A Low Pass RF Filter in an Altoids Box

Design and Construction of a Low Pass Filter for the Low Frequency RF Spectrum

by Dr. Carol F. Milazzo, KP4MD (posted 28 August 2011)
E-mail: kp4md@arrl.net

INTRODUCTION

Monitoring non-directional beacons (NDBs) and other transmissions on frequencies below 530 kHz may be difficult in urban locations due to the overloading of the receiver front end by nearby powerful AM broadcast transmitters in the 530-1700 kHz medium frequency band.  I reside in the Sacramento, California area, within 15 miles of 50,000 watt broadcast stations on 1140 kHz (KHTK) and 1530 kHz (KFBK), and several stations operating at 25,000 watts and lower levels with high field strengths due to their close proximity1 (see Table 1). These all produce strong images and intermodulation products that have interfered with the reception of signals below 540 kHz.


This article describes the construction of a low cost low pass radio frequency filter that was designed to reduce interference from AM broadcast stations.  It is suitable for use with long wave receivers and low power LowFER transmitters.  This construction technique may be adapted for filters on other frequency ranges.

CONTENTS
  • Introduction
  • Design
  • Construction
  • Operation
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  • DESIGNING THE FILTER

    In 1984 I had written, published and explained two BASIC language computer programs2,3 for the design of Butterworth and Chebyshev filters.  Since then numerous filter design programs and calculators have become available on the internet.  From these, I selected the online LC Filter Design calculator by Tony Fisher of the University of York.4  I specified a 5-pole low pass Chebyshev filter with 0.01 dB ripple, 3 dB attenuation at 510 kHz and 50 ohms impedance.

    The calculator yielded the circuit in Figure 1 with component values of 4720 pF, 20 μH and 9840 pF.  I would substitute standard value 4700 pF and 0.01 μF ceramic disc capacitors.  The two halves of the filter would be connected with a feed through capacitor through a separating metal shield to further isolate the input from the output.  As I only had a 1000 pF feed through capacitor, I would connect the 0.01 μF disc capacitor in parallel with it.

    Low Pass Filter Circuit Diagram
    Figure 1.  Filter Schematic Diagram

    BUILDING THE FILTER

    The filter was built inside a 20 mm x 58 mm x 93 mm disposable steel Altoids mints box (Figure 4).  This box had appropriate dimensions, was easily machined and had easily soldered surfaces.  Northrup has also constructed various projects with these boxes.5  Figure 2 shows the filter  construction.  Holes were drilled for the single hole BNC female chassis mount connectors and the central metal shield was cut with metal shears from a spare Altoids box, measured, formed and drilled for the feed through capacitor. The shield flanges were sanded to remove the external paint and then easily soldered into the box.

    The online calculator at http://toroids.info/6 was used to determine the number of windings that were required for the inductors.  Using some FT37-43 toroid cores that were available at a local Fry's Electronics store, it was found that 8 turns of wire would give 22 μH inductance.  These were wound with some 22 AWG enameled wire that was on hand and soldered from the center pin of each BNC connector to one end of the feed through capacitor.  The ceramic disc capacitor leads were trimmed and soldered to their respective connections.  Good solder joints were easily made to the box without much heating.


    Low Pass Filter Construction
    Figure 2. Filter Construction

    FILTER OPERATION

    The red line in Figure 3 shows the predicted attenuation in decibels over the frequency range from 0 Hz to 2 MHz.  It shows no filter attenuation from 0 Hz through 500 kHz and attenuation increasing to nearly -60 dB at 2 MHz.  The attenuation should be nearly -35 dB at 1140 kHz and -50 dB at 1530 kHz.  With the filter placed in line between the receiver and antenna, there was no perceptible decrease in signal strengths below 500 kHz.  Table 1 compares the strengths of the received signals as they were measured using a FlexRadio FLEX-3000 with and without the use of the low pass filter.  The measured attenuations approximated the values that the model predicted.

    For comparison purposes, in the near future I intend to build a 7-pole low-pass filter similar to the designs of Button7, Gale8,9 and Tucker10 but using the above described construction technique with isolating shields and feed through capacitors. The use of feed-through capacitor coupling and shielding between filter sections should increase high frequency attenuation.

    Filters for low frequency reception are also available commercially 11,12,13,14.

    Table 1.  Measured Signal Strength vs. Frequency
    Frequency
    kHz
    Callsign Power
    Watts1
    No Filter
    5-Pole
    Low Pass Filter
    dBm
    S units
    dBm
    S units
    Attn. dB
    650
    KSTE
    21,400
    -22
    9+51
    -20
    9+53
    +2
    710
    KFIA
    25,000
    -20
    9+53
    -17
    9+56
    +3
    990
    KATD
    10,000
    -36
    9+37
    -63
    9+10
    -27
    1110
    KLIB
    5,000
    -32
    9+41
    -66
    9+7
    -34
    1140
    KHTK
    50.000
    -35
    9+38
    -71
    9+2
    -36
    1210
    KEBR
    5,000
    -26
    9+47
    -64
    9+9
    -38
    1320
    KCTC
    5,000
    -8
    9+65
    -51
    9+22
    -43
    1470
    KIID
    5,000
    -46
    9+27
    -86
    7
    -40
    1530
    KFBK
    50,000
    -22
    9+51
    -73
    9
    -51
    1620
    KSMH
    10,000
    -52
    9+21
    -86
    7
    -34
    1690
    KFSG
    10,000
    -44
    9+29
    -90
    6
    -46
    Filter Attenuation
                  vs. Frequency
    Figure 3.  Model Predicted Attenuation vs. Frequency

    REFERENCES

    1. Broadcast station statistical data from Radio-Locator.com
    2. Butterworth Filter Design, Milazzo, Carol F., KP4MD, QZX, Vol 2:4, March 1984, pp. 3-5
    3. Chebyshev/Butterworth Filter Design, Milazzo, Carol F., KP4MD
    4. LC Filter Design, Fisher, T, University of York
    5. Display of Altoids Projects, Northrup, M, N5ESE
    6. Toroids.info, Dietzer, G, W8DIZ
    7. Receiver Filters for NDB Reception, Button, N
    8. Junk Box NDB Low Pass Filter, Gale, T, VE7BPO
    9. Improved NDB Chebyshev Low Pass Filter, Gale, T, VE7BPO
    10. 500 kHz Low-Pass Filter Enhances NDB Reception, Tucker, R, K5TD
    11. Z10020 Medium Wave (AM Broadcast) Band Reject Filter, Clifton Laboratories
    12. 100, 200, 400, 500 kHz Low Pass Filters, Kiwa Electronics
    13. LF Filters, Par Electronics
    14. RF Low Pass Filters, Rescue Electronics
    Low Pass Filter in an
                  Altoids Box Figure 4. Low Pass RF Filter MiniVNA Pro scanned Low Pass Filter response
                  curves from 0.1 to 2 MHz
    Figure 5.  Attenuation vs. Frequency Measured with VNA
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