On Monday, August 5, 2013 9:52:31 PM UTC-5, ve2pid wrote:
A formula and/or a theoretical explanation would be much welcome..!

I don't know exactly what you are asking for but a tuned feeder increases the electrical length of an antenna system because the forward wave and reflected wave have farther to travel, i.e. additional delays are introduced. A stub can accomplish the same function.

Here is an empirical chart that might help. Given the wavelength of a center-fed dipole, this chart indicates the wavelength of ladder-line needed to resonate the system. One can change wavelength to degrees if desired.

http://www.w5dxp.com/majic.gif

For instance, to bring a 0.4WL (144 degree) CF dipole to resonance, 0.12WL (43.2 degrees) of ladder-line is required. To bring a 0.8WL (288 degree) CF dipole to resonance, 0.3WL (108 degrees) of ladder-line is required. As you can see, it is not a linear relationship and resembles a tangent function..

That is a chart from my no-tuner all-HF-band antenna article:

http://www.w5dxp.com/notuner.htm
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73, Cecil, w5dxp.com