Very true. But the question asked is about the length of the antenna to transmit and receive radio signals.
You have all heard of half wave antenna, quarter wave, eighth wave etc: Well, the lower the operating frequency then the longer the wave length of the signal.
This means that am transmissions use longer in length wires to transmit the radio frequency (rf).
Amplitude modulated signals are found in the HF range of frequencies (3.000 to 30.000 MHz).
Very high frequency (VHF) lies in the 30 to 300 MHz range and is used for frequency modulation.
Now to work out the wave length of a sine wave at any frequency, we divide the speed of light by the frequency of the rf signal.
Antenna are set up to be friendly with a particular wave length. There is a natural resonance when the wave length and the length of antenna are in agreement... are related to each other. i.e 1/2, 1/4, 1/8 etc.
From memory, the speed of light is 3 multiplied by 10 to the eighth power or 30,000,000 meters per second.
When you divide by the frequency (say 25,000,000 Hz or 25 to 10 by power 6 or 25 MHz... The antenna for a one wave length is 30 over 25 or 1.2 meters. The wave length of a 3 MHz signal is 12m long. This is very long; hence the use of fractional antenna.
Omni directional antenna radiate in a full 360 degree pattern. Dipoles are bidirectional. They radiate forward and backwards equally. It is important to aim them in the direction you want to send.
Also... and this is important, the height above the ground plane of the dipole effects the take off angle of the transmission. Hence there is a thing called near vertical transmissions.
With HF: there are ground wave, sky wave and space wave transmissions. Hf travels the furthermost being able to circumnavigate the planet in some instances.