Impedance matching losses.
The efficiency of the antenna is not being questioned.
Only the loss of power when coupling to it.
When attaching the transmission cable to the antenna we try to have equal
impedance values with no reactance. A good example of this is the center
feed half wave Dipole antenna. The antenna's 72 Ohm feed point couples to
50-Ohm coax with a simple balun. Of course using 72 Ohm coax would be better
but then the mismatch would be at the radio. In any case there is a slight
loss of power but nothing to be concerned about.
When we try to use small antennas the feed point impedance generally becomes
very low. An example to this is a small magnetic loop transmitting antenna.
By adjusting the antenna size and material the efficiency of the antenna can
be made to around 90%. But even with this example the feed point impedance
will only be around 1 ohm with some reactance. Coupling the 50ohm coax to
the antenna is done easily enough but what is the amount of power being
lost.
A ground mounted wire half wave vertical fed at its base. Its feed point
impedance will be in the thousands of ohms with some reactance. Coupling to
the 50-ohm coax can be done easily enough but what is the amount of power
being lost.
Rich AA2MF
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