MFJ-269C Instruction Manual LF/HF/VHF/UHF SWR Analyzer
31
the feedline is made longer) if the feedline is a good quality 50 ohm cable. If feedline length changes
SWR at any one fixed frequency, the feedline either has common-mode currents that are detuning the
antenna or the feedline is not a true 50-ohm cable. Common-mode currents occur when a balun hasn't
been installed to "choke" or block them off, or when the feedline parallels the radiator and couples to it.
Note:
Advanced-3 allows you to change the SWR Zo reference. If Zo=75 ohms is selected and SWR is
measured along a 75-ohm cable, the SWR referenced to 75 ohms (as shown on the display) will remain
nearly constant regardless of line length. However, SWR referenced to 50 ohms (as shown on the meter)
will vary wildly. The 75-ohm Zo SWR on the display
is the true SWR on the 75 ohm cable
, and the SWR
shown on the meter is the SWR when a 50 ohm system is connected to the 75 ohm cable.
6.2 Verticals
Verticals are usually unbalanced antennas. Many antenna manufacturers incorrectly downplay the need
for a good radial system with a grounded vertical. Given a good ground system, the SWR of a directly fed
quarter-wave vertical can be nearly 2:1. SWR often
improves
if the ground system is poor, but
performance suffers. Low SWR on a directly fed Marconi might be a sign of inefficiency!
Verticals are tuned the same way as dipoles, lengthening moves the frequency lower and shortening
moves the frequency higher.
6.3
Tuning a simple antenna
Select any mode that indicates SWR, then use the following steps to tune a basic coax-fed antenna:
1.
Momentarily short the center conductor and shield to discharge static, then connect to the MFJ-269C.
2.
Adjust the MFJ-269C VFO to the desired frequency.
3.
Read SWR, and tune the analyzer for
minimum
SWR (Confirm cable Zo matches Analyzer Zo).
4.
Divide the
measured
minimum SWR frequency by the
desired
operating frequency.
5.
Multiply the present antenna length times the result from Step-4. This will approximate the antenna
length you actually need.
Note:
This method works on a full-size vertical or dipole antennas with uniform element diameter. It
won't work for antennas that employ loading coils, traps, stubs, resistors, capacitors or capacitance hats.
Those antennas should be tuned using the MFJ-269C in conjunction with the manufacturer’s setup
instructions.
7.0
TESTING AND TUNING STUBS AND TRANSMISSION LINES
7.1 Testing
Stubs