Antenna Notes #101; Or What Every HAM Really Needs To Know..

Just thought I’d share some notes I’ve made regarding Antenna Theory.  Believe it or not, it took many hours of scouring the web and consulting books such as the ARRL Antenna Handbook to come up with this list.

  • Antenna Resonance is the point / frequency at which an antenna produces a purely resistive load.  That is to say, there is no complex impedance component, eg 73 + j0 Ohms.
  • Non Resonant antennas do not necessarily make bad performers, but care must be taken to ensure that matching components  are correctly placed and efficient.
  • Antenna Efficiency is directly related to how much ‘radiation resistance’ an antenna has.  The shorter an antenna with respect to the driven frequency, the less radiation resistance it will have.
  • Maximizing the current across the element (linear metal segment eg, whip) of an electrically short antenna will result in the greatest possible efficiency (all other things equal).  This is achieved by correct placement of matching components.
  • ‘Short’ Antennas (with respect to wavelength) present a capacitive impedance, they require the use of an inductor or ‘loading coil’ for matching.  The optimal location electrically is at the top.
  • ‘Long’ Antennas present an inductive impedance, they require the use of a capacitor or ‘condenser’ for matching.  The optimal location electrically is at the top.

And that concludes the basics.  Next time we’ll explore the affect matching has on antennas, some of the tradeoffs & other design considerations.

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