Radcom Feb 2019 “cable balun” – a deeper look

This article is a continuing discussion of Radcom Feb 2019 “cable balun”.

The article Baluns in antenna systems explores some different dipole and feed line configurations and the effectiveness of common mode chokes at various locations on the feed line.

Models 1, 2 and 3 particularly show the effect of a quarter wave vertical common mode conductor grounded and isolated, and a half wave vertical common mode conductor grounded.

These illustrate that those common mode conductors can be viewed to some extent as a ‘single wire’ transmission line, and the impedance presented at the dipole feed point is low or high in keeping with simple transmission line analysis of a shorted or open line of quarter or half wave length.

The question then arises with the Radcom “cable balun”, does it behave similarly, to what extent does the folding of the conductor affect its quarter wave resonance.

NEC models

One way to explore this is to construct an NEC model of the structure and a reflection of itself.

Three quarter wavelength a side folded

Above is the serpentine structure of three quarter wavelength folded, and below it, a reflection of itself. The whole structure is fed in the middle and the impedance vs frequency charted. Continue reading Radcom Feb 2019 “cable balun” – a deeper look

Last update: 25th January, 2019, 5:10 AM

ATUs and periodic maintenance

This article is about PM of a traditional manual T match ATU with two variable capacitors, and an air cored variable ‘roller’ inductor.

Above, internals of the stock ATR-30.

It has been about 5 years since the last PM on my ATR-30 ATU, so time for covers off, thorough inspection for signs of heat damage (particularly coil support insulation, more so if it is thermoplastic like the very popular polystyrene), contact and other arcing, cleaning and lubrication of mechanical parts as needed (including the fan in this case).
Continue reading ATUs and periodic maintenance

Last update: 22nd October, 2020, 6:49 AM

VU3SQM directional wattmeter build – #3

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

The parts have arrived and construction commenced.

Above, the PCB populated with the SM parts and soldered. It was soldered in an IR reflow oven. Continue reading VU3SQM directional wattmeter build – #3

Last update: 19th January, 2019, 4:28 AM

Designing a Gamma Match – confirmation of as-built antennas

Much is written about the virtue of the Gamma Match, and near as much about how they work, and the difficulty in design and implementation.

Designing a Gamma match using a Smith chart showed a design method for a simple Gamma Match using a Smith chart as the design tool.

This article visits the implementation on a pair of antennas that I built 50 years ago, and are still in use today (albeit with some small preventative maintenance once during that interval). The basic antenna is a four element Yagi for 144MHz copied from an ARRL handbook of the time, probably based on NBS 688. It was designed to deliver a split dipole feed point impedance of 50+j0Ω.

I built them using a Gamma Match, partly to get some familiarity, but mostly to implement a Gamma Match that was reliable, weatherproof and lasting… features that are alien to most implementations I had seen at that point.

Both antennas were constructed and the Gamma Match adjusted for VSWR<1.1 using a Bird 43 directional wattmeter. The dimensions of each (including the key gamma dimensions) are the same, not surprising, but a confirmation of repeatability. See Novel Gamma Match Construction for more discussion.

Above is a dimensioned drawing of the construction. Continue reading Designing a Gamma Match – confirmation of as-built antennas

Last update: 18th January, 2019, 12:15 PM

Radcom Feb 2019 “cable balun”

A correspondent asked for an explanation of a novel balun described in Radcom Feb 2019 by K3MT.

Above is a diagram of the so-called “cable balun”.

To evaluate it, I have inserted it into one of the NEC models used for the article Baluns in antenna systems, the model used for Model 3 which I will repeat here for convenience. Continue reading Radcom Feb 2019 “cable balun”

Last update: 13th February, 2020, 6:13 AM

VU3SQM directional wattmeter build – #2

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

This article canvasses the issues of the display.

Intention is a digital based display (though not to exclude an analogue meter or bar graph type displays).

So, the output of the AD8307 needs to be digitised.

Let’s first consider the nature of the AD8307.

It is a log detector, so it provides a ‘DC’ voltage proportional to the log of the input signal, but the ‘DC’ voltage can vary very quickly.

The chart above from the AD8307 datasheet shows that the unfiltered response to a burst of RF has a rise time of well under 1µs. Continue reading VU3SQM directional wattmeter build – #2

Last update: 19th January, 2019, 11:41 AM

KA0KA’s Youtube test of ATU balance

K0KA made a quite polished presentation published as a Youtube video explaining how to measure balance performance of an antenna system and an ATU.

He did not define what he means by balance… but it will become apparent.

Midway through his video, he measures the ‘balance’ of a Palstar AT5K (though possibly modified) by connecting a symmetric load consisting of two 470Ω 2% resistors in series and connecting the junction to the ground terminal on the ATU.

Above, from KA0KA’s video, his test load. The oscilloscope probes can be seen.

The oscilloscope channel gains are carefully adjusted to be equal.

KA0KA shows two oscilloscope measurements.
Continue reading KA0KA’s Youtube test of ATU balance

Last update: 10th February, 2019, 3:39 AM

VU3SQM directional wattmeter build – #1

VU3SQM offers an interesting directional coupler based on a Sontheimer coupler, and using AD8307 power sensing for a nominally HF coupler. I must say that I am not a fan of Sontheimer couplers… but that is what the board uses.

This article lays out a preliminary design review to assist in selection of appropriate toroids, and ordering of the needed parts.

PCB

Above, the top side of a PCB. Continue reading VU3SQM directional wattmeter build – #1

Last update: 16th March, 2019, 5:51 PM

Elecraft CP-1 directional coupler – magnetics review

Elecraft produces a directional coupler that may interest QRP aficionados. It comes with instructions for 20dB and 30dB coupling factors rated at 25 and 250W respectively from 1 to 30MHz.

This article reviews the magnetics design of the -20dB / 25W coupler.

The coupler uses a type of Sontheimer coupler (Sontheimer 1966) and these are commonly poorly designed. The first question is whether the magnetising impedance of T2 which appears in shunt with the load is sufficiently high to not give rise to poor insertion VSWR. Continue reading Elecraft CP-1 directional coupler – magnetics review

Last update: 3rd January, 2019, 7:02 AM

The first 21 days of Exetel broadband Internet access

We have had wired broadband service delivered to these premises for almost ten years, supplied by five vendors: Telstra Bigpond, iiNet, Amaysim, Southern Phone and now Exetel.

At The first seven days of Exetel broadband Internet access I discussed the performance failure of the service observed over the first seven days.

Following that, a fault report was submitted to Exetel, and at their request, further observation of speed and ping test latency.

Service levels

I have an expectation that “I want it to deliver most of the rated speed, most of the time during the hours that I want to use it”.

As a result of behaviour of the industry, the ACCC gives some guidance on terms used to advertise a service, and a service expectation.

Essentially they say:

Standard Plus Evening Speed—plans using this label will deliver a minimum speed of 30Mbps during the busy period. This plan would be suitable for a higher usage profile (e.g. streaming an ultra-high definition movie and streaming music on one or more other device during the busy period)

So on that measure, how well did they perform?

Above are the results of file transfer tests conducted automatically, the above are filtered for those in the ACCC’s defined “evening hours” on which they base their service level enforcement. All x symbols in the pink area break the 30Mb/s minimum. Continue reading The first 21 days of Exetel broadband Internet access

Last update: 24th February, 2019, 1:09 PM