A recent article (Howarth nd) entitled “If baluns bug you, do yourself a doublet with balanced feed. Antenna matching unit for balanced feedpoint antennas.” described an ATU for two wire line.
The article discusses related matters. Continue reading If baluns bug you…
I see online discussion of specification bending radius for coax cables, and their application to ferrite cored common mode chokes.
A low Insertion VSWR high Zcm Guanella 1:1 balun for HF and follow on articles described a balun with focus on InsertionLoss.
Let’s remind ourselves of the internal layout of the uncompensated balun.
The coax is quality RG58A/U with solid polythene dielectric. The coax is wound with a bending radius of about 10mm, way less than Belden’s specified minimum bending radius of 50mm.
So, the question is does this cause significant centre conductor migration that will ruin the characteristic impedance: Continue reading A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – coax bend radius
This article is principally a short commendation for Jupyter or Interactive Python for ham radio related projects for the quantitative ham. Python is a cross platform programming language that has a very rich set of libraries to support scientific and engineering applications, and a good graph maker.
The exercise for this demonstration is to decompose three measurements of currents on a two wire transmission line at a point into the differential and common mode components at that point, and to plot a phasor diagram of a solution to the measurements. Remember that common mode current and differential current in an antenna system are usually standing waves.
Above is a diagram explaining the terms used, I1 and I2 are the magnitudes of currents in each conductor measured using a clamp on RF ammeter, and I12 is the magnitude of the current when both conductors are passed through the clamp on RF ammeter, i12 is the phasor sum of the underlying i1 and i2. Continue reading Jupyter: one for the toolbox – decompose common mode and differential mode current components
Since the widespread takeup of the NanoVNA, a measure of performance proposed by (Skelton 2010) has become very popular.
His measure, Common Mode Rejection Ratio (CMRR), is an adaptation of a measure used in other fields, he states that he thinks the application of it in the context of antenna systems and baluns is novel and that “CMRR should be the key figure of merit”.
Skelton talks of different ways to measure CMRR, but essentially CMRR is a measure of the magnitude of gain (|s21|) from Port 1 to Port 2 in common mode, with the common mode choke (or balun) in series from the inner pin of Port 1 to the inner pin of Port 2.
Note that this is the same connection as used for series through impedance measurement, but calculation of impedance depends on the complex value s21.
Above is capture of a measurement of a Guanella 1:1 common mode choke or balun. The red curve is |s21|, the blue and green curves are R and X components of the choke impedance Zcm calculated from s21. Continue reading CMRR and transmitting antennas
I have found you can never have enough of these things. It is very convenient to leave some measurement projects set up while work continues on some parallel projects.
Above is the kit as supplied (~$8 on Aliexpress). Note that it does not contain any male turned pin header… more on that later. Continue reading nanoVNA-H – Deepelec test jig #2
NE6F’s common mode current tester – Part 1 ended with the following:
Common mode current adjacent to a small choke
Consider a straight section of coaxial feedline not close to other materials, and with a small common mode choke inserted in the feedline. A “small” choke means one that is a very tiny fraction of a wavelength, say λ/100, from connector to connector.
Q1: Ask yourself that if say 1A of common mode current flows into one connector, what is the common mode current at the other connector?
Q2:What is your answer if you were told the balun was specified to have a CMRR of 20dB?
The answer to Q2 is relatively easy, CMRR is not a meaningful statistic for a common mode choke deployed in a typical antenna system, it would not change the answer to Q1.
The answer to Q1 needs a longer explanation… let’s do it!
Common mode current distribution is almost always a standing wave.
Above is a plot of current distribution of an example dipole antenna system with coax feed. The dipole is slightly off centre fed to drive a significant common mode current on the vertical coax feed which is grounded at the lower end. Continue reading NE6F’s common mode current tester – Part 2
A correspondent asked my thoughts on a Youtube video featuring…
Above is the schematic of NE6F’s common mode current tester.
The concept is that current probes A and B are placed either side of a current mode choke, and by calibrating and switching between them, a relative reading of current on one side compared to the other may be found. Continue reading NE6F’s common mode current tester – Part 1
This article presents a simple way to make measurements of a prototype Guanella 1:1 current balun, measurements that can guide refinement of a design.
The usual purpose of these transmitting Guanella 1:1 current baluns is to reduce common mode feed line current. Not surprisingly, the best measure of a device’s effectiveness is direct measurement of common mode current (it is not all that difficult), but surprisingly, it is rarely measured.
Above is the prototype balun being a Fair-rite 5943003801 (FT240-43) wound with 11t of solid core twisted pair stripped from a CAT5 solid core LAN cable and wound in Reisert cross over style. Note that Amidon #43 (National Magnetics Groups H material) is significantly different to Fair-rite #43. Continue reading Baluns: you can learn by doing!
VNA fixture for measuring Zcm of a common mode choke – coax wound discussed issues with common ham practice for measuring coax wound common mode chokes.
The article left readers with some homework:
This article addresses those questions.
Let’s treat the common mode choke as a black box with two input terminals at left and two output terminals at right with voltages as annotated above. Continue reading VNA fixture for measuring Zcm of a common mode choke – twisted pair wound
A common online question is what sort of fixture is appropriate to measure the common mode impedance of a common mode choke.
Above is a screenshot from a Youtube video by Trx Lab, probably about 2016 vintage. I see many problems with the fixture, lets start with the resistors. Continue reading VNA fixture for measuring Zcm of a common mode choke – coax wound