MFJ-259 Instruction Manual
HF/VHF SWR Analyzer
3
OPERATION OF THE MFJ-259
After the MFJ-259 is connected to a proper power source the
red on-off button can be depressed to apply power. When
pressed, the power button should lock into position.
Upon initial power up all segments of the display will be lit
for a few seconds and then will read MFJ before starting to
count.
SWR and the MFJ-259
Some understanding of transmission line and antenna
behavior is necessary in order to use the MFJ-259 properly.
For a thorough explanation the ARRL Handbooks or other
detailed textbooks can be used for reference.
SWR is the ratio of a load impedance to source impedance.
Since nearly all feedlines and radio equipment used in
amateur service are 50 ohms, this instrument is designed to
measure the system SWR normalized to 50 ohms. For example a 150 ohm load placed across
the "ANTENNA" connector will give an SWR reading of 3:1 .
The MFJ-259 measures the actual SWR. The load must be 50 ohms of pure resistance for a
meter reading of 1:1 . The common misconception that 25 ohms of reactance and 25 ohms of
resistance in a load will give a 1:1 SWR is absolutely untrue. The actual SWR in this
condition will be measured as 2.6:1 . The MFJ-259 is not "fooled" by mixtures of reactive
and resistive loads.
Another common misconception is that changing a feedlines length will change SWR. If the
impedance of a feedline is 50 ohms and the load impedance is 25 ohms the SWR will remain
2:1 as the feedline length changes.
If line loss is low
it is perfectly acceptable to make SWR
measurements at the transmitter end of the feedline. The feedline does not have to be any
particular length. However, as line loss increases, and as SWR increases, more error is
introduced into the SWR reading. The error causes the measured SWR reading to appear
better
than the actual SWR at the antenna. Refer to the section on estimating the line loss on
page 10.
If changing feedline length changes the SWR reading one or more of the following must be
true:
•
the feedline is not 50 ohms,
•
the bridge is not set to measure 50 ohms,
•
the line losses are significant,
•
the feedline is acting like part of the antenna system and radiating RF.
Resistance
(ohms)
50
500
100
0
40
10
20
30
SWR