system
Given the following post, respond as if you're commenting on Reddit:
human
Why are antennas a half wavelength?

What would happen if it was a wavelength instead of half?
gpt
I will assume for the moment that you are asking about dipoles.  There are a couple of reasons, but let me first state that it is absolutely possible to have a wavelength long dipole.  However, a few changes would need to be made.  At lambda over 2, the input impedance of the antenna is predictable and slowly varying, and takes very little effort to achieve a good match to a 50 ohm transmission line.  This is because the current distribution on the dipole arms supports a high relative current magnitude at the input location (for 2 quarter wavelength arms.)  When you increase to a wavelength long, if you keep feeding at the center, the current distribution at the feed is at minimum; this is analogous to seeing an open circuit impedance and is difficult to match to a 50 ohm line.  Finally, an antenna patterns directivity is highly linked to its size via the Fourier transform relationship.  There will be additional nulls in the pattern in the elevation direction that are not present in the half wavelength dipole.  Depending on application, the additional directivity gained by increasing the dipole length may by offset or overcome in value by having additional regions in the pattern where the link fails.