has the effect of cancelling out the reactance of the antenna, leaving only the 50 ohms resistive. This
can be looked on as a series R,L,C circuit that is in resonance, whose total impedance is only that of
the resistance. Another term for this approach to maximize power transfer is "conjugate impedance
matching."
In the above example, we used a value of 50 for the radiation resistance. If this value were not 50
but 150 ohms, the impedance after cancelling the reactance out would be 150 ohms. Connecting this
load to the transmitter designed to operate with 50 ohms load would not result in optimum power
transfer. It would, however, be better than leaving the inductive reactance in, since the antenna
current is maximized for the conditions that do exist. To obtain design performance, it is necessary to
transform the 150 ohms to 50. This can be done with a transformer with a turns ratio of 1.73 to 1.
(Impedance transformation is equal to the square of the turns ratio.) It is also possible to accomplish
this transformation with a parallel tuned circuit with primary and secondary taps properly located on
the inductor, or using two or more capacitors in series with taps taken from the series string. Under
these conditions, the transceiver will deliver rated power to the antenna.
One last observation before we go on. The antenna impedance in the above example as that at
the feedpoint. If we now feed the antenna at a different location along the conductor, the impedance
will be different, both resistive and reactive components. There are an infinite number of impedance
choices available, depending on where the tap is made. This factor is helpful in designing and
matching antennas. The factors that determine this impedance are the current and voltage values at
this point, and the phase between them.
THE TRANSMISSION LINE - In the above example, we assumed that the transmitter output was
connected directly to the feed point. This is hardly practical. So that the transmitter can be located at
a distance from the antenna, we use a transmission line to deliver the power. Unless we have a
perfectly matched system, i.e. antenna, line and transmitter output impedances all the same value
without reactive components, the addition of the transmission line completely changes the picture.
The transmitter will not see the antenna impedance of 50 ohms resistive and 100 ohms inductive
reactance, but some other combination. It will depend on the electrical length of the line, its
characteristic impedance and frequency. The impedance at the transmitter end is what we are
interested in, and the inductive component may even be changed to capacitance. (Only when the
electrical length of the line is an exact multiple of the half wavelength will the impedance at the
transmitter be the same as the antenna impedance.)
Briefly, the line characteristic impedance is determined by the physical dimensions of the line -
wire diameter and spacing - and the dielectric of the material in between. The wire also possesses a
resistive component which will dissipate power when current flows through it to the antenna. This
shows up as heat loss and dictates use of low loss cable. Formulas for coax and open wire line
impedances are given in the handbooks.
Since rf currents flow in the transmission line, one may ask if it then becomes an antenna. In the
case of coax type lines, the current should flow on the inside surface of the outer conductor and outer
surface of the inner conductor. The electric and magnetic fields caused by the current flow are
confined between the two, so none can escape and be radiated. If a system configuration results in
some rf current flowing on the outer surface of the outer conductor, such as when a dipole is fed with
coax without a balun or other means of changing the feed line from an unbalanced to balanced
configuration, it will radiate power. In the case of parallel lines, the current in one conductor at a
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