Does common mode current flow inside coax?

The term “common mode current” applied to coaxial transmission lines is bandied about with abandon these days in online fora, awareness of its existence has increased if not understanding.

A simplistic analysis is that in TEM mode, ONLY differential current is supported inside a coaxial line, ie that at any point the current on the outer surface of the inner conductor is exactly equal to a current in the opposite direction on the inner surface of the outer conductor.

But, lets look at the wider context of the meaning of common mode current when a uniform coaxial line is connected to an antenna system. Whilst an antenna might have an obvious two terminal connection to the feed line, in the presence of ground, the current in those two terminals are not necessarily equal and opposite. Continue reading Does common mode current flow inside coax?

Exploiting your antenna analyser #26

Find coax cable velocity factor using a very basic analyser

A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that lacks facility to backout cable sections or measure OSL calibration (as discussed in other articles in this series). The older models and newer budget models often fall into this category.

The manuals for such instruments often explain how to measure coaxial cable velocity factor, and the method assumes there is zero offset at the measurement terminals (whether they be the built-in terminals or some fixture / adapters). In fact even the connectors are a source of error, especially UHF series connectors.

It is the failure to read exactly Z=0+j0Ω with a S/C applied to the measurement terminals that adversely impacts efforts to measure resonant frequency of a test line section.

The method described here approximately nulls out offsets in the instrument, measurement fixture, and even in the connectors used and for that reason may sometimes be of use with more sophisticated analysers.
Continue reading Exploiting your antenna analyser #26

PD7MAA’s BN43-202 matching transformer for an EFHW – full measurement set

I have written some recent articles about or relevant to PD7MAA’s BN43-202 EFHW matching transformer. At about the same time a discussion started on and through that discussion, one ‘online extra expert’ stated that my analysis was bogus (dictionary meaning: not genuine, faked, a misrepresentation).

This article presents detail that was not included in the earlier articles as it distracts from the issue for most readers. Continue reading PD7MAA’s BN43-202 matching transformer for an EFHW – full measurement set

Thoughts on binocular ferrite core inductors at radio frequencies

Binocular ferrite cores are widely used, but not so widely understood.

Understanding inductors is an important first step to understanding transformers are they are coupled inductors.

The usual use of them is to make a winding of several turns around the central limb. One turn is a pass through both sides of the core around the central limb. Figures given in datasheets for Al or impedance rely upon that meaning of one turn.

A common assumption is that L=Al*n^2.

Note that published Al values are obtained by measurement typically at 10kHz and are not directly applicable at radio frequencies for core materials where the permeability µ is significantly different to µi (most ferrites). Notwithstanding this fact, most inductance calculators assume µ is not frequency dependent.

Let us measure a one turn winding on a practical binocular core for reference

Above is a measurement of R,X of a BN43-202 core with a one turn winding at 10MHz. X is  59.57Ω implying inductance of 0.95µH (assuming a simple two component model which does not capture self resonance effects). Datasheets for this core specify Al as 2200nH for one turn, yet we measure 950nH at 10MHz… proof of problems in simple application of Al.

Of course it is possible to make an inductor by passing a conductor once though one side of the binocular, a half turn if you like, but don’t let that label imply the impedance relative to a one turn winding.

Above is a measurement of a BN43-202 core with a ‘half turn’ winding.

If inductance followed the formula L=Al*n^2 and this was truly a half turn winding, we would expect the inductive reactance X ( 37.16Ω) to be one quarter or 25% of that of the single turn inductor (59.57Ω). Clearly it is not, it is 62%, the notion of a half turn or the formula or both have failed badly in this case.

Well on the back of that failure, lets try 1.5t.

Would we be brave or foolish to predict inductance will be 1.5^2 times that for one turn?

Above is a measurement of a BN43-202 core with a ‘one and a half turn’ winding.

If inductance followed the formula L=Al*n^2 and this was truly a half turn winding, we would expect the inductive reactance X ( 155.2Ω) to be 1.5^2 or 2.25 times that of the single turn inductor (59.57Ω). Clearly it is not, it is 2.60 times, the notion of a half turn or the formula or both have failed badly in this case.

Now let us look at Q, the ratio of X/R. The Q of the half turn inductor is 1.051, the one turn inductor is 1.022, and the one and a half turn inductor is 1.016. The quite small decrease in Q may be entirely due to the lower self resonant frequency as more turns are added and may not indicate a significant increase in core loss because of ‘half turn effects’ as sometimes claimed.

The error in conventional n^2 estimates of odd half turns becomes less significant with higher turns.

Conclusions

The traditional formula L=Al*n^2 does not apply to ferrite binocular cores at radio frequencies for odd half turns, and does not account for variation of permeability with frequency or influence of self resonance.

Understanding inductors is the first step to understanding transformers are they are coupled inductors.

Common failings of EFHW matching transformers

I have written many reviews of published EFHW matching transformers, and in most cases the reviews have reported estimated or measured losses that are appalling and not disclosed by the ‘designers’.

Why is it so?

I am asked, why is it so?

