Antennas – disturbing the thing being measured – open wire lines #5

The articles Antennas – disturbing the thing being measured – open wire lines #3 Antennas – disturbing the thing being measured – open wire lines #4 demonstrated an inconsistency between a partial linear circuit model and a complete NEC model for predicting common mode current behaviour.

One of the oft proposed solutions to characterising a balun is to find the Common Mode Rejection Ratio (a term carried over from other applications, eg voltage driven operational amplifiers). (Anaren 2005) explains a method of finding balun CMRR.  (Skelton 2010) goes so far as to say

The Common Mode Rejection Ratio (CMRR) of a balun is defined in professional literature as the ratio of wanted to un-wanted transmitted power. As rejection of common-mode transmission is the primary purpose of a balun, it follows that CMRR should be the key figure of merit.

Let us take the model scenario used in Antennas – disturbing the thing being measured – open wire lines #4 and lower the height of the dipole 10 10m, and compare the model ratio of Ic/Id.

Again, we will use Python to do the complex maths for the without and with scenarios at 20m height, and without and with at 10m height. Continue reading Antennas – disturbing the thing being measured – open wire lines #5

Antennas – disturbing the thing being measured – open wire lines #4

The article Antennas – disturbing the thing being measured – open wire lines #2 did a simple analysis of current flows in the model scenario using ideal voltage balun drive with a current balun.

It was mentioned that solution of the lumped values network is only a first approximation and not as good as the NEC solution which properly models the coupled conductors, and their mutual effect on the distribution of currents in the system.

This article reports the NEC current solution decomposed into differential mode and common mode components.

Ideal voltage balun drive

Above is a zoomed in view of the feed point with a balanced pair of voltage sources feeding the line against the ground electrode. Continue reading Antennas – disturbing the thing being measured – open wire lines #4

nanoVNA-H – measure 40m low pass filter for WSPRlite flex

This article demonstrates the use of a nanoVNA-H to measure the response of a low pass filter designed to pass 7MHz frequencies but attenuate harmonics. The inductors and capacitors make a seven element Chebyshev filter as designed by G3CWI for use in a 50Ω system.

Implementation

Above, the filter is assembled on a piece of matrix board with two BNC connectors. The inductors are fixed with hot melt adhesive, and the whole thing served over with heatshrink tube. It is not waterproof. Continue reading nanoVNA-H – measure 40m low pass filter for WSPRlite flex

Antennas – disturbing the thing being measured – open wire lines #3

The article Antennas – disturbing the thing being measured – open wire lines #2 did a simple analysis of current flows in the model scenario using ideal voltage balun drive.

That begs the question, what difference would a good current balun make?

We can get a good approximation of what happens by inserting the current balun’s Zcm in series with Z3. Let’s take Zcm to be 1130+j1657Ω (11t on a FT240-43). Continue reading Antennas – disturbing the thing being measured – open wire lines #3

Antennas – disturbing the thing being measured – open wire lines #2

The article Antennas – disturbing the thing being measured – open wire lines illustrated the pitfalls of a simplistic model of an antenna presented on two terminals of an open wire line.

A more complete representation of the antenna can be formed by making three impedance measurements (Schmidt nd). Continue reading Antennas – disturbing the thing being measured – open wire lines #2

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. Continue reading Antennas – disturbing the thing being measured – open wire lines