Category: Test equipment

Resolving the sign of reactance – a method – Smith chart detail

Exploiting your antenna analyser #28 gave an example of use of one method to resolve the sign of reactance comparing measurements made with a slightly longer known transmission line.

One way to predict the input impedance to the longer line is using a Smith chart. This article presents a Smith chart prediction of the expected input impedance of a 8′ section of RG8 at 14.17Mhz (vf=0.66, length=0.175λ) for the cases of Zload being 60.3+j26.9Ω and  60.3-j26.9Ω.

60.3+j26.9Ω

The impedance is normalised to 50Ω and plotted on the Smith chart, point 1 above. A radial from the centre through point 1 is drawn to the edge of the chart. Another radial is drawn a distance towards the generator of 0.175λ and using a pair of dividers or ruler, point 2 is plotted on that radial at the same distance from the centre (same VSWR) as point 1.

These points are on a constant VSWR arc but the arc has not been draw because the two arcs would overlap and might be confusing to some readers.  Continue reading Exploiting your antenna analyser #29

Resolving the sign of reactance – a method

Many analysers do not measure the sign of reactance, and display the magnitude of reactance, and likewise for magnitude of phase and magnitude of impedance… though they are often incorrectly and misleadingly labelled otherwise.

The article The sign of reactance explains the problem and dismisses common recipes for resolving the sign of reactance as not general and not reliable.

This article gives an example of one method that may be useful for resolving the sign of reactance.

My correspondent has measured VSWR=1.68 and |Z|=66 and needs to know R and X. From those values we can calculate R=60.3 and |X|=26.9.

Method

The method involves adding a short series section of known line, short enough to provide a measurement difference in R, and that R would be different for the case of =ve and -ve X, all of these measured at the same frequency. Continue reading Exploiting your antenna analyser #28

An Insertion VSWR test gone wrong

We often learn more from failures than successes, this exercise is one of those opportunities.

An online poster tried to validate his newly purchased MFJ-918 by measuring Insertion VSWR.

That is done preferably by measuring a good termination (dummy load) to validate that it has a very low VSWR, then inserting the Device Under Test (DUT) and measuring the VSWR as a result of insertion of the DUT.

The poster did not mention measurement of the dummy load alone, and it is a type that warrants validation.

Above is the poster’s test setup, his Rigexpert AA-170 is connected to the balun’s input jack using a M-M adapter. The output wires on the balun form a rough circle of about 550mm perimeter by eye. Continue reading Exploiting your antenna analyser #27

Find coax cable velocity factor using a very basic analyser

A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that lacks facility to backout cable sections or measure OSL calibration (as discussed in other articles in this series). The older models and newer budget models often fall into this category.

The manuals for such instruments often explain how to measure coaxial cable velocity factor, and the method assumes there is zero offset at the measurement terminals (whether they be the built-in terminals or some fixture / adapters). In fact even the connectors are a source of error, especially UHF series connectors.

It is the failure to read exactly Z=0+j0Ω with a S/C applied to the measurement terminals that adversely impacts efforts to measure resonant frequency of a test line section.

The method described here approximately nulls out offsets in the instrument, measurement fixture, and even in the connectors used and for that reason may sometimes be of use with more sophisticated analysers.
Continue reading Exploiting your antenna analyser #26