MFJ MFJ-249 - Page 7

MFJ-249 SWR Analyzer Instruction Manual
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-249 is not "fooled" by mixtures of reactive and resistive
loads.
Another common misconception is that changing a feedlines lengih will change SWR. If the
impedance of a feedline is 50 ohms and the load impedance 1s 25 ohms the SWR will remain
2:1 as the feedline length changes. Jf 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.
Feedlines with very low losses, such as air insulated transmission lines, will not have much loss
even when operating at extremely high SWRs. High loss cables, such as small polyethylene
dielectric cables like RG-58, will rapidly lose efficiency as the SWR is increased. With high
loss or long feedlines it is very important to:maintain a low SWR over the entire length of the
feedline. |
Any SWR adjustments have to be made at the antenna, since any adjustments at the transmitter
end of the feedline can not affect the losses, nor the efficiency of the antenna system.
Measuring SWR
The MFJ-249 will measure the impedance ratio (SWR) of any load referenced to 50 ohms.
The SWR can be measured on any frequency from 1.8 to 170 MHz. No other devices are
required.
The "ANTENNA" connector (SO-239 type) on the top of the MFJ-249 provides the SWR
bridge output connection. To measure SWR, this connector must be connected to the load or
device under test.
Warning: Never apply power to the "ANTENNA" connector.
To measure the SWR of a 50 ohm coaxial line simply connect the line to the "ANTENNA"
connector. The counter input should be set to "A". Press the "Input" button until the "A"
appears on counter display.