Antenna loading coil: Everything You Need to Know

An antenna loading coil is an inductor placed in series with an antenna element in order to lower the antenna’s resonant frequency.

A standard dipole antenna is resonant if constructed with a length of one-half wavelength. A vertical antenna (effectively half a dipole operating against a ground plane) is resonant at one-quarter wavelength. Resonance may also be observed at each of the odd multiples.

For most VHF and UHF antennas, a half or quarter wavelength is reasonable in size and can be readily accommodated in all but the smallest handheld transceiver antenna designs.

The same is not true of HF antennas on the radio amateur bands or mobile antennas for 27MHz (11-metre) CB radio operation. Mobile antennas are inherently limited by the amount of available space, yet reducing an antenna’s length increases its resonant frequency.

A loading coil may be used, on its own or in conjunction with a capacity hat, to tune the antenna to resonance at a lower frequency. One coil placed in the center or at the base of a single-band vertical antenna is sufficient, but a single-band dipole will require two coils – one in each element of the dipole.

For multiband HF antennas, different loading coils (or adjustable coils) are needed for coverage of each band.

Design calculators

An antenna with a loading coil cannot be modeled electrically as an element in which the inductor has been replaced by the corresponding length of straight wire.

An inductor will behave differently as current at one end of an inductor creates a magnetic field that immediately induces currents at the opposite end. An inductor also adds electrical resistance to an antenna, incurring a penalty in radiation efficiency compared to a full-size antenna.

There are a number of online calculators that can determine a suitable value and position for inductive loading coils based on an antenna’s dimensions and desired resonant frequency.

Verticals:

  • Antenna Factor of short tuned vertical with concentrated loading, VK1OD

Dipoles:

Various antenna calculation programs are also available for download.

Antenna tuners

An external antenna tuner is often used in conjunction with HF antennas to tune antennas to resonance within a band. In most cases, these are π or T networks in which a variable inductor and capacitors are used to tune for the lowest standing-wave ratio as seen by the transmitter.

An antenna tuner must be rated for the full power of the transmitter and both manual and automatically-tuned versions of these units are commercially available. Some late-model transceivers may integrate this functionality, although for best results any tuner or loading coil needs to be at or near the antenna itself.

External links

4 thoughts on “Antenna loading coil: Everything You Need to Know”

  1. Hello, this is not a comment but a question, I have a old Avanti 11 meter base antenna with a load that i would like to upgrade to a bigger loading coil so it could handle more re RF power, at least 500 or so watts, i am new to this type of radio stuff and don’t know anything like this but i like this antenna my dad gave me before he passed away so i would like to keep around a while if possible so i guess i am asking for help doing this correct, i have seen big and small coil antennas with lines that is used to change the frequency but use the whip or stinger to adjust the SWR’s, i would really appreciate it if you could or knew someone who may be able to help me with this.
    Thank you

    Reply
  2. I am considering making an antenna for either the 2200m or 630m amateur bands for the purpose of operating in WSPR mode (at very low power).
    So, how can I figure out what inductance I would need for a loading coil for such an antenna?

    Reply
  3. Loading coil No 5508A Marconi ZA55879.

    A friend wants to buy one of these from me. This was part of my late husband’s stock. Any idea how much I should charge for it?

    All funds for Mike’s stock go to his favourite charity in his memory.

    Carol

    Reply
    • Watch the sale of the radio equipment carefully. Many have held value for years. A loading coil for an antenna if oyu homebrew (made it yourself) is not very expensive. If you purchase one, depending on band and how well made…commercial ones are about $50. I’ve made my own for less than $15 – not counting my time.

      Reply

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