Palstar BT1500A Owner's manual - page 5
16 Understanding Your Tuner
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higher if the impedance has a reactive component.
Your Antenna and Feedline:
The balanced L-network can be used with any
antenna fed with parallel feedline. Parallel feedlines may range from 300 Ohm
TV twin lead, to “windowed” lines in the 400—450 Ohm range, to 600 Ohm (and
higher) ladder lines. The applicable types of antennas include flat-top and Vee’d
all-band doublets, horizontally or vertically oriented loops, end-fed wires, and
arrays such as the lazy-H and the 8JK. There are also a number of designs for
wire Yagis and quad beams that employ parallel transmission lines.
At any given operating frequency, the antenna has a certain feedpoint impedance.
For most multi-band antennas, the feedpoint impedance will change with the
operating frequency. On most bands, the impedance will be complex, that is, a
combination of resistance and reactance. However, unless your feedline happens
to be an exact multiple of a half wavelength (accounting for the line’s velocity
factor) or unless the feedpoint impedance is identical to the characteristic imped-
ance of the feedline, your antenna tuner will not encounter the antenna feedpoint
impedance.
For any condition where the feedpoint impedance does not exactly match the
characteristic impedance of the feedline, the impedance will vary continuously
along the feedline, returning to the feedpoint value at every half wavelength
along the line. The precise values that you will encounter at some specific point
along the line depend upon the characteristic impedance of the line, its velocity
factor, and the feedline impedance itself. The range of variation in both resistance
and reactance is a function of the degree of difference between the feedpoint
impedance and the characteristic impedance of the feedline.
Many users of multi-band antennas are surprised to learn that even very high
feedpoint impedances can result in very low impedances at certain regions along
the feedline. An end-fed wire at any frequency, or a center-fed wire that is close
to a multiple of a wavelength long will present a very high impedance. If your
feedline is the right length, you may find that the impedance at the antenna termi-
nals is very low. Alternatively, at other lengths, you may discover that the reac-
tance at the antenna terminals is outside the range for which the output capacitor
can compensate. Without careful computation, you may not know which condi-
tion applies. You may only know that the tuner seems unable to provide 1:1
SWR for the line to the transmitter.
A Simple Work-Around:
There are many ways to correct the problem of being
unable to effect a good match on one or more bands of operation when using a
balanced feedline. One of the simplest and cheapest techniques is to insert extra
feedline into the length from the tuner to the antenna. Since the losses on the
parallel line are very low, a few extra feet of transmission line will not be detect-
able by the station you are working.
The sketch shows the general idea. You can insert the loop manually or with a
General Description 5
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The Palstar BT1500A Antenna
Tuner is an American made im-
pedance matching network.
The BT1500A optimizes the per-
formance of your antenna and
transmitter by providing adjust-
able impedance matching using
balanced dual tandem roller in-
ductors with a shunt capacitor
that can be switched to the input
(transmitter) side or the output
(antenna) side.
The BT1500A also measures
the power and Voltage Standing
Wave Ratio (VSWR or SWR)
which allows you to tune the
SWR to the lowest ratio possible
for the selected transmission
frequency.
A switch allows the user to
choose Average or Peak meter-
ing. Also, a Peak Hold function
holds the peak reading for ap-
proximately 2 seconds for easier
reading.
Front panel controls allow for
selection between two ranges of
variable capacitance, and allow
the shunt capacitor to be
switched between the input
(transmitter) side (low Z, high
pass) and the output (antenna)
side (high Z, low pass).
Tuning is achieved with the front
panel controls. The Vernier ca-
pacitor dial allows for fine tuning
with precision and accuracy,
while the Inductor crank handle
provides coarse adjustments.
The range of the power meter
(300W/3000W) is selected by a
push button switch located on
the front panel.