MFJ-269D Instruction Manual LF/HF/VHF/UHF SWR Analyzer
32
Baluns can be tested by connecting the 50-ohm unbalanced side to the MFJ-269D
Antenna
connector.
The balun must be terminated with two equal value load resistors in series and the resistor combination
must have total resistance equal to balun load impedance. For example, a pair of 100 ohm carbon
resistors are required to properly test the 200 ohm secondary of a 4:1 balun (50 ohm input).
Measure SWR while moving a jumper wire from point "A" through point "C".
Balun
>
A
C
B
Clip Lead
50 Ohms
Unbal
R1
R2
Balun
Clip Lead
50 Ohms
Unbal
R1
R2
<
A
C
Fig A
Fig B
A properly designed
current balun
is the type most effective for maintaining current balance. It has the
highest power capability and lowest loss for given materials. It should show a low SWR over the entire
operating range of the balun with the clip lead in
any
of the three positions.
A well designed
voltage balun
should show a low SWR over the entire operating range when the clip
lead is in position "B". That SWR should not change when the clip lead is removed. It will show a very
poor SWR when the clip lead is in position "A" and "C". SWR should be about the same in either
position “A” or “C”. If the balun does not follow these rules, the balun has poor balance and is of
questionable benefit.
A 4:1 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.
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:
1.
Impedance from end to end in the choke becomes very low.
2.
The voltage at the center of the resonant point becomes very high, often causing severe arcing.
3.
The current in the winding becomes very high, often resulting in severe heating.
Troublesome series-resonance can be detected by installing the choke in the operating location, and
connecting
only
the MFJ-269D from end-to-end of the choke through a short 50 ohm 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 close to and along the choke, you will find a point
where the series-resonant impedance suddenly changes. This is the area that has the highest voltage and
also the area where adding or subtracting a tiny amount of capacitance has the largest effect. By
removing turns to reduce capacitance or adding a small capacitive stub at this point, resonance can be
shifted out of the desired frequency range.
A small change in stray capacitance has a much larger effect than a small change in turns, because the
ratio of L to C is so high. It is often possible to move the series-resonance a large amount without greatly
affecting the overall inductance.