[antennaware] Q: Extended Double Zepp with tuner and open wir e feeders
L. B. Cebik
cebik@utkux.utcc.utk.edu
Wed, 4 Nov 1998 20:54:06 -0500 (EST)
Pat,
No, the tuner does not create anything on the antenna at all.
In the case of the EDZ, there is a natural current node (null) and voltage
loop (maximum) at the antenna end, but that is a function of the antenna
itself--and can be demonstrated by modeling the antenna both with and
without feedlines and matching networks. The center 1/4 wl acts as a
phasing section between two 1/2 wl wire sections, so the current maxima
are 1/4 wl inward from the end, leaving voltage maxima on the ends. In
fact, one can get the same performance as from an EDZ by taking two 1/2 wl
wires and placing them in a line, with 1/4 wl (or threreabouts) between
the ends and then feeding the two 1/2 wires in phase.
In general, tuners match the impedance presented by the feedline to some
other impedance--most usually these days, 50 Ohms to the transceiver.
They also compensate for reactance at the line end by introducing an equal
magnitude but opposite type of impedance to the line. These conditions
often make the difference between whether any or much power can enter the
antenna-feedline system. However, antenna patterns are functions of the
antenna itself.
Some folks like to think of phased elements as exceptions to this rule,
because we very often use a length of transmission line to effect the
change in current magnitude and phase between two elements. However, it
is also possible to feed each element from an independent set of sources
having the requisite current magnitude and phase difference and get the
same pattern. Hence, the phasing line is just a convenient (and often
inconvenient) method of splitting and transforming the current. The
antenna elements do the antenna work.
Tuners often use networks, most commonly of 2 or 3 elements (L, PI, Tee,
etc.). Hence, with the right calculations, they can transform impedance
and effect a phase shift in both the voltage and current (so that they
remain in phase with each other) relative to the input end. This ability
is also used to effect phasing between elements. However, the elements
themselves determine the voltage and current magnitude and phase along
their lengths.
In analyzing feedlines and antenna tuner functions, it is usually best to
start from the load end of the line--the antenna or dummy load (and
hopefully, these are not one and the same) and work backwards to the
source of power (or in reception, from the antenna as source to the
receiver as load). The antenna presents a set of conditions called the
source impedance. The feedline is in series with the antenna feedpoint
and the match or mismatch between the antenna source impedance and the
characteristic impedance determines the voltage and current (their
magnitude and phase) and the resistance and reactance everywhere down the
line until it ends at the matching network or the source. By thinking in
this direction, we come to see that the antenna--by virtue of its
geometry--determines the standing values of current magnitude and phase
along its elements, which are everywhere proportional to the energy
reaching the antenna, but in the pattern determined by the antenna. The
tuner cannot modify these factors, but only have an influence on how much
energy might be getting from the source to the feedline system.
Matters like antenna gain are measured in dB, which is a function of a
power ratio. So the antenna gain is independent of the amount of power in
the system, since the power ratio is a comparison of the antenna at hand
with some standard source, whether an isotropic source (dBi) or a perfect
lossless thin wire dipole (dBd) [both hypothetical or mathematical
standards and not to be confused with an actual dipole or other antenna in
the back yard]. So an antenna with 10 dB gain has that gain whether the
power is 1 watt of 1500 watts. Likewise, the pattern shape is independent
of the power level.
Since the tuner at most--in the course of effecting a match between one
line and another--might determine how much power reaches the
feedline/antenna system, it does not affect the antenna pattern or gain or
current distribution along the antenna elements.
I hope this helps.
-73-
LB, W4RNL
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