This is part 1 in a series of however many parts I think are necessary, and is intended to be a drains up guide to how an antenna works; from the very basics to the most advanced my brain can handle – probably to one of the extremes of relatively basic!
So, I guess the first thing to cover off is the fact that all oscillating electric and magnetic fields propagate energy – so all AC currents will naturally radiate. The purpose of an antenna is to optimise that natural radiation in order to provide output and receive input in a controlled and efficient manner.
Now, imagine a cross-section of your standard household appliance electrical cables, the majority of the time you will have a live & neutral (sometimes also a ground). As these wires run parallel, the current flows effectively cancel each other out.
The way to get the antenna to work and radiate optimally is to change that flow of current – obviously in a closed loop circuit that isn’t possible, as you draw power from a source from positive to negative, so a simple way of doing this would be to change the direction of the wires. By doing this, you can create a single direction of current.
As an alternating current flow means electromagnetic energy, with the above configuration you can effectively radiate energy. This creates a half-wave dipole antenna, where voltage in the centre is at its minimum and the current is at a maximum.
As you have seen, I mentioned that this is a half-wave dipole, if you refer back to my previous post, Antenna Length vs Frequency, the meaning behind this will hopefully become clearer. The resonating frequency of a half-wave dipole is twice the length of the pole. That being said, it will also resonate as a quarter-wave dipole, or any multiple there of.
As you have probably guessed, these antenna are quite simple in their operation and therefore fairly simple to make. You can read in much more detail over at the Radio Society of Great Britain.
To see it in ‘action’ here is a useful animated image from Wikipedia:
These types of antenna are very common and can be found in plain sight whenever travelling, especially in the UK. Most of your household TV aerials, whether they’re on a rooftop or sat on top of your TV, are dipole antenna. This is a very effective and efficient method of radiating RF, despite being simple to produce.
Sometimes you will see a dipole being used as a reference for antenna gain, measured in dBd. This simply means the antenna in questions gain (in decibel) is being compared to that of a standard dipole – however more often than not we use dBi, which uses the imaginary isotropic antenna as a reference point. An isotropic antenna does not exist in real life, and could be described as a single point in space radiating energy in all directions (with a perfectly spherical radiation pattern) – I will hopefully cover these topics in greater detail in a future post.