MFJ-269D Instruction Manual LF/HF/VHF/UHF SWR Analyzer
22
5.5.1.3
DTF Antenna Length
To measure the electrical length of wire antennas (longwires, dipoles, Beverages), connect through a
good broadband matching transformer -- or by connecting directly to the analyzer's
Antenna
port.
To ensure accuracy, avoid using any appreciable lengths of feedline (more than 1/32 wl) between the
analyzer and the antenna. While measurements can be made with a transmission line connected between
the antenna and analyzer, false zero reactance crossings will be introduced from line mismatch.
Watching the SWR meter will help you weed-out false reactance nulls when measuring antennas through
a transmission line.
To determine antenna length, treat the antenna like a transmission line, following the procedure for
measuring
Distance To Fault
. For a dipole, your result will be the electrical length of one leg. For a
longwire or Beverage, it will be electrical length for the entire antenna.
5.5.1.4
DTF Measurement Procedures
Distance to Fault
is the first measurement mode in
Advanced-2
. To enter the mode, press and hold the
Mode
and
Gate
buttons until
Advanced-2
appears on the screen. From other Modes, step through the
menu holding the
Mode
and
Gate
button.
The first menu that appears is:
The
Gate
button increases Vf,
Mode
decreases it.
1.
Set the
Vf
to the factory specified velocity factor of the transmission line. This data is required to for
Physical Length
calculations that will be displayed later. For
Electrical Length in Feet
, set Vf to unity
(1.00).
Note:
An
Incorrect
Vf
setting will not cause an errors in electrical measurements such as
Length in
Degree.
However it
will
cause an error in physical length calculations, such as
Dist. to Fault
.
2.
After setting the
Vf
, press
Gate
and
Mode
simultaneously to lock-in the value. The display indicates:
and after a few seconds changes to (sample screen):
15.814 MHz 1st
DTF Xs= 51
Note that
1st
is flashing on and off. This display is prompting you to use
Tune
to
find the frequency
where the
Impedance Meter
shows the lowest reading for
Xs
(as close to Xs=0 as possible). When you
find that frequency, press and hold
Gate
until the flashing
1st
on the display stops flashing. Then release
Gate
quickly.
The display now indicates the first frequency data point and the blinking
1st
will change to a blinking
2nd
: