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MFJ-204B 1.8 - 30 MHz Antenna Bridge Modifications

Edited on January 1st, 1970, 00:00 UT.

Articles

Preface

Introduction

Wouldn't it be nice if you could use that neat looking MFJ-204B that's been sitting in your drawer for years to read capacitor and inductor values? Well, you can but maybe it's not obvious how to do so. The MFJ-204B Owner's Manual sure doesn't offer much help in how to use the device. If you glance through the schematic in the MFJ-204B owner's manual you'll notice it is essentially a solid state grid dip meter. Essentially, the functions of this device are to read reactance, impedance, and resistance. When measuring an antenna you are measuring a combination of all three. However, the MFJ-204B is just as capable of reading them individually.

But, even if you know how to use a grid dip meter you'l need to memorize a complex formula to calculate the values and it will probably require a calculator or slide rule. Sure, you could buy one of those $400 antenna analyzers which does the calculations for you. But, why not just glue premade charts to your MFJ-204B? They are readily available and not very large. Below I will explain exactly how to do this and provide some premade charts sized to fit the MFJ-204B case.

These charts are figures 2-44 and 2-45 from page 45 of The Radio Amateur's Handbook 39th Edition, Copyright © 1962 ARRL, Inc. I do hope they don't mind me providing this copy. If they do mind then they can let me know and I'll replace it with some other charts more suited to the MFJ-204B frequency ranges.

How to read the value of Capacitors and Inductors with the MFJ-204B

  1. Press the Power Switch to turn on your MFJ-204B.
  2. Rotate the Resistance dial all the way counter-clockwise until it stops near the A position.
  3. Connect your capacitor or inductor across the Antenna connector on top of the unit. I reccomend making a short coax jumper with alligator clips on the shield and center conductor.
  4. Adjust the Band Select switch and Tune dial until the meter at the bottom dips down to 50 Ohms.
  5. Now read the frequency indicated by the Tune dial. I reccomend buying a cheap 0-1000 MHz frequency counter and connecting it to the Freq. Out connection on top of the unit. I'm using an old Radio Shack 22-306 frequency counter.
  6. Cross Reference the diagonal Frequency line to the vertical 50 Ohms line on the respective capacitance or inductance chart I have provided.
  7. The intersection of the Tune dial (or Freq. Counter) Frequency and 50 Ohms will correspond to the Capacitor or Inductor value along the Horizontal axis.

How to read the value of Resistors with the MFJ-204B

  1. Press the Power Switch to turn on your MFJ-204B.
  2. Rotate the Resistance dial all the way counter-clockwise until it stops near the A position.
  3. Connect your Resistor across the Antenna connector on top of the unit. I reccomend making a short coax jumper with alligator clips on the shield and center conductor.
  4. Slowly rotate the Resistance dial until the meter at the bottom reads 0.
  5. Compare the Resistance dial reading with your Resistance Calibration Chart on the rear or side of your MFJ-204B.
  6. Your mystery resistor will be very close to the value indicated on your Resistance Calibration Chart.

Relabeling The MFJ-204B for capacitance and inductance readings

Parts
Avery 8255 Full Sheet Labels for Color Inkjet
Tools
Isopropyl Alcohol (or Denatured Alcohol if you have it)
Paper Towel
pencil (do not use a pen nor marker!)
Sharp Hobby Knife
Computer, inkjet printer, and software to print Acrobat PDF (i.e. Acrobat Reader)

Printing the new Labels

  1. Load one or more sheets of the Avery 8255 full sheet label paper into your inkjet printer.
  2. Open mfj-204b-printable-charts.pdf with Acrobat Reader.
  3. Select Print from the toolbar or File menu.
  4. Carefully set aside the printed label sheet(s). Allow your printed label sheet(s) to dry for about 10 minutes.

If you already have a Resitance Calibration Chart

  1. Please write down your calibrated values for A, B, C, D, E, F, and G positions. We will need them later.
  2. My resistance readings follow however your unit may be calibrated differently:
    1. 0 ohms
    2. 10 ohms
    3. 32 ohms
    4. 72 ohms
    5. 203 ohms
    6. 1000 ohms
    7. infinity ohms

Cleaning the Surfaces

  1. Dab a little alcohol on your paper towel as needed throughout the process.
  2. Thoroughly wipe down the back of the unit. Wipe over the existing calibration chart if you have one. You will be covering up. Your capacitance and inductance charts will be affixed here.
  3. Thoroughly wipe down the left or right hand side of the unit. This is where you will affix your new resistor calibration chart. I suggest placing it out of the way of your fingers.
  4. Your case should now look shiny where you have cleaned it with the alcohol.

Trimming the Labels

  1. You will have 6 sets of cross reference charts. Use your Hobby Knife to cut out one pair of Capacitance/Reactance charts. Your charts should be on one label like Figure 1.
  2. You will have 4 resistor calibration charts. Use your Hobby Knife to cut out one of the Resistor Calibration charts.

Affixing the Charts to the MFJ-204B

  1. Carefully line up the Capacitance/Inductance sheet so it fully covers the existing resistance calibration chart. You want it to touch metal on all four sides so it won't peel away. Press it down firmly stroking from the center outward to remove any air pockets.
  2. Carefully line up your new Resistor Calibration Chart on the side you cleaned with alcohol. Press it down firmly the same as you did with the charts.

Write in your Resistor Calibration Values

  1. Earlier I told you to write down your existing calibration values. Now you should transfer those values to the new resistor calibration label using a pencil (no markers or pens as you may need to erase this if you recalibrate).
  2. I have added some extra markings for common impedance values of 50, 72, 75, 90, 300, 450, 600, 1000 ohms. You may want to mark the Resistance dial readings for each of those values.

Filling in the extra Resistance Values (aka How to Read the Value of a Resistor with the MFJ-204B)

  1. Gather some 1/4Watt resistors of values close to those marked on the chart. You can buy a resistor assortment pack (Cat # 271-309) from Radio Shack for about $5 USD if you have no other source.
  2. Connect two 100 ohm resistors in parallel (a 47 ohm, or a 50 ohm dummy load) and rotate the Resistance dial until the meter at the bottom reads 0. Write the dial reading on the chart next to 50 ohm.
  3. Connect two 150 ohm resistors in parallel (or a 75 ohm dummy load) and rotate the Resistance dial until the meter at the bottom reads 0. Write the dial reading on the chart next to 72 ohm and 75 ohm unless you have a 72 ohm resistor and want to be picky.
  4. Connect two 47 ohm resistors in series (or a 90 ohm or 100 ohm resistor) and rotate the Resistance dial until the meter at the bottom reads 0. Write the dial reading on the chart next to 90 ohm. It's close enough. Guesstimate the dial marking downward ever so slightly if you have no 90 ohm resistor.
  5. Repeat the process for the remaining 450 ohm (use 470 ohm), 600 ohm (two 1k in parallel + one 100 in series), and 1000 ohms.
  6. My Resistance Chart looks like this: