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 Z0.

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 Z0 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

Paccomm Tiny-2 Mk2 capacitor polarity problem

Many Paccomm Tiny-2 Mk2 TNCs have a polarised capacitor installed incorrectly. They become leaky and it degrades performance.

Let’s look at the datasheet for a MAX231 and review the charge pump and related filter capacitors.

Note especially the capacitor connected from pin 3 to ground, it is an electrolytic with -ve terminal to pin 3 and it develops the -ve voltage for the true RS-232 line driver. Continue reading Paccomm Tiny-2 Mk2 capacitor polarity problem

Derivation of the expression for the unknown impedance in an s21 series through measurement

This article presents a derivation of the expression for the unknown impedance in an s21 series through measurement.

The diagram above is from (Agilent 2009) and illustrates the configuration of a series-through impedance measurement. Continue reading Derivation of the expression for the unknown impedance in an s21 series through measurement

Mower deck maintenance – Toro MX 4250

The deck is a fabricated deck on a Toro MX 4250 residential zero turn mower, it has 200h of service over 8 years… which is not very much.

The deck has the recycler option installed.

Deck service included:

  1. Fit new bearings to spindles;
  2. Replace both idler wheels (components are not serviceable);
  3. Clean and apply corrosion protection under deck;
  4. Procure, fit and balance new blades; and
  5. Procure spare drive and deck belts for spares.

Spindle bearings

Jiggling the blade tips showed 2-3mm vertical play in both spindles. New bearings have much less play, undetectable without instruments.

The spindles are easily serviced.

The lower bearing is one spindle had a rusted retainer / separator and collapsed when driving it from the housing. The other lower bearing was rough, but did not collapse. The top bearings were in good condition.

A set of four NSK 6203-2RS bearings was purchased ($35) to replace all the bearings.

Idler pulleys

Both idler pulleys were dry of grease. It was possible to pop the rubber seal and pack a little more grease pending arrival of replacement parts.

Replacement parts were purchased for $60 (inc shipping) and fitted.

The message is that water sprayed over the top of the deck assembly is retained in a shallow cup on the sealed bearings and some will wind up inside the bearings… don’t wash the top of the deck.

Deck clean, descale and corrosion protection

Above, the underside of the deck after washing, de-scaling with a pneumatic needle scaler, power brushing with a cup brush, and another high pressure wash ready for application of corrosion protection. Continue reading Mower deck maintenance – Toro MX 4250

A low cost break-out board for STDC14

It is now near impossible to procure a genuine STLINKV2 or a good clone. Newer versions of STM32CubeProgrammer will not work with poor clones.

In pursuit of a small inexpensive replacement for the STLINKV2, the STLINKV3-MINIE is a candidate. That said, they can be difficult to source… mine is on 9 months wait from element14.

Above is the STLINKV3-MINIE (~$20). Continue reading A low cost break-out board for STDC14

Programming a PIC 12F510 using the DIY-150 programmer

A correspondent asked how to program a PIC 12F510 used in one of my projects using his DIY-150 programmer.

The DIY-150 programmer is quite an old design and there has been no development for more than 10 years, meaning no new chips added, though there are lots of online sellers of the now stale design.

The 12F510 is very similar to the 12F509 which microbrn v150807 does support. The 12F510 can be programmed by selecting chip type 12F509.

Alternatively, download the archive below and extract a modified chipdata.cid file which includes a 12F510, and extract it to replace the existing installed file.

The revised chipdata.cid can be downloaded here: chipdata.7z.