MFJ MFJ-834 Manuel utilisateur

MFJ-834 RF Ammeter Instruction Manual

MFJ-834 RF Ammeter

Introduction

Congratulations on purchasing the MFJ-834 RF Ammeter. The MFJ-834 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, 1 Ampere, and 3 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-834 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.

MFJ-834 RF Ammeter Instruction Manual

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 MFJ-

834.

Operation

1. The RF Ammeter measures RF current in three calibrated ranges: .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 1A 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. 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

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

MFJ-834 RF Ammeter Instruction Manual

Table 1: Actual Current Value

RF Hazard--Important Warnings:

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. Using the MFJ-836H continuously for more than 1

minute on the 30 A range may present these high RF voltages. When operating on the 30 A

range, we recommend operating at 10% duty cycle to prevent damage.

Calibration

The MFJ-834 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. 50-ohm cables capable of handling the power. RG-58/u is recommended. DO

NOT USE RG-59 or RG-11.

Figure 1: RF Ammeter Test Setup

Transmitter/

Transceiver

With

Amplifier

Standard

Reference

Wattmeter

MFJ-834

Ammeter

50-Ohm

Dummy Load

MFJ-834 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-834.

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 20-

meter 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. 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). If results vary significantly from

this reading, adjust trim pot R3 for the correct indication. Trim pot R3 is

located on the pc board below the sense coil.

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.

MFJ-834 RF Ammeter Instruction Manual

Schematic

RF Ammeter

Sense Coil

Test

Line Under

R1 7.5kR3

R4 28k

Det

Calibrate

.3A

R5 90k

.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

[email protected]. Send a complete description of your problem, an

MFJ-834 RF Ammeter Instruction Manual

explanation of exactly how you are using your unit, and a complete description of your

station.

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