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MFJ Enterprises MFJ-249B, Instruction Manual

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MFJ Enterprises MFJ-249B Instruction Manual Download Page 14

MFJ-249B Instruction Manual 

HF/VHF SWR Analyzer 

14 

You can also send questions by mail to MFJ Enterprises, Inc., 300 Industrial Park Road, 
Starkville, MS 39759; by Facsimile (FAX) to 601-323-6551; or by email to 
[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. 
 

Summary of Contents for MFJ-249B

Page 1: ...le Antennas 7 Dipoles 7 Verticals 7 Tuning an Antenna 7 Measuring the Feedpoint Resistance of Antennas 8 Testing and Tuning Stubs and Transmission Lines 8 Velocity Factor of Transmission Lines 9 Impedance of Transmission Lines 9 Adjusting Tuners 10 Adjusting Amplifier Matching Networks 10 Testing RF Transformers 10 Testing Baluns 11 Measuring Inductance and Capacitance 11 Measure Capacitance 12 Me...

Page 2: ... approximately 3 Vpp to any load in series with a 50 ohm internal resistance The unit FREQUENCY switch selects the following frequency ranges 1 8 4 MHz 4 10 MHz 10 27 MHz 27 70 MHz 70 114 MHz 114 170 MHz The MFJ 249B can be used to adjust or measure the following Antennas SWR impedance resistance resonant frequency bandwidth efficiency Antenna tuners SWR frequency Amplifiers Input and output netwo...

Page 3: ... blinking message will be displayed By pressing the MODE or the GATE time button on the unit it will try to continue working with the low voltage WARNING The measurements will be inaccurate when the supply voltage falls below 11 volts BATTERY INSTALLATION MFJ recommends the use of ten AA Alkalines or rechargeable NiCd cells to reduce the risk of equipment damage from battery leakage Avoid leaving ...

Page 4: ...s ON or OFF WARNING Do not use external power supplies with less than 13V if NiCds are already installed in the unit This could discharge the batteries to the voltage supplied It is recommended to charge the batteries with the power OFF establishing periods according to the hour capacity of the battery C NiCds like all batteries self discharge They have a relatively high self discharge rate of abo...

Page 5: ... different operation modes are selected The unit can measure on any frequency from 1 8 to 170 MHz The frequency counter will display a maximum of five digits This will produce an entirely stable reading of the internal oscillator short term drifted signal SWR mode The unit enters the SWR mode when it is turned ON The MFJ 249B will measure the standing wave ratio SWR of any load referenced to 50 oh...

Page 6: ...greater than about 650 ohms the message Z 650 is displayed The SWR value for the connected load will be displayed in the LCD The stray output capacitance around 3pF will be lower than 650 ohms at frequencies higher than 60 MHz This stray capacitance does not affect high frequency measurements considering the connection of a transmission line Frequency Counter Mode To use the frequency counter pres...

Page 7: ...h Verticals Verticals are usually unbalanced antennas Most antenna manufacturers down play the importance of good radial systems with grounded verticals If you have a good ground system the SWR of a quarter wave vertical can be nearly 2 to one The SWR generally gets BETTER as the ground system and performance get worse Verticals are tuned like dipoles lengthening the element moves the frequency lo...

Page 8: ...h a 50Ω noninductive resistor in series to the center conductor of the ANTENNA connector with a coaxial line The shield should be grounded to the connector shell For two wire lines the 50Ω resistor connects in series between the ground shell of the PL 259 and one conductor The other conductor of the balanced line connects directly to the center pin of the connector Coaxial lines can lay in a pile ...

Page 9: ...lated in number 5 Example 27 feet physical length divided by 33 7 feet calculated length equals 80 The velocity factor is 80 or 80 Free space 1 4 wavelength Low SWR frequency Velocity Factor Actual feedline length Free space 1 4 wavelength 246 Impedance of Transmission Lines The impedance of transmission lines between 15 and 150 ohms can be measured with the MFJ 249B a 250 ohm potentiometer and an...

Page 10: ...pacitance is unchanged A non inductive resistor that equals the approximate driving impedance of the tube is installed between the cathode of the tube and the chassis or a resistor should be connected between the anode and the chassis that equals the calculated plate impedance of the tube The appropriate network can now be adjusted The antenna relay if internal can be engaged with a small power su...

Page 11: ...hree positions A well designed voltage balun should show a low SWR over the entire operating range when the clip lead is in position B It will show a poor SWR when the clip lead is in position A and C A voltage balun should also be tested by disconnecting the outer connections of the two resistors and connecting each resistor in parallel If the balun is operating properly the SWR will be very low ...

Page 12: ...nductor with the highest value standard capacitor in series 2 Connect the series LC circuit to ANTENNA connector with a 50 Ω resistor in series 3 Adjust the tune knob through the bands until you get the lowest SWR If you do not get a low SWR change to the next smaller value standard capacitor and try again Repeat the process until you get low SWR 4 Solve this equation using F as the resonant frequ...

Page 13: ...es usually have frequencies where the distributed capacitance and inductance form a low impedance series resonance The troublesome series resonance can be detected by slowly sweeping the frequency of the MFJ 249B over the operating range of the choke Peaks in the voltage measured by the RF voltmeter will identify the low impedance series resonant frequencies 1N34 01 10K Measure the voltag between ...

Page 14: ...l to MFJ Enterprises Inc 300 Industrial Park Road Starkville MS 39759 by Facsimile FAX to 601 323 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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