May 22, 2014

FCC Technician Exam Question Of The Day (T9A05)

Q) How would you change a dipole antenna to make it resonant on a higher frequency?

A) Shorten it

NJ2X Notes:
The key to successfully answering this question is to remember that wavelength gets smaller as the frequency increases.  The physical dimensions of antenna correspond to the wavelength.  So to optimize an antenna for a higher frequency you need to shorten the antenna's size (and wavelength).

The old carpenter's adage, "measure twice, cut-once" doesn't quite work when shortening an antenna for a particular frequency.  That is because antennas are complex and cutting them exactly to spec or formula does not assure an antenna will behave at the desired frequency.  A more appropriate adage might be, "measure the length twice, trim a little, measure the resonance and SWR, repeat as necessary".