I recently came across a VHF locator tag used to find animals in the field. It operates on the 160MHz band. According to the user the locator range is some km line of sight (using Yagi antenna) and some hundred meters "in the jungle".
I disassembled the device and honestly was expecting a much larger antenna. Wavelength at 160MHz is about 1.8m, lambda/2 would be 90cm (dipole) and lambda/4 would be 45cm (monopole). But take a look at the antenna I found:
If pulled straight, the length of the handle is 55mm, the short wire 13mm and the long wire 34mm.
Can someone explain me how this antenna works with the length given? My understanding of an antenna so far is that only at a certain length proper RF transmission will happen.
Best Answer
We designed something like this in the mid 70's for Polar Bear tracking beacons for scientists to study migration patterns.
It consists of an LC series tuned current resonator with enough bandwidth for narrow band increase of SNR in the Rx.
It pulsed very slowly with a narrow width to conserve the battery and of course had to be bear-proof.
The solid coaxial wire loop made the inductance stable and low impedance while the maximum signal vs f was tuned with the ceramic trimcap.
Here I choose a resistance that would represent the transistor switch Rce which indicates the max power with low losses on this transfer function. The tradeoffs are Q, loss and stability from bear motions. Temperature compensation for the small NTC of copper required a PTC cap in parallel with the trim cap.
Tracking was usually done by aerial surveys with a directional Yagi.