MFJ MFJ-834H User Manual

MFJ-834H RF Ammeter Instruction Manual

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

to an electric ground. Do

MFJ-834H RF Ammeter

Introduction

Congratulations on purchasing the MFJ-834H RF Ammeter. The MFJ-834H is designed
for measuring in-line RF feedline current on 1.8-30 MHz while having low interaction on
the primary line. Connect the Coax In-Line RF Ammeter between your antenna and
tuner/ transmitter/amplifier and measure antenna feedline current in 3 calibrated ranges:
3 Amperes, 10 Ampere, and 30 Amperes.

Uses

• 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.

• Tuning for maximum feedline current for any given antenna always gives

maximum radiated power. Any adjustment that increases feedline current
improves power transfer and gives you a stronger radiated signal. Radiated power
increases rapidly because power increases as the square of the current.

• 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-834H 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.

should be connected from the station ground bus
not place any non-coaxial feed wires near metallic or RF sensitive devices.

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MFJ-834H RF Ammeter Instruction Manual

Range

Installation

1. Connect your transmitter to the connector on the back of the unit labeled
TRANSMITTER and your antenna to the connector labeled ANTENNA. It is
important that you use good quality coax 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 MFJ­834H.

Operation

1. The RF Ammeter measures RF current in three calibrated ranges: 3 Amperes, 10
Ampere, and 30 Amperes. Simply turn the Current Range knob to the desired
range.

2. The meter is scaled from 0 to 100. On the 10A range, move the decimal point 1
place to the left (100 = 10 A). To convert meter readings made on the 3A and
30A ranges, multiply accordingly. For example, a reading of 85 on the 30A range
equals 30 x .85, or 25.5 A. See Table 1 below to estimate readings quickly.

3. 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).

3A 10A 30A Meter

Actual Value (Amps)

I

=

P

Reading

R

10 0.3 1.0 3
20 0.6 2.0 6
30 0.9 3.0 9

0 1.2 4.0 12
50 1.5 5.0 15
60 1.8 6.0 18
70 2.1 7.0 21
80 2.4 8.0 24
90 2.7 9.0 27

100 3.0 10.0 30

Table 1: Actual Current Value

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