[TowerTalk] antenna impedence and ground impedence??

[Jim Lux]

> All,
>
> I have a questions regarding calculating ground impedance. How can this
> be measured??
>
> The impedance of a loaded vertical measures about 50 ohms, however model
> programs predict the antenna impedance is in the range of 10 ohms. Does
> this imply ground impedance is 40 Ohms??
>
> Thanks,
> Dan
>
>
Yes and no...

First off, impedance is complex (it has both a real (resistance) and
imaginary (reactance) part).  A loaded antenna is using a reactive component
to "counteract" the reactive component of the non-resonant short antenna.
Once you've cancelled the reactances, you're left with what is fundamentally
the sum of the radiation resistance and the loss resistance.

When you modeled the antenna, did you use ideal ground or some "finite
ground"?

A resistance of 10 ohms sounds like the idealized lossless short radiator
with the reactance cancelled out. So, if you measure 50 ohms resistive
impedance, and the ideal model over ideal ground shows 10 ohms, yes, the
extra 40 ohms is loss resistance.

When one speaks of ground impedance, you're really talking about a bulk
property of the soil, composed of permittivity (dielectric constant) and
resistivity.  This can be related to the DC (or RF) resistance between, for
example, two ground rods.  It can also be used to estimate the loss
resistance of the radiator.