N6THN’s novel balun – flux leakage

N6THN’s novel balun presented measurement of the Insertion VSWR of the subject balun, and N6THN’s novel balun – an explanation gave explanation that included mention of flux leakage as a contributor to the quite high inductance per unit length of the transmission line formed by the two windings.

A correspondent suggested that with a ferrite core, flux leakage is insignificant. This article calculates the coupled coils scenario.

The balun as described

Above is the ‘schematic’ of the balun. Note the entire path from rig to dipole. Continue reading N6THN’s novel balun – flux leakage

N6THN’s novel balun

One sees lots of articles and videos on how to make a current balun suited to a low VSWR antenna. This one was recommended in an online discussion on QRZ.com. N6THN might not have invented this balun, but he made a video of it.

In this case, it is described in the referenced video as part of a half wave dipole antenna where you might expect the minimum feed point VSWR to be less than 2.

Apologies for the images, some are taken from the video and they are not good… but bear with me.

The balun as described

Above is the ‘schematic’ of the balun.Note the entire path from rig to dipole. Continue reading N6THN’s novel balun

Comment on KN5L on balun CMRR – series through impedance fixture

In recent articles, I flagged that on some of John’s VNWA plots he showed flawed impedance calculations using VNWA’s t2s inbuilt function.

The function t2s is documented in the VNWA help.

t2s is a VNWA built in function intended to solve the so-called s21 series through fixture for impedance measurement of two terminal Zx connected between Port 1 and Port 2.

None of John’s test fixtures were equivalent to the circuit above required for valid t2s transformation. Continue reading Comment on KN5L on balun CMRR – series through impedance fixture

Comment on KN5L on balun CMRR – two wire line example

The article Comment on KN5L on balun CMRR dealt with model and measurement of John’s coaxial choke in fixture, dealt with first because it is a simpler model. This article builds on that and models the balun wound with a pair of wires.

Above is the subject balun in fixture.

John’s schematic shows the balun as coupled coils, but that does not capture the transmission line transformation that occurs in the actual device. Again the test fixture is used without explanation. Continue reading Comment on KN5L on balun CMRR – two wire line example

Comment on KN5L on balun CMRR – coax example

One of the ham fashions of 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. Anaren gives a definition of CMRR:

Common Mode Rejection Ratio is defined and the ratio between the differential mode insertion loss/gain versus the common mode signal loss or gain.

Note that in a passive system, CMRR in dB will usually be positive, and the larger the better.

Anaren does not mention applying the CMRR statistic to antenna systems. I have commented elsewhere on the lack of utility of CMRR in analysing common antenna systems.

John, KN5L, has published his own solution to balun characterisation in some online forums. Continue reading Comment on KN5L on balun CMRR – coax example

Transmission line filter for a field day station – implementation

Transmission line filter for a field day station – designs laid out some designs for a transmission line filter for harmonic reduction of a field day station on 7MHz. This article describes Bruce’s, VK4MQ, implementation of the “two stubs are better than one” option. Huber+Suhner RG214 coax was used.

Firstly two quarter wavelengths OC stubs were tuned to 14.2MHz by iterative cut and measure. The coax was 20mm longer than prediction, I am not convinced that the transmission line models in Simsmith are better than that. Then the tees were made up and the connecting section and tuned by cut and measure for minimum |s11| at 7.1MHz.

Above is the VNA sweep for the completed filter. Rejection around 14.2MHz exceeds 50dB with bandwidth of over 0.6MHz. Continue reading Transmission line filter for a field day station – implementation

Transmission line filter for a field day station – designs

Bruce, VK4MQ, was canvassing ideas of a simple way to reduce second harmonics from a 40m field station interfering with operations on 20m at the same site.

A shunt OC stub of 90° electrical length was proposed to start thinking. My thoughts were that online experts often propose such as a cheap and effective solution… but I suspect they had read about it rather than speaking from actual experience.

The models and calculations assume that linear circuit theory applies, that the source is well represented by a Thevenin equivalent circuit with Zth=50+j0Ω. Most ham transmitters are not well represented by such a circuit, and the calculated results may not apply exactly. The calculated results should be observed when measuring with a good VNA.

Here is the problem

Above is a Simsmith model of a shunt stub in a linear matched 50Ω system. The stub achieves a reduction of more than 20dB over about 900kHz, and a maximum reduction of around 35dB at 14.2MHz.

But, it ruins the VSWR seen at G at 7.1MHz, VSWR is 2.6. Continue reading Transmission line filter for a field day station – designs

A tale of three VNAs

In researching the article Analysis of output matching of a certain 25W 144MHz PA  , I made measurements using a recently ‘upgraded’ nanoVNA-H v3.3 with oneofeleven firmware v1.1.206 nanoVNA-App.exe and default supplied firmware.

Some unexpected ‘bumps’ on the measured response of a short SC transmission line section were concerning, there was no apparent explanation.

The bump around 80MHz had no obvious explanation, and appeared to be an artifact of the measurement fixture, or the instrument. The s11 values from 70-150MHz are suspect. Continue reading A tale of three VNAs