MFJ-269D Instruction Manual LF/HF/VHF/UHF SWR Analyzer
27
3.
The small flashing
SWR
symbol on the lower left means the on-screen reading is referenced to a new
Zo
(see screen below). The meter, however, continues to indicate SWR normalized to 50-ohms.
4.
Pressing
Gate
(by itself) changes the function back to the Zo setup mode. Pressing the
Mode
by itself
changes the measurement mode to
Coax Loss
(see 5.6.2 below):
5.6.2
Coax Loss
Please re-read and apply the method presented in section 4.2.2 Coax Loss, before using this advanced
function. It explains the loss measurement in detail.
Access this mode via the
Z Characteristic
screen (5.6.1) by pressing
Mode
. In this mode,
Zo
flashes in
the upper right and
Coax Loss
appears on the display.
Coax Loss is measured for the feedline
Zo
selected in 5.6.1.
It is important that the line is not terminated
with a short or a resistive (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 near the desired frequency range is the correct loss
reading.
To return to
Z Characteristic
, press
Mode
. Pressing
Gate
returns the analyzer to the
Zo
setup menu.
Press and hold both
Gate
and
Mode
for an extended period to cycle the analyzer back to the
Main
or to
another
Advanced
mode.
6.0
ADJUSTING SIMPLE ANTENNAS
Most antennas are adjusted by varying element length -- and most homemade 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 (and good engineering practice) 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 using 50-ohm coaxial
cable.
In general, the only adjustment available is the length. If the antenna is too long it will resonate low in
frequency, and if it is too short it will resonate 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 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.