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.CAUTION: The driving impedance of most amplifiers changes as the drive level is varied.Do notattempt to adjust the input network with the tube in an operating condition with the low level of RF fromthe MFJ-269.7.6 Testing RF TransformersRF transformers designed to operate with 10-1000 ohm termination on one of the windings can be tested with theMFJ-269.The 10 to 1000 ohm winding is connected through very short (less than one electrical degree long) leads to the"ANTENNA" connector on the MFJ-269.The other winding(s) of the transformer is terminated with a lowinductance resistor equal to the desired load impedance.The MFJ-269 can then be swept through the desiredtransformer frequency range.The impedance and bandwidth of the RF transformer can be measured.Transformer efficiency can be measured by comparing the source voltage from the MFJ-269 to the load voltage,and using standard power level conversions.A second method is to NOT terminate the transformer and measurethe winding at it s design operating impedance in Advanced 2 s Coax Loss mode.Set the analyzer at the windingoperating Zo value.Approximate loss can be measured using the same method as measuring a transmission line.7.7 Testing BalunsBaluns can be tested by connecting the 50 ohm unbalanced side to the MFJ-269 "ANTENNA" connector.Thebalun must be terminated with two equal value load resistors in series.The resistor combination must have totalresistance equal to balun load impedance.For example, a pair of 100 ohm carbon resistors are required toproperly test the 200 ohm secondary of a 4:1 balun (50 ohm input).Measure SWR while moving a jumper wire from point "A" through point "C".Voltage and current balun test:38MFJ-269 Instruction Manual HF/VHF/UHF SWR AnalyzerTo the MFJ-259B'sA"ANTENNA"BALUNBconnectorCClip LeadA properly designed current balun is the type most effective for maintaining current balance.It has the highestpower capability and lowest loss for given materials.It should show a low SWR over the entire operating rangeof the balun with the clip lead in any of the three positions.A well designed voltage balun should show a low SWR over the entire operating range when the clip lead is inposition "B".That SWR should not change when the clip lead is removed.It will show a very poor SWR whenthe clip lead is in position "A" and "C".SWR should be about the same in either position  A or  C.If the balundoes not follow these rules, the balun has poor balance and is of questionable benefit.A 4:1 voltage balun should also be tested by disconnecting the outer connections of the two resistors andconnecting each resistor in parallel.If the voltage balun is operating properly the SWR will be very low with theresistors connected from either output terminal to ground.Voltage balun test only:To the MFJ-259B's"ANTENNA"BALUNconnector7.8 Testing RF ChokesLarge RF chokes usually have frequencies where the distributed capacitance and inductance form a lowimpedance  series-resonance.This series resonance occurs because the choke acts like a series of back-to-backL networks.This causes three problems:First, impedance from end to end in the choke becomes very low.Second, the voltage at the center of the resonant point becomes very high, often causing severe arcing.Third, the current in the winding becomes very high, often resulting in severe heating.Troublesome series-resonances can be detected by installing the choke in the operating location, and connectingonly the MFJ-269 from end-to-end of the choke through a short 50 ohm jumper cable.By slowly sweeping theoperating frequency range of choke, dips in impedance identify low impedance series-resonant frequencies.By moving a small insulated screwdriver s blade close to and along the choke, you will find a point where theseries-resonant impedance suddenly changes.This is the area that has the highest voltage and also the area whereadding or subtracting a tiny amount of capacitance has the largest effect.By removing turns to reducecapacitance or adding a small capacitive stub at this point, resonance can be shifted out of the desired frequencyrange.39MFJ-269 Instruction Manual HF/VHF/UHF SWR AnalyzerA small change in stray capacitance has a much larger effect than a small change in turns, because the ratio of Lto C is so high.It is often possible to move the series-resonance a large amount without greatly affecting theoverall inductance.8.0 TECHNICAL ASSISTANCEIf you have any problem with this unit first check the appropriate section of this manual.If the manual does notreference your problem or your problem is not solved by reading the manual, you may call MFJ TechnicalService at 601-323-0549 or the MFJ Factory at 601-323-5869.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 FAX to 601-323-6551; or by e-mail to mfj@mfjenterprises.com.Send a complete description of yourproblem, an explanation of exactly how you are using your unit, and a complete description of your station.40 [ Pobierz całość w formacie PDF ]