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 describe 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.

This is a simple transmission line problem, and we would expect that the input VSWR of a length of approximately 50Ω line to be approximately 1.0, but it is substantially different at 1.5, why?

There could be many factors contributing, but if we assume that he used a validated good VSWR meter and dummy load for his published measurements (and they could be checked easily), a possible explanation is that the RG213 is not 50Ω. Could that be the case?

He has specified a radius of curvature of the coax on the form of 63mm, half the minimum specified by Belden for their 8276 RG213 coax, so it is quite possible, likely even, that the tight wrap of RG213 has deformed the shape of the coax profile and altered its characteristic impedance. (This deformation and conductor migration is an even greater issue for foam dielectric cables.)

If we assume that velocity factor remains approximately 0.67 by virtue of the solid polythene dielectric, we can calculate the electrical length of 21′ at 3.5MHz to be 40.8°, and find the value of Zo that would account for an input VSWR(50)=1.5.

Above is a Smith chart solution to the problem. If the sole reason for input VSWR(50)=1.5 is deformation of the coax altering its Zo, then the Zo was approximately 37Ω.

Migration of inner conductor is a problem with small bend radius, and the greater distance subject to small radius bends, the greater to contribution to impedance transformation.

Sure it is cheap, but is it a good implementation?

Properly executed “Ugly Baluns” baluns or common mode chokes have three notable features:

  • very low insertion VSWR;
  • a very narrow, very high impedance peak that gives excellent common mode reduction in that very narrow band; and
  • low cost.

They are fairly ineffective as a wide band common mode choke, and this one on the author’s reported measurements has poor Insersion VSWR. It probably does have very high common mode impedance over a narrow frequency range, but no measurements were given to show that band coincided with any ham band.

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.

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

LP-100A impedance measurement

A correspondent wrote seeking clarification of the Telepost LP-100A claims re impedance measurement in the context of some of my previous articles on the sign of reactance.

I could see several mentions in the LP-100A manual and the LP_100Plot documentation and they do seem a little inconsistent.

The LP-100A manual states very clearly:

Note: The LP-100A cannot determine the sign of X automatically.

and;

If you QSY up from your current frequency, and the reactance goes up, then the reactance is inductive (sign is “+”), and conversely if it goes down, then the reactance is capacitive (sign is “-“). A suitable distance is QSY is about 100 kHz or more. The LP-Plot program has the ability to determine sign automatically, since it can control your transmitter’s frequency. When it plots a range of frequencies, it uses the slope of the reactance curve to determine sign, and plots the results accordingly.

The first part states clearly that the instrument cannot directly measure the sign of reactance, and presumably measures the magnitude of reactance |X|.

Lets explore the second part in light of the overarching statement of the first part.

Above is the calculated R and X looking into 7m of Belden RG58C/U with a load 25+j0Ω. Also shown is |X|(as would be measured by the LP-100A) and calculated magnitude of phase of R,X, |φ|. Continue reading LP-100A impedance measurement

Rigexpert Antscope v4.3.1 released

It seems yet another new version of Rigexpert Antscope has been released, and it maintains the scale limits available for R,X plots to +/-2000Ω, it still does not allow the range permitted by v4.2.57 (+/-5000Ω).

No change details provided by Rigexpert.

Back to v4.2.57, though it is very likely it has undisclosed defects fixed in later releases.

Bottom line is that if you want an analyser with direct graphing of impedances over 2000Ω (eg measuring common mode choke impedance), think of a different analyser.

 

Tytera MD-390 DMR portable evaluation

Recent advertising of the Boafeng DM-5 DMR portable prompted a review of available products at the low end of the market.

The Boafeng DM-5 was dismissed on a desk study that revealed that advertisements were misleading and deceptive in that the claimed Tier 2 support was not yet delivered and was not included in the advertised price. Previous experience with Boafeng also factored against that solution.

Tytera have produced several DMR portables, and the MD-380 has gained a good reputation. I borrowed one for a trial, and it performed quite well… well enough to proceed with purchase of an MD-390 which appears to have similar internals but revised packaging to obtain IP67 protection.

The MD-390 was purchased on eBay for $162 delivered. It came with a US power pack (earning bad feedback), programming cable, two antennas, an earpiece mic set, a disk of all sorts of drivers which aren’t needed, out of date CPS software and Chenglish manual. Continue reading Tytera MD-390 DMR portable evaluation