MFJ-202B RX Noise BridgeAntenna System Measurements with the MFJ-202B RX Noise Bridge

RX Noise Bridge Calibration and Applications in Measurement of an Antenna and a Zip Cord Transmission Line

by Dr. Carol F. Milazzo, KP4MD (posted MAR 2011)
E-mail: kp4md@arrl.net
 

INTRODUCTION

In the 1970's and 80's, antenna noise bridges, also known as RX noise bridges such as the Heathkit HD-1422, Palomar Engineers R-X and the MFJ-202 became commercially available to the amateur radio community.  These devices are used with an external receiver to measure unknown impedances and consist of a broadband noise generator and a bridge circuit with a calibrated variable capacitor and resistor.  These noise bridges can directly determine the resonant (zero reactance) frequency of an antenna or other unknown, but determining other impedance values often requires complex calculations.  As microprocessor technology has made possible digital antenna analyzers and vector network analyzers that perform these calculations, RX noise bridges are now commonly found for sale at reduced prices.  These notes describe the use of spreadsheet utility for calibration and measurements of an antenna and a zip cord transmission line with an RX noise bridge.
CONTENTS
  • Introduction
  • Calibration
  • Transmission lines
  • Impedance calculations
  • References
  • Return to KP4MD home page
  • In operation, a receiver is connected through a 50 ohm coaxial cable to the "RECEIVER" connector on the noise bridge, tuned to the desired frequency and set to the AM mode..  If the radio is a transceiver, the transmitter should be disabled by reducing the power level to zero to prevent accidental transmission that could damage the noise bridge.   The unknown impedance is connected to the "UNKNOWN" connector and the resistance and reactance controls are adjusted for a null (minimum noise) in the receiver.  The receiver's automatic gain control (AGC) should be set to the "slow" or "off" position because the AGC action may render the null difficult to detect.  If a null still cannot be detected, one may also need to activate the attenuator and decrease the RF gain of the receiver.

    CALIBRATING THE NOISE BRIDGE

    Table 1 lists the manufacturer's specification for impedance measurement range for an MFJ-202B noise bridge1 that I recently purchased.


    Range expander off Range expander on
    Frequency
    1 MHz
    100 MHz
    1 MHz 4 MHz
    6-100 MHz
    R ohms
    0-250
     0-2000
    0-2200
    0-3800
    XL ohms
    0-500
    0-5
    0-360
    0-1800
    0-1900
    XC ohms
    0-15,000
    0-150
    Table 1. MFJ-202B RX Noise Bridge Specifications

    The manual states that the MFJ-202B is not a precision grade laboratory instrument, but its accuracy may be improved by calibrating the dials.  The schematic diagram shows the controls are a 250 ohm variable resistor and a 10-300 pF variable capacitor.  The resistance dial is marked from A through G, and the reactance dial is marked in pF from 150 XL, through zero, to 150 XC.  The range expander switch inserts a 200 ohm resistor in parallel with the unknown impedance to be measured.

    To zero the reactance dial, the receiver was set to 2 MHz and a short piece of wire was connected directly between the center conductor and the ground threads on the "UNKNOWN" connector.  With the range expander switch in the "Normal" position, both resistance and reactance controls were adjusted for a null in the receiver noise.  The resistance control was near "A" or "zero" ohms.  If the reactance knob did not read "zero", the knob set screw was loosened and reset to the "zero" mark.  Grebenkemper2 described a case in which the "zero" reactance setting varied with frequency due to bridge imbalance and a method to remedy this and to accurately calibrate the reactance scale.

    To calibrate the resistance dial, a few non-inductive (carbon composition or metal film) resistors of  known value throughout the range from 0 to 250 ohms were used.  Alternatively, a small potentiometer may be used.  A digital multimeter was used to accurately measure the values of the resistors.  The resistance knob set screw was loosened and reset to the "A" mark at the limit of counterclockwise rotation.  With the expander switch in the "Normal" position and the reactance control at "zero", each resistor was connected using the shortest possible lead length between the center conductor and the ground threads on the "UNKNOWN" connector and the corresponding resistance knob position was recorded for each null in the receiver noise.  The resistance dial readings were converted from the alphabetical  "A" through "G" to a corresponding numeric scale "0" through "6".  These measurements listed in Table 2 and plotted graphically in Figure 1.

