MFJ mfj-204b User manual

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MFJ-204B Antenna Bridge Instruction and Technical Manual
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DISCLAIMER
Information in this manual is designed for user purposes only and is not intended to
supersede information contained in customer regulations, technical manuals/documents,
positional handbooks, or other official publications. The copy of this manual provided to
the customer will not be updated to reflect current data.
Customers using this manual should report errors or omissions, recommendations for
improvements, or other comments to MFJ Enterprises, 300 Industrial Park Road,
Starkville, MS 39759. Phone: [phone]; FAX: [phone]. Business hours:
M-F 8-4:30 CST.

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MFJ-204B Antenna Bridge Instruction and Technical Manual
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TABLE OF CONTENTS
TOPIC PAGE
1. TABLE OF CONTENTS 2
2. LIST OF FIGURES AND TABLES 2
3. INTRODUCTION AND FEATURES 3
4. USES FOR THE MFJ-204B 4
5. SYSTEM CONTROLS AND INDICATORS 5
6. THEORY OF OPERATION 7
7. MFJ-204B EASY-START INSTRUCTIONS 8
8. IN CASE OF DIFFICULTY 13
9. TECHNICAL ASSISTANCE 13
APPENDICES
A. ZERO BEATING AGAINST A RECEIVER 12
LIST OF FIGURES
Figure 1 Front Panel Jacks and Controls 5
Figure 2 End Panel Jacks 6
Figure 3 Block Diagram 7
Figure 4 MFJ-204B to Tuner Hook Up 9
Figure 5 Schematic Diagram 11

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MFJ-204B Antenna Bridge Instruction and Technical Manual
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INTRODUCTION & FEATURES
INTRODUCTION:
The MFJ-204B gives you a direct readout of your antenna’s SWR without the need for
formulas or indirect readings. The MFJ-204B can also be used to adjust a tuner to match
your antenna without the need for transmitting. The frequency coverage for the MFJ-
204B is from below 1.75 MHZ to Above 30 MHZ in 5 user selectable bands.
FEATURES
Portable – The MFJ-204B’s light weight, small size and internal battery capability
makes it great for Field Day, Travel or any time you need to check out a new antenna
installation quickly easily.
Great Value – The MFJ-204B’s low cost puts a great instrument in your hands for those
who only need to adjust their antennas occasionally.
Stable – With built in AGC the MFJ-204B is accurate across the bands. No false
readings.
Wide Range – Determine the RF Resistance of your antenna from 10 to over 600 ohms.
Rugged Construction: Attractive all-metal cabinet, conservative component selection
and extensive RF filtering ensure solid performance for years to come. Fully covered by
MFJ’s “No Matter What” one year limited warranty.
Before attempting to operate your MFJ-204B, please read the manual thoroughly. It
contains important detail about setting up your unit to obtain the best performance.

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MFJ-204B Antenna Bridge Instruction and Technical Manual
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USES FOR THE MFJ-204B ANTENNA BRIDGE
Your MFJ-204B ANTENNA BRIDGE has many uses. It can be used to find the
resonant frequency of your antenna, to find the SWR of your antenna at a particular
frequency, and to find the frequency at which your antenna has the lowest SWR. You
can also use the Antenna Bridge to adjust your antenna to a low SWR and to adjust an
antenna tuner to match the transmitter to your transmission line. You can even find out if
your antenna is resonant on more that one frequency.
The added benefit of using an Antenna Bridge over a SWR meter is that you can adjust
antennas to impedances other than 50 Ohms. This is handy if you are using CATV
hardline, Ladder line or any other non 50 ohm coax.

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MFJ-204B Antenna Bridge Instruction and Technical Manual
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SYSTEM CONTROLS AND INDICATORS
Front Panel Jacks and Controls
1 2 3 4 5
FIGURE 1: Front Panel Jacks and Controls
1. Power On LED: Instantly know if the MFJ-204B is on or off with this bright LED.
2. Resistance Scale: This is where you read the resistance of your antenna using the
calibration chart for your unit.
3. Band Select Switch: This 6 position switch turn the power on and selects any one of
five over lapping bands.
4. Tune Control: This variable capacitor instantly sweeps across the desired band to
find that SWR fast and easy.
5. Null Meter: Instantly determine the null point of the RF Bridge.
Frequency Coverage of the Bands
NOTE: The frequency coverage may vary slightly from this chart and the label on the
cover of the unit and is for reference only. A frequency counter can be connected to the
Freq. Out connector shown in Figure 2 to get a more accurate reading of the frequency.
As an alternative to a frequency counter you can Zero Beat the output with an HF
receiver. See Appendix A.
BAND A: 1.40 - 2.50 Mhz
BAND B: 2.40 - 4.50 Mhz
BAND C: 3.90 - 7.20 Mhz
BAND D: 7.00 - 14.00 Mhz
BAND E: 14.00 - 31.00 Mhz

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MFJ-204B Antenna Bridge Instruction and Technical Manual
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SYSTEM CONTROLS AND INDICATORS
End Panel Jacks
FIGURE 2: End Panel Jacks
1. ANTENNA: This SO-239 Antenna connector allows you to easily hook up your
station’s antenna to the MFJ-204B.
2. FREQ. OUT: For additional accuracy you can hook up a frequency counter to this
phono connector or feed the signal over a short piece of wire to your radio and zero
beat against your radio as described in Appendix A.
3. PWR: This 2.1 mm jack allows you to attach a MFJ-1312D AC adapter or feed
power from your station supply.

