my coil-loaded vertical antenna experiment:

[last updated: 2024-02-14]
ham radio home page
vertical antenna with loading coil

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• I bought this from Amazon ($20.14 ordered 9/24/23 delivered 11/03/23):
  "18.4 ft telescoping antenna" with 3/8-24 base mount - However...
  may have mis-read it, as when delivered it's actually M10-1.5
        As my adapter has a 3/8-24 thread, I need a coupler/reducer to go between them.
        In the end I got a 3/8-24 coupler and tapped out one end to M10-1.5. Works great.

  
  18.4 ft antenna on Amazon
  This calculates to resonate (full-wave) at 53.5 MHz (6m) without a loading coil,
  or 13.5 (20+m) at 1/4-wave.

• Calculation parameters:
  for: (link to): Coil-shortened Vertical
  • antenna height: 18.4 ft
  • distance from base to coil = 0 (base-loaded)
  • conductor diam = 0.102" (coil wire diam for #10 AWG)
  • Desired freqs and calculated uH:
    40m         7.000 MHz         8.5 uH
    40m         7.300 MHz         7.5 uH
    30m         10.100 MHz         2.2 uH
    30m         10.150 MHz         2.1 uH
        note that, without a coil, the 18.4 ft antenna is tuned (1/4-wave) for 13.500 MHz
            meaning that, to use it on 20m 14-14.35 MHz freqs, not only will the loading coil need to be completely bypassed/shorted-out,
            but in addition the telescoping antenna itself will need to be shortened
            by sliding one of the telescoping sections "in".

    20m         14.000 MHz
    20m         14.350 MHz

  • In summary:
    To use this antenna on the frequencies I want,
    a coil will be needed with total inductance of 8.5 uH,
    with taps at 7.5, 2.2, and 2.1 uH.

    
    OTOH to give some slop for inaccurate calculations, I'll start with an overall inductance of 10 uH.

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• Coil Design for this antenna:
  • using #10 wire
    with a 1.5" diam coil form,
    gives total coil diam = 1.6"

    inductance	numTurns	coilLength
    10	34.6	6.95
    8.5	29.8	5.97
    7.5	26.6	5.32
    2.2	9.3	1.79
    2.1	8.9	1.72

  • Note: Values in this table were calculated from:
    (link to:) calculate coil dimensions