Month: February 2022

On testing two wire line loss with an analyser / VNA – part 5

This article series shows how to measure matched line loss (MLL) of a section of two wire line using an analyser or VNA. The examples use the nanoVNA, a low end inexpensive VNA, but the technique is equally applicable to a good vector based antenna analyser of sufficient accuracy (and that can save s1p files).

Article On testing two wire line loss with an analyser / VNA – part 2 showed a 1:1 transformer for measuring two wire lines without encouraging significant common mode current.

Online experts suggest that the required transformer is one from 50Ω to Zo of the line being measured. It is often said that: Continue reading On testing two wire line loss with an analyser / VNA – part 5

Last update: 28th February, 2022, 12:40 AM

NanoVNA-H4 – a ferrite cored test inductor impedance measurement – s11 reflection vs s21 series vs s21 pi

This article documents estimation of common mode choke impedance by three different measurement techniques.

The test uses a small test inductor, 6t on a BN43-202 binocular core and a small test board, everything designed to minimum parasitics. This inductor has quite similar common mode impedance to good antenna common mode chokes.

Above is the SDR-KITS VNWA testboard. Continue reading NanoVNA-H4 – a ferrite cored test inductor impedance measurement – s11 reflection vs s21 series vs s21 pi

Last update: 15th June, 2024, 11:11 AM

The Smith chart, a thing of beauty… and great utility

A recent post online provides an interesting demonstration of the value of the Smith chart in analysing a measurement problem.

I have 5.175m of “JSC 1320 300 Ohm Ladder Line 300 Ohm 20 AWG / 7 Strands Bare Copper”. … The first step is to sweep it to determine the velocity factor. Yet, when I sweep from 12-17MHz, I get the Smith chart attached. There’s no point when the impedance is close to zero.

It helps to understand the nature of what one is measuring, indeed the expected outcome if possible. Continue reading The Smith chart, a thing of beauty… and great utility

Last update: 21st February, 2022, 8:21 PM

Loop in ground (LiG) for rx only on low HF – #11 three terminal equivalent Z

The Loop in Ground project is about a receive only antenna for low HF, but usable from MF to HF. The objective is an antenna of that is small, low profile, and can be located outside the zone where evanescent modes dominate around noise current carrying conductors, like house wiring to minimise noise pickup.

To some extent, the project was inspired by KK5JY’s Loop on Ground (LoG).

This article presents measurements and the three terminal equivalent impedance model.

Above is the three terminal equivalent impedance model. Elements Z1, Z2 and Z3 are derived from measurements Za, Zb, and ZC as discussed at Find three terminal equivalent circuit for an antenna system. Continue reading Loop in ground (LiG) for rx only on low HF – #11 three terminal equivalent Z

Last update: 19th February, 2022, 4:59 PM

On testing two wire line loss with an analyser / VNA – part 4

This article calculates and compares three models for matched line loss (MLL) based on measurement of a transmission line section with short and open termination.

This article follows on from:

Measurements

The measurements permitted calculation of MLL vs frequency over the measurement frequency range of 10-200MHz.

The measurement frequency range was chosen as appropriate to the intended application range and the available / manageable sample length. To make measurements down to 100kHz with similar measurement noise would have required a test length of hundreds of metres.

Curve fitting

The measurement data was fitted to three popular models for MLL.

Above is a plot of MLL (dB/m) calculated from the measurements saved as s1p files (raw), and fits to three models: Continue reading On testing two wire line loss with an analyser / VNA – part 4

Last update: 20th February, 2022, 8:36 AM

The oft asked question of how much an LNA improves a 70cm weak signal station – Rules of Thumb

The article The oft asked question of how much an LNA improves a 70cm weak signal station solicited some comment on optimal configurations. This article deals with the notion / Rule of Thumb that optimal LNA gain is just sufficient to offset line losses.

This article explains with graphs the relationship between Signal / Noise degradation (see Signal to noise degradation (SND) concept) and LNA gain in the configurations discussed in the original article. See The oft asked question of how much an LNA improves a 70cm weak signal station for documentation of the scenario assumptions.

The critical value for SND is a personal choice, but for the purpose of this discussion, let’s choose 1dB. That is to say that the S/N at the receiver output is less than 1dB lower than the ultimate that could achieved with the antenna system given the external noise environment.

The total line loss in the example configurations was 2.6dB. The model assumes that LNA Noise Figure is independent of LNA Gain, though in the real world, there is typically some small dependence.

Often the choice of LNA Gain drives the choice of a single stage or two stage LNA, which has cost implications.

Terrestrial external noise – 495+5K

Above is a chart showing SND vs LNA Gain. It can be seen that as LNA gain is increased, SND improves rapidly with a knee around 15dB LNA gain above which SND improvement is slower. Continue reading The oft asked question of how much an LNA improves a 70cm weak signal station – Rules of Thumb

Last update: 15th February, 2022, 7:33 AM

The oft asked question of how much an LNA improves a 70cm weak signal station

A recent online post seeking opinions on the chap’s 70cm weak signal configuration is an interesting subject for study, and one that should be of interest to many weak signal DXers.

This article focusses on just one question in a quite similar configuration, what is the advantage given by the LNA?

Study configuration

The scenario will be evaluated for both terrestrial and satellite paths.

Above is the assumed ambient noise environment, it has great bearing on the results. More on that later. Continue reading The oft asked question of how much an LNA improves a 70cm weak signal station

Last update: 15th February, 2022, 4:53 AM

An admittance graph for NanoVNA-App

Often one finds that a cartesian plot of the components of admittance (conductance and susceptance) would be a convenient plot in understanding / solving a problem.

Let’s work through an example of designing an antenna shunt match to illustrate.

The example is based on measurement of the feed point impedance of a real antenna, an M40 1.2m long vehicle mounted helical whip for the 40m band.

Above is a plot of the VSWR. The minimum VSWR is a bit high, let’s drill down on it. Continue reading An admittance graph for NanoVNA-App

Last update: 12th February, 2022, 6:14 PM