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MFJ-204B Antenna Bridge Instruction and Technical Manual
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THEORY OF OPERATION
Block Diagram
FIGURE 3: Block Diagram
Refer to Figure 3 and the Schematic diagram Figure 5 for this discussion of the theory of
operation. RF is developed in the VCO consisting of L1 through L10 and the associated
FETs Q1 and Q2 along with the variable capacitor. The Band Switch progressively
shorts out the inductors as the frequency range is increased. The RF is then passed to Q3
and amplified. This RF is sensed by D2 and the resulting voltage is fed to U2 where it is
amplified and fed to Q4. Q4 varies the voltage on the source of Q1 and Q2 maintaining
the stable RF level. The RF then goes to the bridge consisting of two fixed legs and one
variable leg the RESISTANCE Pot. The 4
th
leg of the bridge is the unknown leg which is
the Antenna input to the unit. When the bridge is unbalanced then D1 rectifies the
unbalanced voltage and this is amplified by U2 and deflects the meter. When the 4 legs
are equal in resistance then the meter nulls to zero and the Unknown resistance is equal to
the resistance shown by the RESISTANCE pot.

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MFJ-204B Antenna Bridge Instruction and Technical Manual
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EASY START INSTRUCTIONS
The MFJ-204B requires the optional MFJ-1312D power supply adapter or a nine-
volt transistor battery. If using a battery with the MFJ-204B ensure it is an ALKALINE
nine-volt battery. Additionally you can use a 13.8 volt power supply by connecting a 2.1
mm plug to the cable with the positive wire connected to the center pin and the ground
wire connecter to the outer pin.
To install the battery, remove the screws holding the cover onto the case. Insert
the battery into the battery holder. Tuck the battery snap wires out of the way so they do
not interfere with the tuning capacitor rotation. Reinstall the cover and screws.
I. How to measure the antenna impedance at the antenna feed point.
1. Turn on the MFJ-204B
2. Attach the MFJ-204B to the antenna as close to the feed point using the
Shortest length of RG-58U or RG-8U that you can.
3. Set the BAND switch to the appropriate band.
4. Adjust the FREQUENCY DIAL of the ANTENNA BRIDGE to the frequency
at which you want to check the SWR. For added accuracy connect a frequency
counter to the Freq. Out jack or zero beat against your receiver as described in
Appendix A.
5. Turn the Resistance control until a maximum dip on the meter can be
achieved. Note that a complete dip to “0” on the meter may not be possible.
In any case try to obtain the minimum meter reading.
6. Read the antenna resistance on the Resistance control.
II. Adjusting the antenna for resonant frequency.
1. Follow the steps from 1 thru 6 on section I.
2. If the resistance does not read 50 ohms, set the Resistance control to 50 ohms.
3. Adjust your antenna until the meter of the MFJ-204B reads minimum. At this
point the feed point impedance of the antenna is 50 Ohms at the frequency
which was selected.
HINT: If you are adjusting an antenna for the first time, it is recommended that you start
with elements a little too long, then shorten to resonance. Again the Null for
your particular antenna may never go to “0”. Always adjust for the
lowest reading.

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MFJ-204B Antenna Bridge Instruction and Technical Manual
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EASY START INSTRUCTIONS
III. Using the MFJ-204B to adjust an antenna tuner.
1. Turn on the MFJ-204B
2. Attach the MFJ-204B to the tuner using the shortest length of RG-58U or
RG-8U that you can.
3. Set the BAND switch to the appropriate band.
4. Adjust the FREQUENCY DIAL of the ANTENNA BRIDGE to the frequency
at which you want to check the SWR. For added accuracy connect a frequency
counter to the Freq. Out jack or zero beat against your receiver as described in
Appendix A.
5. Set the Resistance control to 50 Ohms.
6. Adjust the antenna tuner until the meter on the MFJ-204B reads minimum.
7. Your antenna tuner is now set for 50 ohms at that frequency.
8. Disconnect the MFJ-204B completely and connect the transmitter to the tuner.
9. It is always best to check the settings with a SWR meter as some slight
retuning might be necessary.
10. Always turn the Band switch to OFF when finished.
CAUTION
Never transmit into or through the MFJ-204B. Take the MFJ-204B completely
out of line before transmitting! Damage to your MFJ-204B will result if left in
line.
We recommend using the MFJ-1702 coax switch as shown in the diagram below for ease
of use when changing bands.
FIGURE 4: Hook-up for using the MFJ-204B to Tune Antenna Tuner