Review of “The Truth About Magnetic Loop Antennas – MYTH BUSTING!”

A reader referred me to a video of a presentation to a radio club, the subject being “The Truth About Magnetic Loop Antennas – MYTH BUSTING!

The presentation includes prediction and measurement of a small transmitting loop. This article tries to reconcile the claimed radiation efficiency between prediction and measurement.

Radiation Efficiency

The presentation liberally uses the term “efficiency”, let us take that to mean Radiation Efficiency:

the ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter.

Note that Radiation Sphere requires that radiated power must be measured / determined / summed in the far field.

Radiation fields decay inversely proportional to distance, other fields immediately around an antenna decay more quickly and are insignificant for the purpose of radio communications at great distances. Hence, Radiation is the usual objective of radio communications antennas.

The calculation tool used in the presentation defines:

Efficiency (%): The percentage of input energy that is actually radiated and not lost as heat.

The prototype loop antenna

The prototype loop antenna is described:

A table is given which appears to be predicted performance based on https://miguelvaca.github.io/vk3cpu/magloop.html . Continue reading Review of “The Truth About Magnetic Loop Antennas – MYTH BUSTING!”

200h maintenance – Toro MX 4250

My Toro MX 4250 residential zero turn mower, has 200h of service over 8 years… which is not very much.

Above is a recent capture of battery voltage (red) and battery current (blue) using a Hantek 1008C that was under evaluation. (The image colours were inverted for the article.)

Valve lash measurement

Starter current measurement hinted that valve lash was incorrect in at least one cylinder

The exhaust valves should be 0.15-0.2mm lash, and were measured within that range for one cylinder and between 0.2 and 0.25mm (Cylinder #1). This probably accounts for the difference in cranking current at compression, the compression relief is degraded by excessive valve lash.

The inlet and exhaust valves were adjusted as needed.

Above is a capture of starter current after the valve lash adjustment. Note that the peaks in each pair are approximately equal.

Spark plugs

Spark plugs were removed and inspected. The gap was good, the electrodes had sharp corners, Nevertheless, new plugs were reinstalled (per the service manual).

Leak down test

A leak down test was conducted using an OTC 5609. Leak down was less than 1% @ 400kPa in both cylinders, excellent.

Oil change

Oil and oil filter changed.

Fuel filter

Fuel filter changed.

Air filter

The prefilter was washed and re-oiled. The paper cartridge was pretty clean, but was blown out with compressed air.

Control dampers

The right hand control damper is not effective, and sufficiently bad to impact driving.

Replacements are unavailable at this time.

Toro MX 4250 starter current captured using a Picoscope

A recent exercise was to become familiar with a recent acquisition, a Picoscope USB oscilloscope.

It has quite different software to those of other USB oscilloscopes that I have used, so a bit of learning and adaptation.

The test scenario here is capture of battery current (CC-650 probe) and battery voltage whilst cranking my lawnmower which as a 24.5HP V twin petrol engine.

Above is the captured data expanded to explore the initial part of cranking (with disconnected spark plugs). Continue reading Toro MX 4250 starter current captured using a Picoscope

Determination of transmission line characteristic impedance from impedance measurements – eighth wave method

For a lossless line, the reactance looking into short section and open circuit terminated line sections is \(X_{sc}=Z_0 \tan \beta l\) and \(X_{oc}=Z_0 \frac1{\tan \beta l}\).

Noting that when \(\beta l= \frac{\pi}{4}, \tan \beta l=1\) so when the line section is π/4ᶜ or 45° or λ/8, then \(|X_{sc}|=|X_{oc}|=Z_0\).

We can use this property to estimate Zo of an unknown practical low loss transmission line by finding the frequency where \(|X_{sc}|=|X_{oc}|\) and inferring that \(Z_0 \approx |X|\).

 

Above is a chart created using Simsmith’s transmission line modelling of the reactance looking into short section and open circuit terminated 10m sections of RF174. The blue and magenta lines intersect at X=51.16Ω whereas red R0=51.85Ω, about -1.3% error. The error depends on line loss, line length, frequency and the characteristics of the terminations. Continue reading Determination of transmission line characteristic impedance from impedance measurements – eighth wave method

Determination of transmission line characteristic impedance from impedance measurements #2

Determination of transmission line characteristic impedance from impedance measurements discussed issues with the short circuit and open circuit terminations used with measurement of Zoc and Zsc for calculation of characteristic impedance of a line section.

