# Why would a very electrically short helical antenna change significantly by adding 1 inch to both leads?

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.

• 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? Feb 28, 2023 at 16:40
• @MarcusMüller freedom units is a funny term ... it's more shoot yourself in the foot units Feb 28, 2023 at 17:10
• @jsotola you mean lose a mars sonde units? Feb 28, 2023 at 17:17
• Please show a picture of the antenna/coil with dimensions. Feb 28, 2023 at 18:21
• @MarcusMüller I have changed "1 inch" to the appropriate fraction of a wavelength at 13.56 MHz. Mar 14, 2023 at 19:39