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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

Moree launch: 401.3MHz 292km

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.

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

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?”

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

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