Which is better, a helical antenna or a wire or telescopic antenna?
A helical antenna is a physically shortened quarter-wave antenna: more compact, but with lower gain and bandwidth and more sensitive to the body and the hand. A full-size wire or telescopic antenna at the same frequency is typically a few dB better; given that a 6 dB difference doubles the range outright, that difference is felt directly in the field.
The difference comes from electrical length. A quarter wave is about 17.3 cm at 433 MHz and 8.6 cm at 868 MHz (16.4 and 8.2 cm with a velocity factor of 0.95). A helical antenna winds that length into a few centimetres; the space gained is paid for with lost efficiency and narrowed bandwidth.
Selection criteria:
- Helical: makes sense where portability and durability come first, as in a handheld remote control. The risk of breakage is low and it fits in a pocket.
- Wire / telescopic: should be preferred on fixed-mounted receivers; if range is your priority, this is the right choice.
- The performance of a helical antenna is sensitive to the hand and the body; if the operator takes the antenna into their palm the range drops noticeably.
- Narrow bandwidth means the helical antenna is more sensitive to manufacturing tolerance and to the mounting environment.
In practice the best balance is a compact antenna on the remote control side and a full-size external antenna on the receiver side. That is also the reason a separate antenna and cable glands are supplied for the receiver side in AXI's IP67 sets: being able to move the receiver antenna outside the enclosure, to a free position, is the biggest gain in the link budget.
Full topic: 433 MHz or 2.4 GHz?