Palstar ZM-30, Owner'S Manual

Page: 17 / 37

17
Practical measurements are best done in the
mid-tuning range of the instrument where
accuracy is optimum and feedline lengths are
reasonable; so this procedure will be
performed between 7 and 21 MHz.
The measurements need to be done with a
transmission line over frequencies where the
feedline is at about 1/8 wavelength at the low
frequency end and something over ¼
wavelength at the high frequency end, so it is
recommended that a length of about 16 feet is
used.
Connect the near end of the feedline to the
ZM-30. Connect a 1000-ohm carbon or
Cermet potentiometer to the far end with leads
no longer than an inch or so. Initially set the
pot to its highest value. See Figure 8.
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 over the range of 7-to-21
MHz while noting the resistive (R) and
reactive (X) values. More than likely they
will vary widely over the tuning range. Now
readjust the potentiometer to a slightly lower
value and do another sweep while observing
the variation of R and X values. At some
potentiometer setting the R value will vary
very little over the tuning range while the X
value will remain near zero. This is the
estimated characteristic resistance.
Transmission Line Loss
Transmission line loss for 50-ohm feedlines
can be easily measured using the analyzer.
The basic operating principle is that loss in
transmission lines attenuates RF sent through
them. When the line is connected to the
analyzer and the far end is short or open-
circuited there is a theoretically infinite SWR.
If the feedline had zero loss this would be the
case. However since any real line has some
loss both the forward and reflected power are
attenuated and a finite SWR is measured.
For most good quality new coaxial feedlines
the loss at HF frequencies will not exceed
several dB per hundred feet; however as they
age the dielectric becomes lossy to it is a good
idea to periodically check the loss.
Measurement is simple. All you have to do it
is to remove the load, short-circuit the far end
of the feedline, and then connect the near end
to the analyzer’s RF output connector.
Measure the SWR and refer to Table 1 for the
approximate corresponding loss. If the
measured SWR is above 9:1 that’s good news
since the SWR then is less than 1 dB. If you
vary the analyzer frequency you will see that
SWR decreases with frequency indicating that
loss increases at higher frequencies.
Table 1 – SWR vs line loss (infinite load SWR)
Approx Loss Measured SWR
1 dB 9:1
2 dB 4.5:1
3 dB 3:1
4 dB 2.3:1
5 dB 2:1
6 dB 1.7:1
7 dB 1.6:1
8 dB 1.5:1
9 dB 1.4:1
10 dB 1.3:1
Transmission Line Stub Lengths
Measurement of quarter and half wave
transmission line stubs can be performed
regardless of the transmission line
characteristic impedance. The method relies
on the fact that an open-circuited quarter
wavelength line or a short-circuited line acts
like a precise short circuit at the chosen
frequency of operation.
With either type of feedline first cut it about
10% longer than the desired length, taking the
appropriate velocity factor into account. The
velocity factor of common feedlines is
available from manufacturer’s literature or