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I wanted to know is it possible to create a loop antenna that functions in a way like a high frequency helmholtz coil. Because a helmholtz coil is usually made with multiple turns of wire having high inductance, but for high frequency much lover inductance is optimal. The aim is to create a homogeneous axially symmetric B field inside the antenna loop which would be the antenna near field. From what I know a loop antenna radiates due to B field perpendicular to it's plane. So it functions as a single loop coil. I was wondering is it possible to make the loop physically larger than the wavelength of current that feeds it and still retain the homogeneous B field at the middle? Such antenna would be electrically long. Say a 50cm diameter loop driven at 2.4 Ghz, which has a wavelength of roughly 12cm.

Would I also be able to parallel many such loops and drive them together to double, tripe of multiply the total magnetic field strength in the middle?

The main objective is to create a homogeneous and symmetric B field within a region enclosed by said loop.

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  • \$\begingroup\$ No, it's not possible. Once the circumference of the loop exceeds an appreciable fraction of the wavelength, the RF current at various points along the loop will not be uniform. In your example the loop is 4 wavelengths in diameter and over 12 wavelengths in circumference. There will approximately 6 points where the current is completely opposite in sign to 6 other points. The B field will vary correspondingly and it will not be axially symmetric. \$\endgroup\$
    – Mark Leavitt
    Commented Jul 11 at 23:36
  • \$\begingroup\$ @MarkLeavitt Would it -in principle-help If I split up the loop into segments with capacitors in between? That is practice in MRI coils, but maybe only to equalize local e fields along the necessary capacitance. \$\endgroup\$
    – Carsten
    Commented Jul 13 at 9:45

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Putting these loop coils in parallel is equivalent to adding more windings. You could put them some distance apart but that would reduce the constructive interference. Maybe there is a sweet spot.

Adding more windings seems beneficial at first glance (after all B ~ N*I for a solenoid), but

  1. long wires lead to phase shifts along the wire which brings destructive interference

  2. Adding windings adds resistance, which reduces the current. My guess: For low frequency, this is still beneficial, but for high frequencies, proximity effects spoil the show.

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