Revision Notes

Frequency scaling Any antenna will be designed to operate at particular frequency or range of frequencies and its physical size will in some way be related to that frequency. The most obvious examples of this are the element antennas whose defining characteristic is the element length which is generally a fraction of a wavelength. (eg: one arm of a dipole being λ/4 long). It stands to reason that all you would need to do in order to make an antenna work at different frequency (or wavelength) is to make the antenna either larger or smaller. Generally speaking this is precisely what you would do and it is termed frequency scaling. An extreme example of this is again the case of the dipole antenna. Going from the smallest which is designed to work up to about 18GHz and whose length is just a few mm long ( you can actually get them even smaller !), as shown below:

to the sort of antenna that you would need in order to communicate with a submarine at a depth of tens or hundreds of metres in sea. Since this would require extremely low frequencies in the range of 30 to 300 Hz and would require using a antenna which is hundreds of kilometres long (this has actually been tried !) as illustrated below.