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
The article Low power Guanella 1:1 balun with low Insertion VSWR using a pair of Jaycar LF1260 suppression sleeves describes a current balun with low Insertion VSWR for operation at modest power levels. The design was based on Jaycar LF1260 cores which are readily available in Australia.
This article presents the workup of a balun with similar design objectives using a pair of low cost Fair-rite 2631540002 cores (FB-31-5621) which are similar in size to the LF1260 and have higher µi (1500 vs 1000).
Above, a pic of the cores from Amidon’s catalogue. Continue reading Low power Guanella 1:1 balun with low Insertion VSWR using a pair of Fair-rite 2631540002 suppression sleeves – design workup
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
Readers of my articles occasionally ask for explanation of the distinction between meanings of:
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 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 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 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
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.
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
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
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
In a recent QST article, (Gable 2021) gave the following advice:
Self-SWR is commonly known as Insertion VSWR. The article contains several errors in definition of SWR, Return Loss and Insertion Loss… but suffice to say that he uses \(InsertionLossdB=-|s21dB|\). Continue reading QST 2/21 on Insertion Loss
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.
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
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