MFJ-269 Instruction Manual HF/VHF/UHF SWR Analyzer
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This mode measures coax loss for the line Zo selected in 5.6.1. it is important that the line is not terminated in
any sort or dissipative load when making this measurement.
To use this mode, sweep the desired measurement frequency range. Watch the loss reading carefully, and tune for
minimum loss. The minimum loss reading obtainable near the desired frequency range is the correct loss reading.
To return to Z Characteristic, push the
MODE
button one time. Pressing the
GATE
button returns the analyzer
to the Zo setup menu.
Pressing and holding both
GATE
and
MODE
buttons for a long time cycles the analyzer back to the
“MAIN”
or
“Advanced”
modes.
6.0 Adjusting Simple Antennas
CAUTION: THERE IS A ‘‘UHF’’ SWITCH LOCATED AT THE UPPER LEFT-HAND
SIDE OF THE ANALYZER. THIS SWITCH SHOULD BE PRESSED AND
LOCKED FOR UHF OPERATION ONLY WHEN UHF OPERATION IS DESIRED
AND ONLY AFTER THE UNIT IS POWERED UP. FOR INFORMATION ON
UHF OPERATION, SEE SECTION 5.7
Most antennas are adjusted by varying the length of the elements. Most home made antennas are simple verticals
or dipoles that are easily adjusted.
6.1 Dipoles
Since a dipole is a balanced antenna, it is a good idea to put a balun at the feedpoint. The balun can be as simple
as several turns of coax several inches in diameter, or a complicated affair with many windings on a
ferromagnetic core.
The height of the dipole, as well as it's surroundings, influence the feedpoint impedance and feedline SWR.
Typical heights result in SWR readings below 1.5 to 1 in most installations when using 50 ohm coaxial cable.
In general, the only adjustment available is the length of the dipole. If the antenna is too long it will resonate too
low in frequency, and if it is too short it will resonate too high.
Remember feedline length, when the antenna is not exactly the same impedance as the feedline, modifies the
impedance
along the feedpoint.
SWR
will remain constant (except for a small reduction in SWR as 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 are caused by lack of a balun or other installation errors, such as a
feedline paralleling the antenna.
Note: Advanced 3 allows you to change the SWR Zo reference. If 75 ohms Zo is selected, and SWR
is measured along a 75 ohm cable, SWR referenced to 75 ohms shown on the display will
remain nearly constant regardless of line length. SWR referenced to 50 ohms (shown on the
meter) will vary wildly.