Category: Measurement

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

Last update: 1st April, 2021, 6:46 AM

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 (or CMR) in dB will usually be positive, and the larger the better. You might even think that the plain English meaning of the words Common Mode Rejection Ratio would suggest that a large ratio (or high +ve dB value) would mean most rejection, goodness. Such a meaning would be quite consistent with that of CMRR (CMR) applied to operation amplifiers (for a very long time).

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.

Then immediately after the above definition, they give a formula which implies the inverse:

\(CMRR=\frac{S_{1c}}{S_{1d}}\) and goodness would be a tiny fractional value, or a small (-ve) dB value.

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

Last update: 23rd January, 2022, 3:04 PM

Return Loss Bridge – some woolly thinking – a Simsmith model of a reflection bridge

Return Loss Bridge – some woolly thinking discussed some online opinions on the practical measurement range of nanoVNA, and underlying reasons… but both were flawed.

Reflection Bridge and Return Loss Bridge are somewhat synonymous, in practice to measure Return Loss one is interested in the magnitude of the response, and to measure the complex reflection coefficient or s11, both magnitude and phase are of interest.

He derives a flawed expression for bridge response, then plots a dodged up version to demonstrate the asymmetry of the response.

Above is Oristopo’s graph. Continue reading Return Loss Bridge – some woolly thinking – a Simsmith model of a reflection bridge

Last update: 19th March, 2021, 4:56 PM

Return Loss Bridge – some woolly thinking

Some discussion on groups.io nanovna-users attempts to explain the behavior of the RF Return Loss Bridge used in some VNAs and other instruments, proof if you will that the instruments are not capable of measuring more than a few hundred ohms.

Oristopo gives a diagram and explanation.

Above is his diagram. He gives an expression that he states applies when R1=R3=R4=Rm: im = sqrt(Vf*(Rm – R2)/(12*Rm + 4*R2)). Continue reading Return Loss Bridge – some woolly thinking

Last update: 18th March, 2021, 10:19 AM

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

Last update: 1st August, 2022, 9:34 PM

On Insertion Loss

Readers of my articles occasionally ask for explanation of the distinction between meanings of:

  • Insertion Loss;
  • Mismatch Loss;
  • Loss (or Transmission Loss).

These terms apply to linear circuits, ie circuits that comply with linear circuit theory, things like that impedances are independent of voltage and current, sources are well represented by Thevenin and Norton equivalent circuits.

Insertion Loss

Insertion Loss is the ratio of power into a matched load (to mean that the load impedance is the complex conjugate of the Thevenin equivalent source impedance) to the power in the load with the subject network / device inserted.

Insertion Loss can also be expressed in dB.

Mismatch Loss

Mismatch Loss is the ratio of output power of a source into a matched load to the output power under a given mismatch.

Mismatch Loss can also be expressed in dB.

Loss

Loss is simply \(\frac{Power_{in}}{Power_{out}}\).

Loss can also be expressed in dB.

Loss is sometimes called Transmission Loss to distinguish it from other qualifications, but it is unnecessary. Recent hammy Sammy practice is to label |s21| graphs Transmission Loss which is an error on two counts.

Let’s illustrate these with some examples using Simsmith. Whilst these are models, you would expect to measure similar results using a good VNA or like test equipment. Continue reading On Insertion Loss

Last update: 30th August, 2023, 10:13 AM

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

Last update: 15th March, 2021, 6:21 AM

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

Last update: 15th March, 2021, 7:44 AM

Analysis of output matching of a certain 25W 144MHz PA

Andrew, ZL2PD, contacted me regarding the matching scheme in a 25W 144MHz amplifier published in (ARRL 1977). The design no doubt appeared in many editions of the handbook. He was resurrecting an old build that just didn’t work as expected, and trying to understand why… which starts with understanding how it works, or should work.

Above is the schematic of the amplifier, analysis here is of the 25W configuration using a 2n5591. Continue reading Analysis of output matching of a certain 25W 144MHz PA

Last update: 12th March, 2021, 5:52 AM