (>10) then you can achieve a similar outcome with VSWR plots. Note that low end analysers tend to be less capable in this respect.

The initial focus of investigation is to examine whether the antenna system through-life continues to show a similar maximum VSWR out-of-band because it is quite sensitive to small increases in coax loss.

Links / References

    IEEE. 1988. IEEE standard dictionary of electrical and electronic terms, IEEE Press, 4th Edition, 1988.