VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

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:

  • Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?
  • Are the resistors beneficial?
  • Do they degrade fixture behavior?
  • Then, why are the used so often?

This article addresses those questions.

Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?

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

VNA fixture for measuring Zcm of a common mode choke – coax 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

Voltage baluns are making a comeback for HF antennas – some real antenna measurements

Voltage baluns are making a comeback for HF antennas discussed the application of a common Ruthroff 1:4 (voltage) balun design to a slightly unbalanced theoretical scenario.

This article applies the same analysis to two reported measured dipole cases at 3.6MHz.

G3TXQ dipole system at 3.6MHz

(Hunt 2015) reported measurements of his dipole antenna system at 3.6MHz. Continue reading Voltage baluns are making a comeback for HF antennas – some real antenna measurements

Voltage baluns are making a comeback for HF antennas?

I planned this article to be a Youtube video, but recent behaviors of Google / Youtube give me pause to consider whether or how I use Youtube in the future. So, this article is a simpler presentation of the underlying concepts.

For most purposes, users of HF antennas would like the feed line to perform that function alone, ie to not participate as radiating or pickup conductors.

For that end, we want the common mode component of feed line current to be very very low.

Discussion of antennas tends to represent them as two terminal devices in free space, ie ignoring the presence of ground in close proximity. This applies whether the feed line is coax or two wire line.

A more complete representation (or model) is a three terminal network that includes a terminal to permit current to flow to ground. Above is a Wye or Delta equivalent circuit, and that can be transformed to an equivalent Tee circuit. Continue reading Voltage baluns are making a comeback for HF antennas?

Simsmith model of Ruthroff 1:4 voltage balun

Ruthroff described some baluns in 1959, and which have had continuing application. This article discusses the 1:4 Voltage Balun.

Voltage Balun

From Definition: Current Balun, Voltage Balun:

An ideal voltage balun delivers voltages that are equal in magnitude and opposite in phase.

A good voltage balun will approach the ideal condition. It will deliver approximately equal voltages (wrt the input ground) with approximately opposite phase, irrespective of the load impedance (including symmetry).

Common mode voltage ((V1+V2)/2) will be small.

If the load impedance is not symmetric, then the currents flowing in each output terminal will not be equal in magnitude and opposite in phase.

An ideal voltage balun has zero common mode impedance, a good one has very low common mode impedance (ohms).

Ruthroff’s 1:4 voltage balun

Above, Fig 5 from (Ruthroff 1959). Continue reading Simsmith model of Ruthroff 1:4 voltage balun

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Seventh part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

  • This article describes a measurment of common mode impedance Zcm of the packaged balun.

Packaging

The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a PTFE membrane vent was added later).

AtuBalun201

Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Overheating balun cores – how much power does it take?

Overheating balun cores – an explanation discussed a scenario where an operator reports unstable VSWR after 30s of ATU adjustment.

Where the antenna system incorporates ferrite elements, a possible / likely explanation is that loss in a ferrite core has been extreme and raised core temperature to the Curie temperature at which it quickly loses its magnetic properties.

In that scenario, theoretically, the complete temperature curve would look like this.

The initial rate of temperature increase here is 5°/s, and we can safely assume that almost all of the power absorbed by the core is stored as heat energy, little energy is lost the the air when the temperature difference is very small. Continue reading Overheating balun cores – how much power does it take?

Overheating balun cores – an explanation

Correspondents raise instances of damage to baluns with me from time to time, and there is a steady stream of reports online.

One of the very common reports is of something unexpected happening while adjusting an ATU, after perhaps 30s of power applied, VSWR suddenly becomes unstable, changing for some unknown reason, and attempts to find optimal settings of the ATU fails.

A likely cause of this is non-linear behavior of the ferrite core in a balun in the system.

Let’s talk about that.

A theoretical model of temperature rise

A simple model that gives useful insight is to consider the case of a toroid core in still air, being heated by constant applied RF power giving rise to core loss.

Core temperature rises quickly initially, then more slowly as the core heats up and loses more and more heat to the surrounding air.

We can write and expression for core temperature T: \(T=T_{max}\left(1-\mathrm{e}^{-\frac{t}{\tau}}\right)\) where τ is the thermal time constant and Tmax is the final temperature if things continued without disruption.

Above is an example where τ=20s and temperature rise is 100°.

Note that at the beginning (t=0), the slope of the line is pretty constant, but as temperature increase, slope decreases until eventually it is almost zero. Continue reading Overheating balun cores – an explanation

Power dissipation of a sealed plastic balun enclosure

Correspondents raise instances of damage to baluns from time to time, and there is a steady stream of reports online.

The thinking is often “my balun is rated at 5kW, how can I damage it with a transmitter that is only 100W?”

This article discusses heat dissipation from a small sealed plastic enclosure. This is not an unusual problem, it exists widely in industry and is solved routinely for enclosures ranging from small ones like these to large switchboard enclosures.

An example balun deployment

Above is a balun enclosure located under the soffit of the building, so outside in free air and where the sun will not shine on it. Continue reading Power dissipation of a sealed plastic balun enclosure

PVC speaker twin – loss model applied

One of the many gems of ham lore that I was fed as a beginner almost sixty years ago was that 23/0.076 (0.67mm^2) PVC insulated twin flex was suitable as an RF transmission line at HF, and that it had a Characteristic Impedance Zo close to 75Ω.

It seems that these claims have been extended to apply to lighter gauge cables often called speaker cable or bell wire.

This article explores two cases of the application of a light grade of speaker twin to a G5RV antenna. The scenarios is a G5RV Inverted V with 7m of speaker twin from dipole to the coax section, and loss is calculated for the speaker twin section at 14.1 and 3.6MHz.

PVC speaker twin copper / PVC 0.2mm^2 characteristics

The following articles report measurement of a sample of speaker twin, and derivation of a simple loss model:

From those articles, the loss model is copied for reader convenience.

Above is a plot of the calculated MLL (red dots) based on the s11 measurements, and a curve fit to the model \(MLL = k_1\sqrt f+k_2f \text{ dB/m}\). Continue reading PVC speaker twin – loss model applied