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

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Gary, if your coil-loaded 8-feet vertical had been mounted above a ground
radial system then the radial system would have to be classified as "poor".

Radiation resistance is about 4 ohms for a height of 8 feet at 7.2 MHz.
Coil loss = 3 ohms. Subtract 4+3 from your measured 36 ohms which leaves 29
ohms for the ground loss resistance. Which is a poor value for a radial
system. But it may not be poor for your particular vehicle-mounted system
because the vehicle itself forms a large physical part of the antenna and so
increases radiation resistance.

( The vehicle plus whip behaves as a short, off-centre fed, half-wave
resonant vertical, there being fewer ground losses to worry about.)

But the calculating procedure is the same for all vertical ground-mounted
antennas. Subtract calculated radiation resistance from measured input
resistance to obtain ground loss resistance. If there's a loading coil,
subtract its loss resistance too.
Also subtract something for wire-loss of a long wire.
---
Reg, G4FGQ