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

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, it 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

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

Measuring OC and SC transmission line sections

Failure estimating transmission line Zo – λ/8 method – nanoVNA discussed the potential for failure using this ‘no-brainer’ method of estimating differential mode characteristic impedance Zo, providing an NEC-4.2 model to demonstrate effects.

This article reports nanoVNA measurement of a two wire line where no common mode countermeasures were taken.

A little review of behavior of practical transmission lines

Above is a Smith chart of the complex reflection coefficient Γ (s11) looking into a length of nominally 142Ω transmission line of similar type to that in the reference article, the chart is normalised to Zref=142+j0Ω. Note the locus is a spiral, clockwise with increasing frequency, and centred on the chart prime centre Zref. More correctly it is centred on transmission line Zo, and the keen observer might note that the spirals are offset very slightly downwards, actual Zo is not exactly 142Ω, but 142-jXΩ where X is small and frequency dependent, a property of practical lines with loss. Continue reading Measuring OC and SC transmission line sections

Estimating transmission line Zo – λ/8 method – nanoVNA – success

Failure estimating transmission line Zo – λ/8 method – nanoVNA discussed the potential for failure using this ‘no-brainer’ method of estimating differential mode characteristic impedance Zo.

Well, as the article showed, it is not quite the no-brainer but with care, it can give good results. This article documents such a measurement of a 0.314mm cable.

The nanoVNA was carefully SOLT calibrated from 1 to 201MHz. Care includes that connectors are torqued to specification torque… no room here for hand tight, whether or not with some kind of handwheel adapter or surgical rubber tube etc.

Above is the Smith chart view over the frequency range from a little under λ/8 to a little over λ/8. It is as expected, a quite circular arc with no anomalies. Since the DUT is coax, and the connector is tightened to specification torque, we would expected nothing less. The situation may be different with two wire lines if great care is not taken to minimise common mode excitation. The sotware does not show Marker 2 properly, it should be between ‘c’ and ‘i’ of the word Capacitive. Continue reading Estimating transmission line Zo – λ/8 method – nanoVNA – success

A magnetics review of the VK3AMP Sontheimer directional coupler

This article documents a review of the magnetics of the ‘voltage’ transformer in the VK3AMP Sontheimer directional coupler. It is typically the most important component in determining InsertionVSWR and ReturnLoss at the lowest frequencies.

The transformer of interest is the one to the left, and if you follow the tracks, the multiturn winding is connected between ground and a track that routes across to the through line. The transformer primary appears in shunt with the through line. Continue reading A magnetics review of the VK3AMP Sontheimer directional coupler

Basic measurements of the VK3AMP Sontheimer directional coupler for a N2PK wattmeter

This article documents measurement and analysis of a VK3AMP Sontheimer directional coupler in an implementation of a 400W wattmeter design by N2PK (Kiciack nd).

I purchased one of the couplers for use with a DIY digital display, and although I have had it longer, it isn’t yet realised!

A common failing of almost all hammy Sammy designs is appalling InsertionVSWR at the lower end of the specified frequency range. This coupler is specified for 1.8-54MHz, and differently to most, has meaningful published characteristics.

In this implementation, 60mm lengths of solder soaked braid coax similar to Succoform 141 were used between the PCB and box N connectors. The expected matched line loss of both of these is about 0.01dB @ 50MHz.

The measurements here were made by VK4MQ using an Agilent E5061A ENA, data analysis by myself.

Above are the raw s parameter measurements plotted. It is a full 2 port measurement, and it can be observed that the device is not perfectly symmetric, quite adequate, and quite good compared to other ham designs that I have measured. Continue reading Basic measurements of the VK3AMP Sontheimer directional coupler for a N2PK wattmeter

Failure estimating transmission line Zo – λ/8 method – nanoVNA – Smith chart perspective

Failure estimating transmission line Zo – λ/8 method – nanoVNA discussed traps in using the λ/8 method to estimate Zo… it is not the no-brainer that is often suggested.

This article shows the use of the Smith chart to look for departures from pure transmission line behavior in that test, or any other that depends on measuring purely Zin of a length of line in purely differential mode with short circuit or open circuit termination.

Above is a Smith chart plot of what we should see looking into a line of similar characteristic swept from 1 to 20MHz. There is no magic there, this is basic transmission lines and Smith chart. Continue reading Failure estimating transmission line Zo – λ/8 method – nanoVNA – Smith chart perspective