Prototype 2el Yagi for 401MHz (Radiosonde) Fox Hunting – measurement

This article documents measurement of a prototype two element Yagi for Fox Hunting / portable direction finding: Prototype 2el Yagi for 401MHz (Radiosonde) Fox Hunting . See the original article for the design criteria.

A prototype was constructed and tuned to approximately mid band.

Above, the antenna with MySondy custom firmware for a TTGO.

The MySondy links to an app on a phone and displays the position of the tracked radiosonde on a map.

Be aware that small changes in element lengths and spacing may be required for optimal performance, don’t commit these until optimised.

Above are the measured |s11| and VSWR curves with the antenna pointing to the zenith.

Observations included that the Front/Back ratio was consistent with the model, the antenna was quite symmetric (it could be rolled about the boom axis and boresight followed the boom), and had deep nulls at approximately 110° from boresight.

For Foxhunting, Front/Back is useful for identifying the approximate direction of the signal, the side nulls are useful for quite precise bearing if you have calibrated that angle (ie, you know the angle from observation of a known source).

Whilst the map display guides the chaser to the radiosonde, at the end of the chase, it often comes down to conventional DF techniques.

Off-air measurements

The following tables document measurements from street level using the two element Yagi, a quarter wave whip and quarter wave with radials. The Rx levels are dBm reported by the MySondy Go.

Brisbane launch: 401.5MHz 154km

Rx (dBm) F/B (dB) Main null
Yagi V -101 10 ±110°
Yagi H -116
λ/4 whip -111
λ/4 whip + radials -106

Moree launch: 401.3MHz 292km

Rx (dBm) F/B (dB) Null
Yagi V -117
Yagi H -127 (no decode)
λ/4 whip -125
λ/4 whip + radials -120

The Yagi in vertical orientation appears to have about 10dB of gain over the quarter wave vertical that is commonly used with the MySondy Go.

Prototype 2el Yagi for 146MHz Fox Hunting – measurement

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

Toro MX4250 rear tyre mid life maintenance

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

“CM choke acts like the cut-off end of a wire antenna?”

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.

A half wave dipole above average ground

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?”

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 of Radiosondes on 401MHz.

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 401MHz (Radiosonde) Fox Hunting

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

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

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

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