    Resistance
    Dial
    Ohms
    0
    2
    1
    27
    2.2
    72
    2.9
    100
    4.2
    140
    4.9
    180
    5.6
    210
    Graphic plot of
                    resistance dial readings for known resistances
    Table 2. MFJ-202B resistance dial 
    readings for known resistances
    Figure 1.  Graphic plot of resistance 
    dial readings for known resistances

    This data was entered into a spreadsheet (Figure 2) to perform a linear regression analysis.3  Download Microsoft Excel spreadsheet.  This determined the relationship between the unknown resistance and dial scale to be:

    R(ohms) = R(dial) × 37.29 - 6.3 and R(dial) = (R(ohms) - 6.3) / 37.29


    Figure 2.  Resistance calibration linear regression spreadsheet



    TRANSMISSION LINE MEASUREMENTS

    In his article, Grebenkemper2 described various R-X noise bridge applications in various measurements of antenna and transmission line characteristics.  Banz4, Duffy5, Jurgens6, Maguire7, Schmidt8, and Straw9 have published computer programs and De Stefano10 has published a series of video tutorials for these applications.  Figure 4 shows the spreadsheet used to calculate the characteristics of a 36 foot (11 m) length of Pfanstiehl 18-gauge AS-18/50Z PVC speaker wire used as transmission line.11  Hall12, Parmley13, and Wiesen14 have previously discussed and characterized the use of other zip cords as transmission lines.

    To determine its velocity factor (VF), a 78.25 inch (1.987 m) random length of the speaker wire was attached to the UNKNOWN connector of the noise bridge.  The frequency at which the line was a quarter wavelength long (zero reactance) was then determined for when the conductors far end were open and when they were shorted together, and the mean of the two frequencies was calculated (24.99 MHz).  The velocity factor (VF) at that frequency was then calculated as the ratio of the length of the line to a free space quarter wavelength, or 0.66.  The dielectric constant (k) is the reciprocal of the square of the velocity factor, or 2.3.


    VF = 4 × λ/4 length(m) × fMHz/299.79
    and k = 1/(VF2)
    Pfanstiehl AS-18/50Z Speaker Wire
    Fig. 3. Pfanstiehl 18-gauge AS-18/50Z Speaker Wire

    The characteristic impedance (Z0) of this random length of speaker wire transmission line was then determined by measuring its impedances with the far ends open and shorted, and taking the square root of the product of those numbers.  It was found that these impedances fell within the range of the noise bridge at 5 MHz and 7 MHz.  The characteristic impedance was calculated as 114.4 ohms at both these frequencies. In the particular case of a parallel transmission line, this result was confirmed using the formula for calculating Z0 from the physical dimensions of the parallel transmission line.  Substituting 114.4 ohms for Z0 and the 0.512 mm radius (r) of the 18 AWG conductors, gave a center to center conductor separation distance (D) of 2.2 mm which was confirmed by physical measurement of the wire.

    Z0 = √(Zoc × Zsc) and  Z0 = 276/√k × log(D/r), D = 10((Z0×√k)/276+log(r))

    The electrical length of the full 36 foot speaker wire transmission line used with the 40 meter horizontal loop antenna15 was determined by disconnecting it from the antenna and shorting those ends together and attaching the noise bridge to the station end of the feed line.  The feed line was determined to be 0.5, 1 and 1.5 wavelength long at 9, 18.25 and 27.7 MHz, the first 3 frequencies at which the reactance was zero and the resistance near zero.  These frequencies were observed not to be exact multiples, reflecting the velocity factor variation with frequency.  Using the previously calculated velocity factor of 0.66 at 24.99 MHz, the physical length line of this feed line (one electrical wavelength long at 27.7 MHz) corresponded to 35.3 feet (10.8 m).  The feed line loss at 27.7 MHz αl(f0) was calculated as 8.69 times the ratio of the measured resistance Ri to R0, the resistive part of the characteristic impedance Z0.  The feed line loss at other frequencies αl(f) was estimated by multiplying αl(f0) by the square root of the ratio of the frequencies. 