Included was a model of the effect of small delay offset in one of the termination parts on an example scenario.

This article gives a Simsmith model that readers might find interesting to explore the effects of line length, offset, line characteristics, and frequency.

I have issues with Simsmith modelling of transmission lines, but nevertheless the model is informing.

The above example is 6m of RG58A/U with 5mm offset in the short circuit termination. Continue reading Determination of transmission line characteristic impedance from impedance measurements #2

Relationship between radiation efficiency and minimum VSWR for common short helically loaded verticals

This article explores the relationship between radiation efficiency and minimum VSWR for common short helically loaded verticals.

For clarity, \({RadiationEfficiency}=\frac{FarFieldPower}{InputPower}\).

Such antennas are often advertised with a “minimum VSWR” or “VSWR at resonance” figure, but rarely show gain figures. One might wryly make the observation that that is how one might sell dummy loads rather than antennas.

Well, these things do radiate, so they are not very good dummy loads. Lets explore a theoretical example on the 40m band to inform  thinking.

Unloaded vertical

Above is a NEC5.2 model of a vertical on a wagon roof. Continue reading Relationship between radiation efficiency and minimum VSWR for common short helically loaded verticals

Determination of transmission line characteristic impedance from impedance measurements

Measured impedances looking into a uniform transmission line section with short circuit (SC) and open circuit (OC) terminations can provide the basis for calculation of characteristic impedance Zo.

We rely upon the following relationships:

\(Z_{sc}=Z_0 \tanh (\alpha + \jmath \beta )l\\\) and

\(Z_{oc}=Z_0 \coth (\alpha + \jmath \beta )l\\\)

Rearranging the formulas and multiplying, we can write:

\(Z_0^2=\frac{Z_{sc}}{\tanh (\alpha + \jmath \beta )l} \frac{Z_{oc}}{\coth (\alpha + \jmath \beta )l}\\\) \(Z_0^2=\frac{Z_{sc}}{\tanh (\alpha + \jmath \beta )l} Z_{oc}\tanh (\alpha + \jmath \beta )l\\\)

The tanh terms cancel out… provided the arguments are equal. Focus on length l, l for the short circuit measurement might not equal l for the open circuit measurement if the termination parts are not ideal (and they usually are not).

If the tanh terms cancel, we can simplify this to \(Z_0=\sqrt{Z_{sc}Z_{oc}}\). This is commonly parroted, apparently without understanding or considering the underlying assumption that l is equal for both measurements.

Another big assumption is that it is a uniform transmission line, ie that the propagation constant β is uniform along the line… including any adapters used to termination the line.

The third assumption is that the measured impedance values are without error.

Above is a plot of calculated Zo for a theoretical case of a line of ~10m length of Belden 8267 (RG213A/U) around the frequency of first resonances. This calculation essentially imitates perfect measurements of perfect DUTs. Continue reading Determination of transmission line characteristic impedance from impedance measurements

Review of mower starter motor current using Hantek 1008C and Hantek CC-650

Review of mower starter motor current using Owon HDS242S and Hantek CC-650 reported measurement of starter motor current in “normal” operation (ie engine starts and runs). This article reports measurement of starter motor current and battery voltage with the spark plug connectors removed, the mower is Toro MX4250 mower with the Toro (Loncin) 0.708l 18kW V-twin engine.

Above is capture of battery voltage (red) and battery current (blue) using a Hantek 1008C that was under evaluation. (The image colours were inverted for the article.) Continue reading Review of mower starter motor current using Hantek 1008C and Hantek CC-650

NanoVNA-App v1.1.209-OD15 released

Most of the changes I have made to NanoVNA-App have been to align it with accepted standards and conventions.

This change is to the format of saved Touchstone, .s1p and .s2p, files.

Though the relevant specification is silent on the permitted decimal separator, the only one shown in examples is “.” so it is reasonable to interpret that the required separator is “.” which makes the file format locale independent (as were the first instruments using Touchstone format.

This release of NanoVNA-App writes “.” decimal separator, independent of locale.

The original reading code which was tolerant of either “.” and “,” is maintained, so it will continue to open files which might have been (incorrectly) saved using “,”.

NanoVNA-App-Setup-v1.1.209-OD15