How does shortening or cutting the antenna affect range?
Antenna length sets the resonant frequency; a shortened antenna moves to a higher frequency and its impedance in the operating band moves away from 50 Ω. The result is rising SWR, returned power and reduced radiated power. At 433.92 MHz the correct λ/4 length is 17.3 cm (16.4 cm with the velocity factor), and 8.6 cm at 868 MHz; even cutting off a few centimetres noticeably reduces range.
Cutting the antenna often looks like a "practical" solution: the remote does not fit into the case, the antenna keeps catching on things. But an antenna is not a cable, it is a resonant circuit element.
Shortening breaks three things at once:
- Resonance shifts. If you cut a 17.3 cm 433 MHz antenna down to 14 cm, resonance rises to about 535 MHz; the antenna is no longer resonant in the band you operate in.
- Impedance is degraded. With a load moving away from 50 Ω, SWR rises and part of the power returns to the transmitter. A 2:1 SWR is only about 0.5 dB of loss, but at higher values both the loss and the stress on the output stage become serious.
- Radiation efficiency falls. The radiation resistance of an electrically short antenna gets smaller and a growing share of the power turns into heat.
The total effect cannot be given as a single number, but losses of 6–10 dB are commonplace in the field; 6 dB halves the range.
If space is the problem, the right solution is not cutting but using a helical or short-bodied antenna designed for the same band. Those antennas also fall a few dB behind a full-length wire antenna, but not as far behind as a butchered cut antenna.
Full article: 433 MHz or 2.4 GHz?