Re: [TenTec] (no subject)

[Ken Brown <ken.d.brown@hawaiiantel.net>]

> I was always under the impression that the coil added inductance to
> cancel out the Xc due to the shortened antenna,  Then they tapped up
> from the ground end to get  R=50.    Never thought about it increasing
> the electrical length to 3/4λ (resonance).   I will have to read up on
> this one, for some reason my old mind doesn't grasp the 3/4λ part..
>   
An end fed wire has an impedance that is purely resistive when it is a 
multiple of 1/4 wavelengths long. Low impedances at odd multiples of 
quarter wavelengths, and high impedances at even multiples. Adding a 
series inductance to an end fed wire that is a bit short of an odd 
multiple of quarter wavelengths long can be thought of as compensating 
for the capacitive reactance, or as adding length to bring it up to a 
full multiple of quarter wavelengths. Either way you are adding 
inductance to get a purely resistive impedance. They are just two 
different ways of thinking about the same thing. The resulting end fed 
impedance of the system (wire and series inductor) is seldom 50 ohms. 
Tapping up a few turns on the inductor is one way of getting a 50 ohm 
load for the transmission line.
DE N6KB