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How to use MFJ Antenna analyzer

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA2SQQ, Feb 7, 2011.

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  1. WA2SQQ

    WA2SQQ XML Subscriber QRZ Page

    I'm wondering if:
    1. Anyone know how to use the antenna analyzer to measure the resistance (at 1.8 mhz) of a choke balun I constructed?

    1. Anyone know of any sites that discuss how to use the analyzer? I've already looked at MFJ's posts on YouTube - these are great, but I'm sure hams have posted their own tips.
  2. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Which analyzer do you have? MFJ has made at least six different models.

    Resistance?? Do you mean impedance?
  3. AC0FP

    AC0FP Ham Member QRZ Page

    I used a network analyzer to measure a ferrite bead choke on a 1' piece of coax that I made.

    Resistance measured zero.

    Impedance measured ~ 1000 ohms from 3 to 28 MHz peaking around 10k ohms at 14 MHz.

  4. NA7U

    NA7U Ham Member QRZ Page

  5. VK1OD

    VK1OD Ham Member QRZ Page

    As noted by someone else, your objective is to measure the impedance, it is a complex quantity with resistance and reactance.

    Join both wires at the input and at the output of the balun, and connect them between the analyser's ground connection and centre pin. There should be nothing else connected to the balun.

    Now put in in impedance measuring mode, and write down / plot the impedance components over a range of frequencies. It is most likely that the reactance component at the low frequency is positive, but at much higher frequencies, it will probably become negative.

    The impedance of a good common mode choke may be higher than you can measure with the MFJ259B for example.

  6. KM1H

    KM1H Subscriber QRZ Page

    If those are Fair-Rite ferrites then go to their web site and look up the impedance curves over the frequencies you want. Im not aware of any MFJ instrument that will be satisfactory.

  7. W5DXP

    W5DXP Ham Member QRZ Page

    The MFJ-259B is limited to measuring impedances less than ~650 ohms. If your choke balun is a 1:1, just short the input wires together and the output wires together, and measure the impedance. If it is less than 650 ohms, the MFJ-259B should yield a reasonable result. Hopefully, the impedance will be too high for the MFJ-259B to measure.:)
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    Do you mean that the DC resistance measured zero? It would be very unusual if a choke had zero resistance from 3 to 28MHz.

    Steve G3TXQ
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Unless I'm mistaken, all you'll find of relevance on the Fair-Rite web site is curves of μ' and μ'' vs frequency. You need more information than that to predict the choking impedance.

    Steve G3TXQ
  10. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    If the Z is above the MFJ's range, you can extend the measurement range by using other known passive devices such as a resistor across the terminals in parallel with the unkown part under test. With the impedance now within range of the MFJ unit, you can mathematically remove the known resistance revealing the higher unknown impedance of the part in question. My homebrewed impedance measuring instruments and some of my ancient commercial instruments are limited in the same way so if I can do it, anyone can:) Plus it is a good exercise in learning AC circuit theory:) For example, my old noise bridge (applicable for a Palomar as well) can only reach an Rp (parallel equivalent resistance) of 250 Ohms so I would place a 330 Ohm low reactance resistance in parallel to the unknown and null the bridge under this condition and then remove the 330 ohms mathematically. If any one would like to work thru an example, let me know and I'll go thru it step by step. The useful formulas here are the series to parallel and parallel to series conversions. I programmed these into my HP calc and have also loaded everything into a spreadsheet. 73, Pete
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