I've stumbled upon this module. It is a set of receiver and transmitter module at 433MHz.
This shows the antennas:
With the lengths being given as:
- "external antenna: 25cm ordinary multi-core or single-core line" (transmitter)
- "External antenna: 32CM single core wire, wound into a spiral" (receiver)
What is the rationale behind this? As far as I understood it (only basic knowledge), a different length of the antenna causes a different resonant frequency.
For example, if you use a LoRa module at 868MHz, Adafruit recommends a 8.2cm long "quarter wavelength" antenna. Since λ = c / f, here λ/4 is ca 8.63cm, which make sense (slightly longer but maybe they're accounting for something else?).
For this product I don't see relation since 25cm and 32cm don't have a nice factor between them (like one is λ/4, the other λ/2).
Why is it that it is apparently better that the receiver has a longer antenna than the transmitter? Where do the antenna lenght numbers come from?
Btw, I've read this question, which however doesn't give insights on why there are different length antennas.