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line, simply
open
the far end of the line (Fig.3) and find the lowest frequency where Z reaches a minimum.( See Fig. 4) This is F-1/4.
You should mark each line by, say, wrapping a piece of masking tape around the near (shack) end and writing F-1/4 on the tape.
MEASURING LINE LOSS
How lossy is your transmission line? Has weathering ruined it? Now you can tell with a very simple measurement. In fact there are two ways to do it. In
both
cases, connect
the meter to
either an open or shorted line as shown in Fig. 3. (We’ve found more accurate results with an open line.) Line loss increases with frequency (often as the square root of frequency.) So you may see twice the loss at 28 MHz as at 7 Mhz. Use a reasonable length of line, say 25ft. or more, since loss is proportional to length. The longer the better.
1.
SWR METHOD
Simply measure the SWR of the cable versus frequency. If you
get H, the SWR is greater than 20, or Z is greater than 1000. If
SWR displays, simply read the loss at the frequency of use from
One problem with this method is that Z varies widely as you vary frequency, and SWR accuracy degrades at small and large Z. So the SWR will appear to vary widely. Note: You must select the appropriate line impedance also. If you’re using 50 ohm line there is no problem. If not, see page 2..CHANGING LINE IMPEDANCE.
Either open or short the line and find the minimum Z at the nulls (See Fig 4.) The cable loss at that frequency is given by:
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(Note: You need not change the VA1 line impedance for this
measurement, since Z does not depend on line impedance.) For 50 ohm line the loss is
For example, if you measure a 4 ohm minimum Z with 50 ohm line, the loss is 0.68 dB.
The Z method works well for any line impedance. Overall, we recommend the
Z
method for its greater accuracy. The SWR method can be used for a quick estimate.
Please note that all
loss
values
are
for a line terminated in the line impedance
( SWR
=
1.
)
The loss is higher at higher SWR’s.
POWER REACHING THE ANTENNA
Fig. 6 shows the effect of SWR on power reaching the antenna. For example, if you measure 6 ohms minimum Z with 50 ohm line, or calculate 1 dB loss with any line impedance, Fig. 6 shows that about 80% of a transmitters power reaches the antenna if the SWR is 1. However, if the SWR is 4, only 55% reaches the antenna. The lost power mostly heats up your transmission line. Lossier lines are affected more by high SWR. Note that SWR is always lower at the transmitter end of a transmission line because of line losses. This is where the SWR of Fig. 6 is measured.
One way is to terminate the far end of the line with a resistor and measure its impedance as you vary frequency over a wide range. If the impedance stays the same as the resistor, or close to it, the line has the same impedance as the resistor. For example, for 50 ohm line, connect a 50 ohm resistor to the far end of the line.
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