MFJ-269D Instruction Manual LF/HF/VHF/UHF SWR Analyzer
9
Remember to set the top
Frequency MHz
selector fully counterclockwise (
155-230
) when setting up for
UHF HI (470-490MHz)
or set to the second to last switch position
(113-155)
when setting up for
UHF
LO (415-470 MHz)
measurements. The analyzer converts the analyzer's VHF oscillator up to the UHF
band for those measurements.
4.0
MAIN (OR OPENING) MODE
IMPORTANT WARNING: Never apply RF or DC voltages to the Antenna port of this unit. It uses
zero bias detector diodes that are easily damaged by any external voltages over a few volts. Also,
confirm the power supply voltage and polarity are correct, as described in Section-2.0.
A basic understanding of antenna theory and transmission line behavior will prove helpful for making the
best use of the data provided by your MFJ-259C. The ARRL Handbook and ARRL Antenna Book
provide concise peer-reviewed explanations that should suffice for most applications. When it comes to
the finer points of antenna design, there is (unfortunately) a fair amount of misinformation circulating on
the web and over the airwaves. When it comes to RF networks and antenna systems, there's no black
magic. Stick with the fundamentals as presented by credible professional sources.
4.1
General Connections
When conducting
SWR
and
Impedance
measurements, follow these practical guidelines:
1. Connectors:
If connector transitions (RF adapters) are needed, use only high-quality parts and check
them for wear, oxidation, dirt, and tight pin contact before proceeding.
2. Lead Length:
Make all connection electrically secure and keep all leads as short as possible. This
precaution is especially important when measuring electrical components that are not part of a 50-ohm
coaxial system.
3. Coaxial Cable:
Always use good quality 50-ohm cable and connectors when making SWR
measurements. Contaminated, mismatched, or damaged cable will introduce significant error.
4. Calibration Plane:
When making
Complex Impedance
measurements,
(R+X) or (Z)
, remember that
any length of transmission line you install between the load and the analyzer will displace the load
from the analyzer's calibration plane.
For simple handheld analyzers like the MFJ-269D, the c
alibration plane is always located at the
analyzer's RF connector
. This is the point where Zo=50 Ohms and Phase shift = 0 degrees.
It is the only
test point where the analyzer will be calibrated for complex impedance measurements.
Displacing the
load away from the analyzer's calibration plane through random lengths of coax should have little or no
impact on SWR readings, but will introduce significant error through phase shift and transformer action
to invalidate virtually any complex impedance measurement you might make.
When measuring Complex Impedance, always connect the MFJ-269D as close (electrically) as
possible to the DUT (device under test)!
4.2
Antenna SWR and Impedance
Use the N-Female
Antenna
connector located on top of the MFJ-269D for all RF measurements (except
those using the
Frequency Counter
mode). Follow the procedure outlined below for measuring
SWR
: