VU3SQM directional wattmeter build – #4

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

At long last, some PTFE rod arrived to permit assembly of the transformers.

For reasons discussed in an earlier article, the transformers use a larger core than the original VU3SQM. They need to stand above the board, and whilst that compromises the mechanical strength of the assembly, it should have better performance. Continue reading VU3SQM directional wattmeter build – #4

Measuring common mode current with a scope

I wrote recently of a flawed test of balance performance of an antenna system and an ATU, and some readers have taken up the issue, basically asking the question “then, how do you measure balance of a two wire line with a scope?”

The first step is that you must define what you mean by “balance”.

For most wire HF antennas, the balance objective should be equal but opposite currents in the adjacent wires at all locations along the line (recalling the currents may vary along the line). This reduces radiation from the feed line (which can cause EMC problems with nearby appliances / systems), and reduces very local noise pickup on receive (from those same appliances / systems).

Let’s take KA0KA’s scope display from the reference article, but assume that they were taken from current probes so that we are directly measuring feed line currents rather than voltage. Current probes allow the scope to measure current on a conductor placed through the probe, an RF current probe (or current transformer) can be as simple as a suitable ferrite toroid with the primary conductor passing once through the center of the core, and a secondary winding of 10-30 turns loaded with a low value resistor, and the scope input connected across the resistor.

The obvious measurement method

Above, the first measurement shows both channels, and the currents appear almost equal in magnitude and almost opposite in phase, but it does appear that there is a slight phase difference, perhaps 5-15° from exactly opposite phase. Each channel is almost 2div peak to peak, and let’s assume the calibration factor is 1A/div. Continue reading Measuring common mode current with a scope

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

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

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

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

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

An explanation of W5DXP’s ‘line extender device’

A correspondent wrote seeking explanation of W5DXP’s no-tuner tuner which purports to obtain a near match by adjusting the length of the transmission line using relays or switches of some kind.

The particular device that is of interest is one using a single double pole knife switch as a three position On-Off-On switch.

The accompanying explanations states that this “is a way to use a single DPDT knife switch to obtain one, two, or three feet of ladder-line depending on the position of the switch”. Continue reading An explanation of W5DXP’s ‘line extender device’

An RF choke for a 1.8-30MHz coax power injector – LF1260 core

This article describes a prototype RF choke (RFC) for use in a power injector for 50Ω coax over range 1.8-30MHz. Power injector / extractors are often used to connect power and / or signalling on a shared common RF coax feed line to accessories such as remote antenna switches and ATUs.

Design criteria are:

  • Insertion VSWR of the RFC in shunt with 50+j0Ω < 1.1;
  • Dissipation < 2% of a 100W transmitter.

The core chose is a LF1260 ferrite suppression bead from Jaycar. It is a medium / high µ core readily available in Australia at $7.50 / 6.

 

Above is the prototype RFC wound with data cable wire for the purpose of measurement. In application it could be wound with 1mm enamelled copper or PTFE insulated wire (Curie point is lowish at 120°+, but it still benefits from higher temperature insulation). Continue reading An RF choke for a 1.8-30MHz coax power injector – LF1260 core