MFJ-209 SWR Analyzer Instruction Manual
5
The MFJ-209 measures 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-209 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.
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-209 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.
To measure the SWR on a predetermined frequency adjust the "TUNE" and "FREQUENCY"
knobs to the desired frequency. To accurately measure the MFJ-209's frequency connect a
counter to the "FREQ. OUT" jack or zero beat the MFJ-209 with a receiver. Read the SWR
from the "SWR" meter.
To find the lowest SWR adjust the frequency until the SWR meter reaches the lowest reading.
Read the approximate frequency of the lowest SWR from the "TUNE" scale or measure the
exact frequency with a counter.