Up front, I do not know the answer definitively, but let me offer some thoughts based on the designer’s own articles and discussions by ‘online experts’.

Apparent reasons include:

  1. lack of understanding of ferrite and powdered iron core material behaviour;
  2. lack of understanding of coupled coils, and mutual inductance;
  3. use of inductor design tools that are inadequate at radio frequencies;
  4. lack of competency in basic linear circuit theory analysis for AC circuits;
  5. failure to make meaningful measurements of the built article;
  6. focus on input VSWR as a single metric indicating goodness;
  7. reliance on QSOs for evidence of performance;
  8. an attitude that antenna system radiation efficiency doesn’t matter, particularly for QRP (if the term antenna system radiation efficiency is even understood as a quantitative metric).

Continue reading Common failings of EFHW matching transformers

PD7MAA’s BN43-202 matching transformer for an EFHW – measurement of a prototype

At PD7MAA’s BN43-202 matching transformer for an EFHW I gave an estimate of the core loss in PD7MAA’s transformer.

This article reports measurement of a prototype built to his design.

 

Above is PD7MAA’s graphic for his transformer. It is a little confusing as an 11t wind will start and finish with ends as the blue wind, so the red winding must have and odd number of half turns which suggests the windings are actually 1t and 5.5t (pity he did not show a picture of the real transformer).

PD7MAA gives some measurements for his transformer with a 3300Ω load, but he does not give loss measurements. This experiment is to replicate his configuration, measure the loss and compare it to the estimate given at PD7MAA’s BN43-202 matching transformer for an EFHW.

The prototype uses 1t primary and 5.5t secondary. The secondary load is a 3300Ω resistor in series with the VNA 50Ω input port.

Above is a screen shot of a sweep from 6 to 8MHz. The key data is that shown for the marker at 7.1MHz. Continue reading PD7MAA’s BN43-202 matching transformer for an EFHW – measurement of a prototype

PD7MAA’s BN43-202 matching transformer for an EFHW

End Fed Half Waves have certainly captured the minds of QRP aficionados, and there is a steady stream of ‘designs’ appearing on the ‘net.

A recent article by PD7MAA describes such a transformer using a BN43-202 balun core for up to 20W PEP from 7-29MHz.

Above is PD7MAA’s graphic for his transformer. It is a little confusing as an 11t wind will start and finish with ends as the blue wind, so the red winding must have and odd number of half turns which suggests the windings are actually 1t and 5.5t (pity he did not show a picture of the real transformer). Let’s proceed under that assumption. Continue reading PD7MAA’s BN43-202 matching transformer for an EFHW

Exploiting your antenna analyser – contents

A convenient list of ‘Exploiting your antenna analyser’ and short subject sub-titles, a table of contents for the series as it grows.

Exploiting your antenna analyser #29 Resolving the sign of reactance – a method – Smith chart detail

Exploiting your antenna analyser #28 Resolving the sign of reactance – a method

Exploiting your antenna analyser #27 An Insertion VSWR test gone wrong

Exploiting your antenna analyser #26 Find coax cable velocity factor using a very basic analyser

Exploiting your antenna analyser #25 Find coax cable velocity factor using an antenna analyser without using OSL calibration

Exploiting your antenna analyser #24 Find coax cable velocity factor using an antenna analyser with OSL calibration

Exploiting your antenna analyser #23 Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser Continue reading Exploiting your antenna analyser – contents

Earth electrodes in parallel

I came across an article giving guidance to hams about antenna / station grounding, presumably for lightning protection.

The question is, what is the ground resistance improvement of one electrode over the two shown above. Let’s ignore the issue of earthing conductor size and deal only with the issue of parallel electrodes.

We don’t know the soil type, and we need to guess the spacing… it appears to be one house brick which is 9″ or 225mm in a lot of the world, perhaps that applies to the pic.

By way of an example, let’s make some assumptions that are likely to apply in lots of practical implementations. Continue reading Earth electrodes in parallel

VY Commodore key repair

There is lots of advice from online experts on dealing with a flat battery in a Commodore VY key. The battery is not “user replaceable”, it is embedded inside a key shell that is glued together.

In my case, the battery had leaked.

The following procedure was for the exact key, there may be other key types used in VY production, and it is also possible that other models used a key with the same internals and could be repaired in the same way.

NO WARRANTY: if you break it, you get to keep both parts!

I am an experienced technician, it is inadvisable to attempt this unless you have the necessary competencies.

Enough of the fear, let’s get on with it.

The key uses an ordinary CR2032 lithium cell, though with tabs. Above are two batteries which I prepared with 4mm tabs, but as it turned out, while the +ve one is 4mm the -ve tab is 3mm, I should have used 3mm.

Holden’s answer to the problem of the $2 battery being flat / leaking is to buy a new key head and get it programmed for a total cost of around $150.

Above, the underside of the board. The small chip is a 93S46 EEPROM, which hints that this thing has non-volatile memory and unless the controller chip does something clever like erase the EEPROM on power up, it should be possible to replace the battery carefully without sustaining power to the board. Continue reading VY Commodore key repair