MFJ-249C Instruction Manual
HF/VHF SWR Analyzer
6
3.) Frequency Counter:
The third mode converts the analyzer into a discrete frequency counter. Connect the RF
source (DUT) to the BNC connector labeled
Frequency Counter Input.
As with many counters, the sensitivity
threshold for a locked-in reading gradually decreases with increasing frequency. The measurement threshold at
0.53 MHz is around 10 millivolts -- and this level gradually increases to around 200 millivolts at 230 MHz.
The
"never exceed" limit for safe testing is 2-volts peak-to-peak.
The counter's default gate time is 0.1 second, but
you may reset it to either .01 second (very fast) or 1.0 second (very slow) by tapping the
Gate
button. A 1.0-
second gate times provide increased frequency resolution (more digits to the right of the decimal point), and the
.01 gate provides very fast response with less resolution (see sample screens below):
IMPORTANT WARNING: Never apply more than two volts of peak voltage -- or any dc voltage -- to
the Frequency Counter BNC port.
3.4
Blinking “VOLTAGE LOW” display warning
If the external dc source or battery voltage drops below 11 volts, a blinking
Voltage Low
warning will come up on
the display. Pressing
Mode
during a low-voltage warning will disable it and allow you to continue testing.
Caution: Measurements made with supply voltages below 11 volts may not be as reliable.
4.0 MAIN (OR OPENING) MODE
IMPORTANT WARNING: Never apply RF or any other external voltages to the Antenna port of this
unit. This unit uses zero bias detector diodes that are easily damaged by external voltages over a few
volts. Also, confirm the power supply is correct, as described in Section-2.0, before operating this unit.
A basic understanding of antenna theory and transmission line behavior will be helpful for making the best use of
the data provided by your MFJ-249C. 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 mis-information circulating on the web and over the airwaves.
When it comes to antenna systems, there's no black magic. Stick with the scientific fundamentals as presented by
credible professional sources, and everything your analyzer tells you should make sense!
4.1
General Connection Guidelines
When conducting
SWR
and
Impedance
measurements, follow these practical guidelines:
1.) If connector transitions (RF adapters) are needed, use only high-quality parts and check them over for wear,
oxidation, dirt, and tight pin contact before proceeding.
2.) 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.
