MFJ MFJ-9218 Manual de usuario

SWR Meter, Wattmeter, & Dummy Load

MFJ-9218

INSTRUCTION MANUAL

MFJ Enterprises, Inc.

300 Industrial Park Rd. Starkville, MS 39759

P: (662) 323-5869

F: (662) 323-6551

DISCLAIMER

The information in this manual is for user purposes only and is not

intended to supersede information contained in customer regulations,

technical manuals or 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: (662) 323-5869

Fax: (662) 323-6551

Business hours: M-F 8:00 AM - 4:30 PM CST.

Contents

1 RADIO-FREQUENCY RADIATION 1

2 THE MFJ-9218 2

2.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2 THE WHEATSTONE BRIDGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.3 FEATURES......................................... 4

2.4 CONTROLS & CONNECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 SYSTEM SETUP 5

4 SYSTEM OPERATION 5

4.1 THERMAL WARNING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

5 TECHNICAL ASSISTANCE 6

List of Figures

1 Horizontally-polarized Electromagnetic Plane Wave . . . . . . . . . . . . . . . . . . . 1

2 The Electromagnetic Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 Relevant Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4 MFJ-9218 Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

5 MFJ-9218 Basic Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

6 Safe transmitting times as a function of RF power . . . . . . . . . . . . . . . . . . . 6

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MFJ-9218 SWR Meter, Wattmeter, & Dummy Load Instruction & Technical Manual

1 RADIO-FREQUENCY RADIATION

Radio-frequency (RF) radiation is one type of electromagnetic radiation. Electromagnetic waves

and associated phenomena are discussed in terms of energy, radiation, and ﬁelds. Electromagnetic

radiation is deﬁned as waves in electric and magnetic ﬁelds moving together, or radiating, through

space (Figure 1). These waves are generated by the movement of electrical charge. For example,

the movement of charge in a radio station antenna creates electromagnetic waves that are radiated

away from the antenna. The waves then induce charge motion in the receiving antenna, which is

detected and converted into signal by the radio. The term electromagnetic ﬁeld refers to the

electric and magnetic environment existing at some location due to a radiating source such as an

antenna.

Figure 1: Horizontally-polarized Electromagnetic Plane Wave

An electromagnetic wave consists of oscillating orthogonal electric ( ~

E) and magnetic ( ~

B) ﬁelds.

These ﬁelds propagate together with direction and velocity ~v. In a vacuum this is the speed of

light, c. In Earth’s troposphere, |~v| ≈ 0.9999997c. The two deﬁning characteristics of an

electromagnetic wave are its wavelength (λ) and frequency (f). The wavelength is the distance

between two adjacent peaks in the wave, and the frequency is the number of peaks passing a given

point in space during a second. Wavelength and frequency are reciprocal with the speed of light

(fλ =c), so if you know one quantity, you can easily ﬁnd the other. For example, a typical radio

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MFJ-9218 SWR Meter, Wattmeter, & Dummy Load Instruction & Technical Manual

wave transmitted by a 2-meter VHF station has a frequency of about 145 MHz. Dividing the speed

of light (∼3×108m/s) by the frequency in Hz, we ﬁnd that the wavelength in atmosphere of the

signal from our station is 2.06 m. Since wavelength and frequency are reciprocal, an increase in

wavelength corresponds to a decrease in frequency, hence, the 160 m band has a rather low

frequency of 1.8 MHz.

The electromagnetic spectrum (Figure 2) includes all of the various energies of electromagnetic

radiation ranging from extremely low frequency (ELF) ranges (with very long wavelengths) to all

the way up to x rays and γrays, which have very high frequencies and correspondingly short

wavelengths. In between these extremes lie radio waves, microwaves, infrared radiation, visible

light, ultraviolet radiation, and the entirety of the FCC spectrum allocation chart. The RF part of

the electromagnetic spectrum is generally deﬁned as that part of the spectrum from about 3 kHz

to 300 GHz.

Figure 2: The Electromagnetic Spectrum

2 THE MFJ-9218

2.1 INTRODUCTION

The MFJ-9218 is a simple resistive QRP SWR Meter, Wattmeter, and Dummy Load. It is

designed to be used with QRP transmitters (<5W) but can handle up to 20W for short periods.

The MFJ-9218 uses 100Ω 2W surface-mount resistors in the network will work well up to 54MHz

with a very low insertion SWR. The power measurement ranges are set by the METER SET pot

and the reference marks on the front panel. These ranges are 0.5W, 1W, 5W, and 20W. Note these

marks are only for reference and the power readings are only approximate.

2.2 THE WHEATSTONE BRIDGE

This section is solely for the enjoyment of those who wish to learn some of the theory behind

amateur radio. Knowledge or understanding of it is not required to operate and enjoy the

MFJ-9218.

