Antennas – disturbing the thing being measured – open wire lines

A common question in online forums relates to inability to reconcile analyser measurements of an antenna system with the transmitter system antenna facing VSWR meter.

The cause is often that the antenna system was changed significantly to connect the analyser.

Seeing recent discussion by the online experts of how the measure the impedance of an antenna system looking into a so-called balanced feed line gives advice that is likely to cause reconciliation failure.

I will make the point firstly that the line is not intrinsically balanced, it is the way the it is used that may or may not achieve balance of some type. I will refer to that type of line as open wire line.

Let’s explore the subject using some NEC models.

I have constructed an NEC-4.2 model of an approximately half wave dipole at 7MHz, it is 20m above the ground, and fed slightly off centre with open wire line constructed using GW elements. At the bottom, I have connected a 2 segment wire between the feed line ends, and two sources in series.

The impedance seen by the sources is 71.66-j217.8Ω. This is what you would expect if you connected your trusty nanoVNA or other stand alone analyser to the lower end of the open wire line.

Job’s done!

Wrong! The measured scenario does not represent what might be seen by a transmitter, it has ignored the existence of the ‘ground terminal’.

Lets model the case where the two sources used are from an ideal voltage balun with its centre ground immediately below. There is a 3m deep ground electrode used as a ground point (NEC-4.2 can do this).

Zooming in on the geometry above, we can see there is significant common mode current on the ground connection, so connecting the voltage balun source has changed the antenna, and now the impedance measures 85.56-j199.5Ω.

This example shows that:

  • making isolated two terminal measurements might be quite inadequate; and
  • if the measurement scenario does not provide for exactly the same transmission line and common mode current paths, then you are measuring ‘something else’.

Whilst it is often relatively easy to properly maintain the common mode current path when measuring a coax fed antenna, it can be more challenging in the case of open wire feed. If the antenna system includes a balun, it may be more practical to measure looking into the balun (whilst maintaining the common mode current path.

Application of isolated small stand alone antenna analysers to open wire fed antenna terminals is naively over simplified.