For heights less than 0.3 wavelengths, Radiation resistance can be accurately
enough estimated from -
Rrad = Square( 24 * Height / Wavelength ) ohms. Which is near enough exact
when height = 1/4 wavelength.

Hi Reg, I just went outside and measured the impedance of my 8' mobile
antenna. It's center loaded with a high Q loading coil and small capacity hat.
It is mounted on a full size pick-up truck. At 7.2 MHZ it reads 37+j0 ohms.
The antenna analyzer has always been accurate within a few percent. Does this
mean that my truck provides an "excellent ground" at 7.2MHZ? If it does, I
need to use my truck for an rf ground for the rig inside my house. It may be
that the truck does indeed provide a good ground, I get good results on 40M
mobile.

73 Gary N4AST