Palstar ZM-30, Руководство пользователя

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references such as the ARRL Antenna Book.
If you cannot find the value or if you are using
a custom type of feedline, the “Velocity
Factor Measurement” section in this manual
provides a way to determine this value.
The following formulas can be used to
estimate the length of transmission line
required.
For a
half-wavelength
stub the length is:
L = 5904 * VF
F
Where L is the length in inches, VF is the
velocity factor and F is the operating
frequency in MHz for the stub.
Similarly for a
quarter-wave
stub use the
formula:
L =
2952 * VF
F
To determine the length of a
half wave stub
,
connect the near end of the transmission line
through a 51-ohm resistor as shown in Figure
4 to the analyzer’s RF output connector. Short
circuit the two leads at the far end of the half
wave stub.
Ensure that the transmission line is supported
for its entire length in a fairly straight line and
kept several inches from any conductive
surface or material. This is important to
minimize any detuning effects. Ideally the
line should be dressed along to top of a
wooden fence or supported by fiber rope or
string.
Now tune the ZM-30 for minimum SWR and
note the frequency. This is the frequency
where the transmission line is exactly a half
wavelength long. If the initial length was
chosen properly it should be below the desired
frequency. If so, cut off a short length making
sure the far end is still short-circuited, and
repeat until resonance is achieved at the
desired frequency.
For a
quarter wave stub , the above procedure
can be used except, of course that the length is
different and that the far end needs to be open-
circuited.
Transmission Line Velocity Factor
Velocity factor of a transmission line can be
measured using techniques similar to the ones
used for measuring quarter and half wave
stubs.
The procedure can be performed at any
frequency that the ZM-30 tunes but it is most
practical in the vicinity of 10 MHz where line
lengths are reasonable and instrument
accuracy is optimum.
Either a quarter wave or half wave length can
be used; but using the shorter length consumes
less feedline if it will be discarded after the
measurement.
Begin by cutting a quarter wavelength of
feedline using the formula:
L =
2952 * VF
F
for a frequency of 10 MHz and assuming a VF
(Velocity Factor) of 1.
Now connect the near end of the feedline to a
51-ohm resistor as shown in Figure 9 then to
the analyzer’s RF output connector. The far
end must be open circuited.
Ensure that the transmission line is supported
for its entire length in a fairly straight line and
kept several inches from any conductive
surface or material. This is important to
minimize any detuning effects. Ideally the
line should be dressed along to top of a
wooden fence or supported by fiber rope or
string.
Now tune the ZM-30 for lowest SWR and
note the frequency. VF can now be calculated
using the formula:
VF = 10/F
where F is the measured frequency in MHz.