    λ length(m) = VF × 299.79 /fMHz

    αl = 8.69 × Ri/R0 and αl(f) = αl(f0) × √(f/f0)


    Figure 4.  Transmission lines calculations spreadsheet


    IMPEDANCE CALCULATIONS

    Using the resistance calibration data, the spreadsheet in Figure 5 was designed to calculate unknown resistance, reactance and standing wave ratios from readings on the noise bridge dials.  As an example, these are the data measured through the FT140-43 common mode choke used in the parallel line 4:1 balun listed in Table 3 of Zip Cord Transmission Lines and Baluns.11
     
    Figure 5.  Noise bridge impedance calculations spreadsheet


    The spreadsheet in Figure 6 shows impedance and SWR measured at the source through 10 turns of the AS-18/50Z speaker wire wound on an FT114-43 ferrite toroid core common mode choke used as a 1:1 current balun, the 35.3 feet of speaker wire feed line to the 40 meter horizontal loop antenna15 and the calculated impedances at the load using the transmission line equations and assuming a transmission line with no attenuation and a constant characteristic impedance and velocity factor over the frequency range.  Figures 7 through 12 are graphic plots of these data.
     
    Figure 6.  Noise bridge impedance at load calculations spreadsheet


    Impedance at source
    Impedance at load
    Figure 7.  Measured Impedance at Source
    Figure 8.  Calculated Impedance at Load
    Reactance at source
    Reactance at load
    Figure 9.  Measured Reactance at Source
    Figure 10.  Calculated Reactance at Load
    SWR at source
    SWR at load
    Figure 11.  Measured SWR at Source
    Figure 12.  Calculated SWR at Load

    Maguire's program Transmission Line Details7 accounts for factors that influence frequency dependent variations in transmission line characteristics.  Fitting the observed data through the adjusting the parameters (Nominal Z0 = 105, Nominal VF = 0.69, K0 = 0.1171, K1 = 1.61, and K2 = 0.001) produced the relationships represented in Figures 13 through 15. 

    Characteristic impedance Z0
                  vs. frequency
    Velocity factor vs. frequency
    Attenuation vs frequency
    Figure 13.  Characteristic impedance Z0 vs. frequency
    Figure 14.  Velocity factor vs. frequency
    Figure 15.  Attenuation vs. frequency

    Figure 16 shows a Transmission Line Details calculation of the total transmission line loss, antenna impedance and power at the loop antenna end of the transmission line based on impedance read at the transmitter end of the transmission line as 57 ohms resistance and 9.7 ohms inductive reactance at 7 MHz.

    Load calculation with Transmission Line Details
                  program
    Figure 16.  Transmission Line Details program antenna system calculations


    REFERENCES

    1. MFJ-202B RX Noise Bridge, MFJ Electronics
    2. Improving and Using R-X Noise Bridges, Grebenkemper, J, KI6WX, QST, August 1989, pp. 27-32, 52.
    3. Simple Linear Regression, Wikipedia
    4. AA3RL Transmission Line Calculator, Banz, D, AA3RL
    5. Generic noise bridge calculator, Duffy, O, VK1OD
    6. Software for Palomar R-X Noise Bridge, Jurgens, R, KQ6RH
    7. Transmission Line Details, Maguire, D, AC6LA
    8. A Transmission line calculator, Version 0.3, Schmidt, K, W9CF
    9. TLW (Transmission Line for Windows), Straw, D, N6BV
    10. VK3AQZ RX Noise Bridge Video Tutorials, DeStefano, LL, VK3AQZ
    11. Zip Cord Transmission Lines and Baluns, Milazzo, C, KP4MD
    12. Zip Cord Antennas - Do They Work?, Hall, J, K1TD, QST, March 1979, pp. 31-32.
    13. Zip Cord Antennas and Feed Lines For Portable Applications, Parmley, W, KR8L, QST, March 2009, pp. 34-36.
    14. Portable Antenna Notes, Wiesen, R, WD8PNL
    15. Computer Assisted Low Profile Antenna Modeling II, Milazzo, C, KP4MD
    16. The RX Bridge, Nichols, E, KL7AJ

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    Noise Bridge Utilities Microsoft Excel spreadsheet

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