MFJ 209 - Impedance of Transmission Lines; Estimating Transmission Line Loss

MFJ-209 SWR Analyzer Instruction Manual
9
Impedance of Transmission Lines
The impedance of transmission lines between 15 and 150 ohms can be measured with the MFJ-
209, a 250 ohm potentiometer, and an ohm meter. Lines of higher impedance can be measured
with a higher resistance potentiometer if a broad band transformer is used (see the section on
testing transformers) to transform the line impedance to approximately 50 ohms.
1 Measure the 1/4 wavelength frequency of the transmission line to be tested as in Testing
and Tuning Stubs on page 7.
2 Terminate the far end of the transmission line with a non-inductive 250 ohm potentiometer.
3 Connect the transmission line to the MFJ-209 "ANTENNA" connector and set the analyzer
to the 1/4 wave frequency.
4 Observe the SWR as you vary the "TUNE" from end to end of the "FREQUENCY" range
selected.
5 Adjust the potentiometer until the SWR reading varies as little as possible, over the
"TUNE" range. Note that the value of the SWR is not important. Only the change in
SWR as the frequency is varied is important.
6 The value of the potentiometer will correspond closely to the line impedance.
Estimating transmission line loss
The loss of 50 ohm feedlines (between 3 and 10 dB) can be measured with the MFJ-209. It is
a simple matter to find the loss at a known frequency and then estimate the loss at a lower
frequency.
To measure feedline loss:
1. Connect the feedline to the MFJ-209 "ANTENNA" connector.
2. The far end of the feedline is either left unconnected or terminated with a direct short.
3. Adjust the MFJ-209 frequency to the frequency desired and observe the "SWR" meter.
4. If the SWR is in the red area of the scale the loss is less than 3 dB. Increase the frequency
until the "SWR" meter reads 3:1. This is the 3dB loss frequency.
5. If the SWR on the operating frequency is in the black area of the "SWR" meter, pick the
closest SWR point and estimate the loss from the chart below.
SWR LOSS
3.0 : 1 3.0 dB
2.5 : 1 3.6 dB
2.0 : 1 4.7 dB
1.7 : 1 5.8 dB
1.5 : 1 6.9 dB
1.2 : 1 10.3 dB
You can estimate the approximate loss at the operating frequency by remembering that the
feedline loss in dB is reduced by 70 % at half the frequency, and increased by 140 % at twice
the frequency you measured. This method is reasonably accurate if the loss is distributed along
the feedline and not confined to one bad area.