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 #30 Quality of termination used for calibration

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 SOL calibration

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

G4YDM balun

G4YDM described his balun at Ham Radio – What Is a Balun and How to Make One Cheaply.

With a title like that it is sure to have wide appeal, but it isn’t anything too novel, it is simply an air solenoid of 50Ω coax cable as a common mode choke, commonly known as an Ugly Balun.

He gives some instructions for one of several constructions:

When wrapping your coax around the pipe don’t use too much force as it may damage the inner braid and space the turns away from each other by a millimetre or two. R-G-2-1-3 coax around 21 feet used with 5 inch pipe will handle 400 watts pf power.

Above is a pic of the third construction which appears to be 21′ of RG213 on a 5″ PVC former:

He gives some performance measurements adjacent to the pic above:

Using a dummy load connected to the choke and transmitting 100 watts from my transmitter indicated an S.W.R. readings of around 1.5 to 1 at 3.5 Megahertz when testing 28 Megahertz the S.W.R. reading came down to 1.1 to 1 which is an excellent match. …

The test described above seems to simply be a dummy load connected to one end of 21′ of RG213 and the transmitter with VSWR meter feeding the other end. To be meaningful we need to know the impedance of the dummy load, indeed to be meaningful it needs to be 50Ω, so lets assume that is the case. Continue reading G4YDM balun

Another speaker mic modification to reduce RF interference

Remote speaker-microphones and DMR portables discussed RF ingress to Speaker Mics (RSM) used with DMR radios in digital mode.

The problem

I purchased a RSM branded Kenwood but obviously a Chinese fake for an MD-390 for about $5 posted, but it turned out to be lousy with RF interference in the form of the ‘motorboat noise’ on transmit audio.

Dismantling the RSM  I found there is precious little RF filtering, just a single SMD cap near the electret capsule.

A solution

Above is the modified RSM. Continue reading Another speaker mic modification to reduce RF interference

Rigexpert Antscope v4.3.1 dowload link?

At Rigexpert Antscope v4.3.1 released I commented on a new release of Antscope.

Correspondents have asked where I obtained v4.3.1.

Well, it seems the Rigexpert website is broken again, the URL to list the Antscope downloads produces garbage. Nevertheless, you can get a directory listing at https://www.rigexpert.com/files/antscope/ and yes, you will note that v4.3.1 is not listed… so it seems to have been either pulled due to defects or it is just a consequence of the web site problems.

Little loss, I use v4.2.57 on Rigexpert’s advice as it has better scales for impedance plots… and v4.2.57 is still published (at the time or writing) https://www.rigexpert.com/files/antscope/antscope040257.zip .



Regenerative braking and electronic power supplies

Simple DC machines

Simple DC machines includes a DC motor with permanent magnet field and wound armature with commutator. The permanent magnet DC motor is a good case to study.

In simple DC machines, the difference between being a motor and generator is often simply a matter of rotational speed. The motor develops an induced voltage in its windings by virtue of its rotational speed, and current flows in the winding if that voltage is different to the terminal voltage… the direction of current determined by which voltage is higher and the direction of current determines whether the torque assists or resists the rotation.

The counter torque from reverse current is often referred to as regenerative braking as the retarding effect of the current driven by the induced emf of rotation slows the motor, and current flows into the source.

If a simple DC machine is powered from a simple rectifier circuit, the rectifier will block the flow of reverse current, and so there is no regenerative braking… the rotation induced emf simply raises the terminal voltage of the motor (possibly dangerously), but no current flows and there is no counter torque.

If a simple DC machine is powered from an electronic regulated power supply, the situation is a little different. The regulator will commonly block reverse current, and it may sense that output voltage is greater than desired and shut down, it may even be damaged by the excess terminal voltage.

Brushless DC motors

Brushless DC motors use some form of electronic driver to provide commutation of current in the coils, whether derived from sensors fitted to the motor or sensed from the undriven coil at any instant.

Some driver configurations provide a path for regenerative current to flow to the power source. If the power source blocks the regenerative current, the terminal voltage of the motor and power supply may increase, possibly to levels that may damage the motor driver and damage or disrupt the power supply. Electronic power supplies do not usually contain provision for regenerative current.

An example sensorless brushless DC motor used in UAVs

This example illustrates the nature of regenerative current in a particular application where rapid response of the drive is very important.

Above is a supply current graph for a test scenario that subjects the drive to a number of acceleration / deceleration scenarios. The current sensor does not measure negative current, its output is clipped at I=0. Continue reading Regenerative braking and electronic power supplies

RCTimer 4215-530Kv BLDC motor checkout

I purchased an inexpensive BLDC for some tests on a 6S battery pack. The RCTimer 4215-530Kv could be loaded up to about 20A at 24V (the limit of my bench supply) with an 11×4.7 SF propeller (in stock). The motor is a low pole count motor, 12N14P. On 24V, the no load speed should be almost 13,000rpm, and fully loaded perhaps three quarters of that.

Above is the motor as supplied. I used an ordinary M6 propeller nut so that it was easy to remove and replace without wearing out the nyloc nut supplied.

Above, the induced voltage waveform at 940rpm, somewhat the result of the 12N14P configuration.


The ESC was a Hobbyking 40A ESC 4A UBEC 9261000003, SimonK commit 02bd8e4ca36a06722efe51bc7cd5130d72a184b8 with COMP_PWM.

On a steady test on the 24V bench supply, the drive drew just on 20A cold and was clocked at 9200rpm with the Gemfan 1147 SF. Winding speed up and down slowly (to avoid degenerative braking which is incompatible with the power supply), motor starting, acceleration and deceleration were always smooth and without any sign of sync loss.

Tests were conducted with a script that I use consistently with asrg and eLogger to capture current, altitude is 700m. Continue reading RCTimer 4215-530Kv BLDC motor checkout

End fed matching – VK3IL design on LO1238

A correspondent asked about the use of a Jaycar LO1238 ferrite core in VK3IL’s EFHW matching unit for 40m and up. The LO1238 implementation would use 3t primary and 24t secondary on the core.

If the transformer is simply used without an ATU between it and the radio, and we assume that the antenna system is adjusted to present low VSWR(50) to the radio, a simple approximation involves calculating the magnetising admittance of the 3t 50Ω winding, and calculating the portion of total input power that is dissipated in that admittance.

Using the calculator at Calculate ferrite cored inductor, the admittance (G+jB) of the 3t winding is 0.00177-j0.00204S. (The impedance of a sample wind could be measured with a suitable analyser and converted to admittance.) Continue reading End fed matching – VK3IL design on LO1238