This question is in regards to high power transmission at 13.56 MHz.

I kind of understand the concepts of what an electrically short antenna is as well as how radiation resistance affects radiative efficiency (increased radiation resistance means increased efficiency.)

I've run into a situation where, adding one inch to the straight leads of a helical antenna has seemingly greatly reduced performance despite maintaining the same coil turn number, turn pitch, and circumference. The circumference of the antenna is such that it is ≪0.01λ, and the pitch is even smaller. The antenna is surrounded by a ground plane which also much smaller in diameter than the wavelength. If the radiation resistance equation for a small loop area with circumference ≪ λ/3 from Wikipedia page for radiation resistance is to be believed, the radiation resistance is R≪1. The equation for this is $$R_{rad}=320\pi^4\left(\frac{N\,A}{\lambda^2}\right)^2.$$

Why would simply adding λ/871 to the leads make such a difference?

I should add that I'm trying to couple power to something inside the coil.

  • 1
    \$\begingroup\$ Took me a while to realize "1 in" means "an inch"; that's a very useless measure, not because you used freedom units instead of proper SI units that people outside a single country can also work with, but because 1 inch might be much or very little relative to wavelength, and the rest of your antenna. Can you tell replace the "1 in" by something like "1/100 radius" or similar? \$\endgroup\$ Feb 28, 2023 at 16:40
  • \$\begingroup\$ @MarcusMüller freedom units is a funny term ... it's more shoot yourself in the foot units \$\endgroup\$
    – jsotola
    Feb 28, 2023 at 17:10
  • \$\begingroup\$ @jsotola you mean lose a mars sonde units? \$\endgroup\$ Feb 28, 2023 at 17:17
  • 2
    \$\begingroup\$ Please show a picture of the antenna/coil with dimensions. \$\endgroup\$
    – Andy aka
    Feb 28, 2023 at 18:21
  • \$\begingroup\$ @MarcusMüller I have changed "1 inch" to the appropriate fraction of a wavelength at 13.56 MHz. \$\endgroup\$
    – Ender
    Mar 14, 2023 at 19:39

1 Answer 1


Using an inductance calculator here, https://www.allaboutcircuits.com/tools/wire-self-inductance-calculator/

I estimate it should not be a problem with 40 nH at 13.6MHz from 5 cm of wire. However distorting the coil geometry by > 0.1% may impact this. You can see this with light stress on the antenna using a network analyzer looking at s11. If you do not have one then use a splitter in reverse and rectify the reflection for a DVM reading using a hot-carrier or a Schottky diode.

I am surprised it is that small. It would be helpful to show your good test results without the added wire as small Helix antennae are not recommended for this low frequency.

For RFID’s and any magnetic sensing or power transfer, it is best for the coil diameter to exceed the distance to target, otherwise it attenuates like a point source with quadratic losses.

Rigidity is crucial in high gain helix antenna.



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