This article describes VK4MQ’s (Bruce) build of the antenna and documents measurement of the prototype two element Yagi for Fox Hunting / portable direction finding: Prototype 2el Yagi for 146MHz Fox Hunting . See the original article for the design criteria.
A prototype was constructed and tuned to approximately mid band.
Above, the antenna pointing at the zenith for measurement. Continue reading Prototype 2el Yagi for 146MHz Fox Hunting – measurement
The Toro MX4250 is a ZT ride on mower (riding mower).
A recent inspection at 220h revealed that rear tyre wear is uneven (as would be expected for this type of application), tread depth inside to outside was 1.5/2.5mm and 1.8/2.6mm.
To wring a little more life out of the tyres, they were reversed on the rims, and rotated left to right.
Small diameter wheels, especially wide ones can be difficult (these are 18×7.50-8 tyres), but these were much easier than the front castors which needed tubes inserted when the mower was quite new.
Above, a Bead Cheata was used to break the bead. On one side of the wheel, the bead needs to be forced down more than 50mm, the other side is easier. Beyond that, tyre levers were sufficient. I used a little mounting lube on the beads to remount the tyres, that makes the job a lot easier. The tubeless tyres reseated for inflation pretty easily, a few bounces of the wheel and they inflated just fine. Continue reading Toro MX4250 rear tyre mid life maintenance
A poster on social media recently wrote:
Remember that a CM choke acts like the cut-off end of a wire antenna. That includes performing the reflection that creates the standing wave for resonance – it IS your ‘end point’ for the frequency it’s built for and any higher frequency.
Let’s explore that proposition with two simple NEC models.
The baseline model is a half wave dipole 10m above ‘average’ ground (σ=0.005, εr=13).
Above is a graphic of the current distribution on the wire. The current distribution is the classic half wave dipole current distribution, approximately sinusoidal in shape and falling to approximately zero at each end. Continue reading “CM choke acts like the cut-off end of a wire antenna?”
This article documents a design prototype for a two element Yagi for Fox Hunting / portable direction finding of Radiosondes on 401MHz.
Design criteria:
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 401MHz (Radiosonde) Fox Hunting
This article documents a design prototype for a two element Yagi for Fox Hunting / portable direction finding.
Design criteria:
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
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
Earlier articles in this series explored measurement of a test inductor with the NanoVNA concentrating on frequencies below the first self resonance of the inductor.
Above is the test inductor, enamelled wire on an acrylic tube. Continue reading NanoVNA-H4 – inductor challenge – part 8
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:
The first is simpler and lays down important concepts for the second case. Continue reading Common mode current and RF feed lines
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
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