MFJ-259B Instruction Manual
HF/VHF SWR Analyzer
25
A voltage balun should also be tested by disconnecting the outer connections of the two resistors and connecting
each resistor in parallel. If the voltage balun is operating properly the SWR will be very low with the resistors
connected from either output terminal to ground.
BALUN
"ANTENNA"
To the MFJ-259B's
connector
7.8
Testing RF Chokes
Large RF chokes usually have frequencies where the distributed capacitance and inductance form a low
impedance “series-resonance”. This series resonance occurs because the choke acts like a series of back to back
L networks. This causes three problems:
First, impedance from end to end in the choke becomes very low.
Second, the voltage at the center of the resonant point becomes very high, often causing severe arcing.
Third, the current in the winding becomes very high, often resulting in severe heating.
Troublesome series resonances can be detected by installing the choke in the operating location, and connecting
only the MFJ-259B from end to end through a short 50 ohms jumper cable. By slowly sweeping the operating
frequency range of choke, dips in impedance identify low impedance series-resonant frequencies. By moving a
small insulated screwdriver’s blade along the choke, you will find a point where the series resonate impedance
suddenly changes. This is the area that has the highest voltage, and the area that adding or subtracting a tiny
amount of capacitance will have the largest effect. By removing turns to reduce capacitance, or adding capacitive
stub at this point, the resonance can be shifted out of the desired frequency range.
A small change in capacitance has a much larger effect than a small change in inductance, because the ratio of L
to C is so high.
8.0 TECHNICAL
ASSISTANCE
If you have any problem with this unit first check the appropriate section of this manual. If the manual does not
reference your problem or your problem is not solved by reading the manual, you may call
MFJ Technical
Service
at
601-323-0549
or the
MFJ Factory
at
601-323-5869
. You will be best helped if you have your unit,
manual and all information on your station handy so you can answer any questions the technicians may ask.
You can also send questions by mail to MFJ Enterprises, Inc., 300 Industrial Park Road, Starkville, MS 39759;
by FAX to 601-323-6551; or by e-mail to techinfo@mfjenterprises.com. Send a complete description of your
problem, an explanation of exactly how you are using your unit, and a complete description of your station.
