[SOC] Balanced vs Unbalanced Antennas

[John Davies]

Balanced vs Unbalanced Antennas.

 

Many newcomers to the wonderful world of Amateur Radio ask "what is the difference between a balanced and an unbalanced antenna?"  I hope the following explanation will help.

 

Firstly, a balanced antenna is inherently more stable than it's unbalanced cousin. It is very hard for a balanced antenna, like a dipole, to fall down, hence the main reason for the term "balanced". However,  the fundamental reason why we call a dipole a "balanced" antenna has something to do with travelling waves in two equal halves. To make it simpler, try to visualize two water pipes fed from a single standing pipe. The diameter of each pipe is the same. The device which separates the two pipes equally is often called, in technical terms, a "balun". This is a very clever thing and we will not spend too much time discussing it now. Suffice it to say, that it "separates" both halves of the circuit and looks quite nice too, especially if it's in a  shiny black box.

Usually we use ladder line from the balun to the feedpoint of the antenna. Newcomers to the hobby often ask "why is a called a ladder line?" The answer is quite astonishing really - it actually looks like a ladder if you think about it! Again, it's a question of visualization. Like Morse code, it will come to you if you practice this kind of lateral thinking hard enough.

The ladder line ensures the positive electrons equal the negative electrons  on each side. Some really smart Amateur Radio operators use a concept known as J-notation to calculate the number of travelling waves on each side, and their force. This is a very tricky concept to grasp for novices, but to make it simpler I will just say that it involves being able to have the square root of minus one, which otherwise is very difficult to do. This is why we use the term "imaginary numbers", because nobody understands it.

 

Unbalanced antennas, like verticals, on the other hand, only have one pipe with an outer shield called a coaxial cable. To use the water analogy again, imagine the water only goes up the central pipe now. Of course, if it goes too high the antenna will topple over. This is the principal reason why we use the term "unbalanced" when referring to verticals. It is also the reason why you must use stays and guys to stop the antenna, which is inherently unbalanced, from falling down.

 

Next month we will consider the tricky subject of lightning and why it is not good for you. Until then feel free to email with any questions.

 

73