The circuit used in the MFJ-9218 is known as a Wheatstone bridge and is shown in Figure 3a. The

Wheatstone bridge is a simple circuit that was originally designed to determine an unknown

resistance in a DC circuit but also works to determine unknown impedances in RF circuits. Several

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MFJ-9218 SWR Meter, Wattmeter, & Dummy Load Instruction & Technical Manual

(a) MFJ-9218 Circuit Schematic (b) A Wheatstone Bridge

Figure 3: Relevant Schematics

variants exist, but the one used in the MFJ-9218 can be simpliﬁed to the circuit shown in

Figure 3b. The Wheatstone bridge works by measuring the current ﬂowing between points B and

D (IBD ). It is considered balanced when IBD = 0, or due to Ohm’s Law, VBD = 0. In that case,

Rx=R2R3

R1. If we deﬁne R1=R2=R3≡50Ω, then the value of Rxthat will balance the bridge is

also 50Ω.

This tells us that in the case of the MFJ-9218 and others like it, any imbalance (i.e. current ﬂowing

between points) B and D will occur only if Rx6= 50Ω. Assuming that the resistance of the ammeter

is suﬃciently large that IBD is negligible (reasonable for most meters and the MFJ-9218), VBD can

be written in terms of the source potential V0and the resistances. This is shown in Equation 1.

VBD =V0R2

R1+R2

−Rx

Rx+R3(1)

Finally, if we extend our source to include AC (or RF) potentials and replace Rxwith a load, such

as an antenna, we can write Equation 1 in terms of reactances to get

VBD =V0Z2

Z1+Z2

−Zx

Zx+Z3.(2)

Resistors will have negligible reactances, so Z1,Z2, and Z3will only carry their resistances of 50Ω.

This converts Equation 2 into

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MFJ-9218 SWR Meter, Wattmeter, & Dummy Load Instruction & Technical Manual

Zx=R3V0R2−VBD (R1+R1)

V0R1+VBD (R1+R2).(3)

Now that we know the impedance in terms of known quatities (R1=R2=R3= 50Ω, VBD is

measured by the meter, and V0is “programmed in” during SWR calibration), we can determine

the SWR using Equation 4.

SW R ≡Zx

Z0±1

(4)

where in our case, Z0= 50Ω.

2.3 FEATURES

2.4 CONTROLS & CONNECTIONS

Figure 4: MFJ-9218 Controls. 1.) METER SET, 2.) INPUT, 3.) OUTPUT, 4.)TUNE/BYPASS,

5.) SWR/SET, 6.) Multifuntion Meter

1. METER SET: Adjusts the sensitivity of the multipurpose meter

2. RADIO: Connect to the radio

3. ANTENNA: Connect to the antenna

4. TUNE/BYPASS: Bypasses the MFJ-9218

5. SWR/PWR: Toggles between SWR and wattmeter modes and engages the internal load

6. Multipurpose Meter: Displays SWR and power depending on mode

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MFJ-9218 SWR Meter, Wattmeter, & Dummy Load Instruction & Technical Manual

3 SYSTEM SETUP

Figure 5 shows a basic setup that utilizes the MFJ-9218. The necessary connections are simple:

connect the RADIO port of the MFJ-9218 to your radio, and the ANTENNA port to your

antenna. The MFJ-9218 does not use any external DC power, so once those connections are made

the unit is ready to operate.

Figure 5: MFJ-9218 Basic Setup

4 SYSTEM OPERATION

1. Connections

(a) Connect the transmitter to the IN connector.

(b) Connect the antenna or antenna tuner to the OUT connector

2. SWR Measurements

(a) To measure SWR set the SWR/SET switch to OUT and transmit.

(b) Adjust the meter with the METER SET pot to full scale on the meter.

resistive network will temper the SWR on the transmitter to less than 2:1 and depends

on the antenna impedance. Only a fraction of the transmitted power reaches the

antenna in this mode, so be sure to switch back to BYPASS when ready to operate.

(d) Above 5W limit the transmitter time to the time indicated in Figure 6.

3. Power Measurements and Dummy Load

(a) To use as a DUMMY LOAD set the SWR/SET switch OUT. The internal resistive

network is a 50 ohm load which will handle 5W power continuous or higher power for

short periods of time.

(b) To measure POWER the SWR/SET switch is OUT.

(d) Key the transmitter and measure the power on the meter RF PWR scale.

(e) Above 5W limit the transmitter time to the time indicated in Figure 6.

4.1 THERMAL WARNING

The resistor network in the MFJ-9218 is made up of eight 100Ω 2W surface-mount resistors. Six

are used in SWR mode and all eight in the Dummy Load and Power mode. Due to the small size

of the devices the temperature increase is substantial. Above 5W limit the transmit time in SWR,

Wattmeter, and Dummy Load mode to less than the time listed in Figure 6 and allow the resistors

to cool for 2 to 5 minutes between transmissions. It is recommended to keep the power below

100W in BYPASS mode.

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MFJ-9218 SWR Meter, Wattmeter, & Dummy Load Instruction & Technical Manual

Figure 6: Safe transmitting times as a function of RF power

5 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

tec[email protected]. 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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