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

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”

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

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

On Thevenin’s theorem – #2

On Thevenin’s theorem looked at a simple source network to demonstrate some key characteristics and limitations of Thevenin’s equivalent circuit.

The example network used was linear in V,I for all V,I combinations possible. Let’s now look at a network that is not linear for all V,I, but is sufficiently linear over a sub range to be usefully modelled using Thevenin’s equivalent circuit.

Black Box for discussion

For the purpose of discussion, we have a Black Box with just two terminals and is a source of DC voltage and current, and the internal implementation is hidden from us.

A series of measurements is made with different load resistors attached and the voltage and current at the terminals is recorded and plotted uniformly stepped currents.

The V,I characteristic is clearly non-linear, but on closer examination there are two fairly linear regions, from 0.008 to 0.060A and 0.08A to 0.1A. It is a device that is usually used in the region below the knee, and for our application, let us concentrate on 0.008 to 0.030A. Continue reading On Thevenin’s theorem – #2

RFPM2 – calibration files

The RF Power Meter 2 (RFPM2) stores calibration constants in a file located in the (SPIFFS) file system in the microcontroller flash.

The file opened by default when RFPM2 starts is /default.cfg, the following is an example.

{
"name":"dBm",
"hostname":"rfpm201",
"vref":3.3,
"avg":3,
"slope":0.12991,
"intercept":-91.406,
"unit":"dBm",
"lcdfsd":16
}

The parameters above capture the most basic operation of RFPM2 as a power meter directly displaying dBm with bar graph in fixed 2dB increments to 16dBm FSD. These values serve as a basis for some other applications as they capture the basic intercept and slope of the AD8307 module in this instance.

Current probe calibration

Alternative config files can be loaded on the fly from the webserver interface, for example http://192.168.0.86/config?filename=/dBA.cfg will load the dBA config file for a certain current probe. Continue reading RFPM2 – calibration files

RFPM2 – calibration

The RF Power Meter 2 (RFPM2) stores calibration constants in a file located in the (SPIFFS) file system in the microcontroller flash.

The WiFi credentials are stored separately at the default location in the flash.

Calibration constants

The AD8307 outputs a voltage from zero to about 3V for inputs from around -90 to +15dBm. The nominal output has a slope of 25mV/dB and intercept of -84dBm.

A starting point for RFPM2 calibration constants is intercept=-84 and slope=0.129.

Modules such as that used here may have pots to adjust the gain and offset of the output. Adjust the gain so that the maximum output voltage is a little lower than 3.3V (the maximum ADC input), say 2.7V.

Clip 194

The response of the AD8397 has some ripple in the transition between log amp stages. The log cell ranges are 14.3dB, so min error repeats every 14.3dB. My own practice is to calibrate at -62 and -5dBm input as they fall approximately on the mid line of the ripple trend (4 cycles of the error curve). Continue reading RFPM2 – calibration