Antenna Theory

Antennas form the interface from electronic, electron-conducting ‘circuits’ to pure Electro-Magnetic (EM) energy in 3-space (three dimensional space) by ‘radiating away’ the RF (radio frequency) energy applied to the antenna’s input connection.

An AC (Alternating Current) EMF (Electro-Motive Force) varying with time (termed herein “RF energy”) is applied on the input port (the “antenna connector”)  and the RF energy is converted to EM energy which is transferred/propagated into the ‘aether’ (or æther.) Technically speaking, antennas perform an ‘interface function’.

To finite human beings, antenna operation appears almost as magic. Even to engineers the actual ‘process’ of RF radiation is a mystery. To most hams, it is the simple ‘function’ of dipole and verticals (with their attendant ground radial ‘field’).

At this point it may be useful to view a few visualizations of antenna radiation:

ElectricDipoleOscillating_642x781

An animation of radiation from a dipole:

ElectricFiledLinesHWDipole_Ani

or click this link for more images:

http://www.mikomma.de/fh/eldy/hertz.html – or Google English translation

Main page, English translation: http://translate.google.de/translate?u=http%3A%2F%2Fmikomma.de&hl=de&ie=UTF8&sl=de&tl=en

Example of near and radiating (far) fields from a vertical antenna:

NearElectricField_and_GroundPlaneConductionCurrents

From observing the visualizations of antennas radiating, observing the near field E and H fields, these near fields set the stage for fields a bit further away which may then ‘radiate away’, one might conclude that the requirement for efficient radiation from a ‘basic’* antenna might hinge on two dependent factors:

1) Create a strong enough field at 1/4 Lambda away from the antenna and

2) such that this field does not ‘return’ to the antenna in the time it takes for the current (and voltage) appearing on the antenna to ‘reverse’ direction.

How a strong ‘field’ is achieved at roughly 1/4 wave from the antenna varies depending on specific antenna topology; dipole versus quarter wave vertical versus the canonical tuned loop. Dipoles and quarter wave verticals depend on the intrinsic properties of their ‘distributed’ capacitance and inductance for resonance whereas a tuned loop achieves this via ‘lumped’ capacitance and inductance.

 

 

* A ‘basic’ antenna excludes types such as Rhombics which fall more into a travelling wave category. Dipoles, quarter wave verticals fall into the ‘standing wave’ or ‘tuned’ category.

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