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.