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MFJ-835 Balanced Line RF Ammeter Instruction Manual
MFJ-835 RF Ammeter
Introduction
Congratulations on purchasing the MFJ-835 Balanced Line RF Ammeter. The MFJ-835
is designed for measuring balanced RF feedline current on 1.8-30 MHz while having low
interaction on the primary lines. Connect the Balanced Line RF Ammeter between your
antenna and tuner/ transmitter/amplifier and feedline currents are simultaneously
displayed on separate calibrated scales in three calibrated ranges: 0.3 Amperes, 1
Ampere, and 3 Amperes. Balance is clearly revealed on MFJ's exclusive BalancedBar.
When currents are balanced, the dual needles cross in the center. If the needles cross
within the vertical BalancedBar, the balance is within 10%.
Uses
Equal currents in the feeder wires minimize feedline radiation and reduce pattern
distortion, RFI, TVI and wasted power. You could lose up to 10% of your power.
Imbalance is due to feedline not being perpendicular to antenna, if one side runs close to
metal or is shorter or closer to ground. The MFJ-836 helps you solve all your unbalanced
problems.
• Use it for tuning your antenna tuner/transmitter/amplifier for maximum radiated
power, for determining antenna feedpoint impedance, for comparing antennas and
tuners, for troubleshooting and for checking for changes.
• Minimum SWR does not always mean maximum radiated power. Your tuner or
lossy coax may be dissipating power and you may not get peak output power even
if your tuner or amplifier is properly tuned.
• Use the MFJ-835 Balanced Line RF Ammeter to determine best antenna tuner
settings and to compare various tuners. Calculate your antenna feedpoint
impedance by dividing your applied power by the square of the feedpoint current.
• Your antenna can change when you apply power. Insulators break down, traps
heat up, capacitors leak and your feedpoint impedance may be drastically
different from your SWR Analyzer measurements.
IMPORTANT:
Please read entire manual before attempting to operate this equipment. This
unit does not provide a dc or low frequency earth ground. A separate wire
should be connected from the station ground bus to an electric ground. Do
not place any non-coaxial feed wires near metallic or RF sensitive devices.
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MFJ-835 Balanced Line RF Ammeter Instruction Manual
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Installation
1. Connect your transmitter to the Heavy-duty 5-way binding posts on the back of
the unit labeled TRANSMITTER and your antenna to the Heavy-duty 5-way
binding posts labeled ANTENNA. It is important that you use good quality
feedline and properly installed connectors.
2. An internal lamp backlights the RF Ammeter scale. The lamp circuit requires 12
VDC. The rear panel accepts a 2.1 mm coaxial plug with the center conductor
positive (+) and the sleeve negative (-). The negative lead is grounded inside the
meter.
3. To measure the power output capability of a transmitter/amplifier you should
connect a quality 50-ohm dummy load to the ANTENNA connector of the MFJ-
835.
Operation
1. The Balanced Line RF Ammeter measures RF current in three calibrated ranges:
0.3 Amperes, 1 Ampere, and 3 Amperes. Simply turn the Current Range knob to
the desired range.
2. The meter is scaled from 0 to 100. On the 1-A range, move the decimal point 2
places to the left (100 = 1.00 A). To convert meter readings made on the .3A and
3A ranges, multiply accordingly. For example, a reading of 85 on the 300-mA
range equals 300 x .85, or 255 mA. See Table 1 below to estimate readings
quickly.
3. The BalancedBar easily displays when currents are balanced by using the dual
needles. When the needles cross within the vertical BalancedBar, the balance is
within 10%.
4. To prevent meter pegging, begin checks on a high range and work down. To
estimate unknown current levels, use the formula below (I = Unknown Current in
Amps, P = Applied Power in Watts, and R = Load Resistance in Ohms).
Range
I
Table 1: Actual
Current Value
P
=
Meter 300 mA 1 A 3 A
10 30 0.1 0.3
20 60 0.2 0.6
30 90 0.3 0.9
40 120 0.4 1.2
50 150 0.5 1.5
60 180 0.6 1.8
70 210 0.7 2.1
80 240 0.8 2.4
90 270 0.9 2.7
100 300 1.0 3.0
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MFJ-835 Balanced Line RF Ammeter Instruction Manual
RF Hazard--Important Warnings:
1.
RF-current measurements must be conducted with extreme caution. Physical
contact with conductors carrying even moderate RF-energy levels can result in
severe RF burns, or injury.
2.
Avoid measuring RF currents on conductors where high RF voltages may be
present (>500 Volts). Damage to your meter may result.
Calibration
The MFJ-835 RF Ammeter has been calibrated at the factory. If the RF Ammeter should
ever need to be recalibrated, then follow this procedure:
Equipment Needed
1. Transmitter capable of supplying enough power to obtain ½ to full-scale reading
at 14 or 21 MHz.
2. 50-ohm dummy load that is capable of handling full transmitter output power and
has better than a 1.15:1 SWR.
3. Power meter of know accuracy. The calibration will only be as good as the
standard reference meter.
4. Ladder line capable of handling the power.
Transmitter/
Transceiver
Standard
Reference
Wattmeter
Figure 1: RF Ammeter Test Setup
MFJ-835
RF
Ammeter
50-Ohm
Dummy Load
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MFJ-835 Balanced Line RF Ammeter Instruction Manual
RF Ammeter Calibration
1. Refer to Figure 1 for the Test Setup and refer to the PCB layout in Figure 2
for trim pot location.
2. Remove the top of the MFJ-835.
3. Connect the Test Setup equipment as shown in Figure 1. Use a 50-ohm
dummy load for the antenna. Set the Transmitter to the 14 MHz in the 20meter band.
4. Set the Current Range selector to 1A. For meaningful results, your Wattmeter
must be accurate and the 50-ohm load free of significant reactance.
5. To test, apply a 50-watt CW signal to the 50-ohm dummy load. You should
observe a full-scale reading of 1A (100 on the scale) on CURRENT 1. If
results vary significantly from this reading, adjust trim pot R3 on the top pc
board for the correct indication. Trim pot R3 is located on the top pc board
below the sense coil.
6. Reverse the connections of the ladder line and apply a 50-watt CW signal to
the 50-ohm dummy load. You should observe a full-scale reading of 1A (100
on the scale) on CURRENT 2. If results vary significantly from this reading,
adjust trim pot R3 on the bottom pc board for the correct indication. Trim pot
R3 is located on the bottom pc board below the sense coil.
R3
Figure 2: RF Ammeter PCB Layout and Trim Pot Location
IMPORTANT WARNING: Avoid direct physical contact with this device when making
measurements with power sources of over 5 Watts. RF burns or
permanent injury may result from contact with high RF voltages.
If your meter is out of calibration and you lack the equipment or skills to perform the
calibration procedure, you may return it to the MFJ factory for service.
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MFJ-835 Balanced Line RF Ammeter Instruction Manual
Schematic
RF Ammeter
Line Under
Test
Sense Coil
T1
R2
68
1k
Calibrate
Det
D2
.3A
1A
3A
R1 7.5kR3
R4 28k
.3A
1A
3A
Range
R5 90k
Line Under
Test
Sense Coil
T2
R2
68
1k
Calibrate
Det
D2
.3A
1A
3A
R1 7.5kR3
R4 28k
R5 90k
.3A
1A
3A
Range
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 reading the manual does not solve your
problem, you may call MFJ Technical Service at 662-323-0549 or the MFJ Factory at
662-323-5869. 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 Facsimile (FAX) to 662-323-6551; or by email 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.
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