Month: November 2024

Prototype 2el Yagi for 146MHz Fox Hunting

This article documents a design prototype for a two element Yagi for Fox Hunting / portable direction finding.

Design criteria:

  • compact;
  • good symmetry;
  • good front/back ratio;
  • deep nulls at the side;
  • easy DIY construction; and
  • low cost.

The design is for a two element Yagi using a Half Folded Half Wave Dipole driven element and reflector. The driven element was covered by US Patent 6307524 which may have expired.

A model was constructed in NEC-4.2 and adjusted to meet the design criteria.

Above is a diagram of the Yagi elements. The driven element feed arrangement has an integral balun and 1:4 impedance step up ratio allowing a close spaced configuration matched to 50Ω coaxial feed line. Continue reading Prototype 2el Yagi for 146MHz Fox Hunting

Last update: 2nd December, 2024, 5:04 AM

Common mode current and RF feed lines – an interesting thought exercise from WA7ARK

Mike (WA7ARK) recently posted details of an interesting experiment: Measuring currents in an unbalanced dipole.

Above is a diagram of his experiment, and several points at which he measured the magnitude of current with his RF current probe. Continue reading Common mode current and RF feed lines – an interesting thought exercise from WA7ARK

Last update: 28th November, 2024, 4:53 AM

Common mode current and RF feed lines

At any one point along an RF feed line, there are currents in the conductors of sinusoidal waveform with magnitude and phase. These currents can be decomposed into differential and  common mode components to assist analysis of behavior which is often mode specific.

This article explains the composition of feed line current, and its decomposition into differential and  common mode components.

Over the length of the feed line, the differential and common mode components of current are typically standing waves each with different phase velocity, so the magnitude and phase of differential and common mode components may vary along the feed line, as will the phase difference between the components.

We will discuss the principles in two contexts:

  • two wire transmission lines; and
  • coaxial transmission lines.

The first is simpler and lays down important concepts for the second case. Continue reading Common mode current and RF feed lines

Last update: 24th November, 2024, 3:01 AM

Preliminary design of digital display for RF current probe – revised

A significantly revised version of the original article follows.

This article documents a preliminary design for an RF current probe with half wave detector and digital display preferably leveraging a prior design for the digital display.

The requirement is to display with reasonable accuracy, primary current of 10-1000mA.

Above is the schematic of an LTSPICE model of the half wave detector excited by a current source to represent the secondary of a ferrite cored current transformer with 1:10 turns ratio. The model will step the excitation current from 1.414 to 141.4mApk (equivalent to 10-1000mArms primary current). (Additionally to the components in the schematic above, I recommend a 2.2V Zener diode across C1 to limit the maximum voltage that will be fed to an ADC input.) Continue reading Preliminary design of digital display for RF current probe – revised

Last update: 18th November, 2024, 11:20 AM

LED plate driver failure #3 & 4 – 24W round plate

This article documents a third and fourth failure of a 24W LED oyster. The luminaire was purchased complete on eBay for about $45.

About 12 months after the last driver replacement, the driver has again failed. Again it is the mains filter capacitor that has failed.

The driver was replaced with the same type, we will see how long it lasts.

Within a few days of replacing the driver, the lamp failed again. This time it was the LED plate.

The LED plate is a 48S2P configuration of 0.5W 5730 LEDs. The fail either as short circuit or open circuit. If one fails short circuit the plate keeps working just two LEDs are dark. If one fails open circuit, its pair will overheat and go open circuit then the whole plate is dark. Continue reading LED plate driver failure #3 & 4 – 24W round plate

Last update: 17th November, 2024, 9:07 AM

The Mansfield strainer knot for fence wire

Rob Clark from Mansfield in a video by Tim Thomson demonstrated a strainer knot for fence wire. Rob called it his “Everyday Knot”, Tim thought it needed a name and named it the “Mansfield Knot.”

Strainer knots are ones that go min line and must be tied between two strained end ends in a wire span, without releasing much tension from the span when the strainer is removed.

Above is a pic from instructions supplied with a Donald Wire Strainer, a quite old device that I am not sure is still available. The Donald Wire Strainer was my first strainer around 1970. Let’s call the left hand part the standing part and the right hand part the running part. Note that after pulling the running part through the U loop formed near the strainer grip, the running part is wrapped firstly over the standing part. Continue reading The Mansfield strainer knot for fence wire

Last update: 16th November, 2024, 6:18 AM

Measuring RF feed line common mode current – relative or absolute measurement?

Common mode current is one of the very popular topics on social media, but one does not see much quantitative discussion, mostly just a lot of hand waving without supporting measurement data.

There are a a number of commercial instruments instruments purporting to measure RF feed line common mode current (Icm), but many are specious frauds.

There are quite a few articles on the ‘net describing DIY  ‘balance measurement’ techniques / instruments and again, many are specious frauds.

There are a very small number of commercial instruments and DIY designs that would appear to be soundly based, many of them are ‘uncalibrated’ in that they do not give absolute current readings, and even relative readings may be impacted by non-linear response.

So, what are we trying to measure?

Bear in mind that Icm is usually a standing wave, and ‘measurement’ means a survey at several points along the feed line.

In my experience, for HF, Icm with a 100W transmitter feeding a reasonably efficient antenna system is likely to result in Icm greater than about 10mA, and more that 100mA is poor… so the measurement range of interest is really between 10mA and 100mA. Continue reading Measuring RF feed line common mode current – relative or absolute measurement?

Last update: 13th November, 2024, 3:47 AM

NanoVNA: where exactly are Port 1 and Port 2?

NanoVNA is a unidirectional two port VNA. It makes corrected measurements of S11 and S21 of a two port DUT wrt two reference planes:

  • Port 1; and
  • Port 2.

The location of the reference planes is determined by the calibration process and saved data that is subsequently used to correct raw measurements. Note that the reference plane can be displaced from the calibration plane by using the e-delay feature.

The “Ports” are not connectors on the VNA (even if the labels might suggest that), they are the boundaries of the DUT.

For the purpose of this discussion about a unidirectional VNA, let’s call the VNA port that transmits signal the TX port, and the one that receives signal the RX port.

Example: setup to measure SMA(M)-SMA(F) attenuator

Above is the instrument setup. The instrument is reflection (SOL) calibrated using the parts shown on the upper SMA(F)-SMA(F) adapter which defines the DUT attachment as Port 1. Thru (T) calibration is done with the lower SMA(M) connected to the  upper SMA(F)-SMA(F) adapter, so defining that SMA(M)  DUT attachment as Port 2. Continue reading NanoVNA: where exactly are Port 1 and Port 2?

Last update: 13th November, 2024, 3:48 AM

Flashover and the popular ~132′ dipole with two wire line feed and longer feed line

Flashover and the popular ~132′ dipole with two wire line feed worked up the voltage seen looking into the feed line for a particular system configuration,

For convenience:

An NEC-4.2 model of a 40.23m dipole at 9m height above good ground, and a quarter wave of lossless 600Ω line was run to obtain a .s1p file of the impedance looking into the feed line.

Above is a plot of peak input voltage @ 1500W around the 80m band. If you are using 100W, voltage is approximately a quarter of that shown, but that is still quite substantial.

Now as mentioned, this is dependent on many factors, including feed line length.

Let’s look at a wider sweep with feed line length 40m, about twice that of the configuration above. Continue reading Flashover and the popular ~132′ dipole with two wire line feed and longer feed line

Last update: 9th November, 2024, 4:04 PM