Hey Warren, no squabble and I'm happy you asked for an explanation
With a vertical antenna over an ideal sheet metal ground plane of 1/4 wavelength all around in free space, the ground plane makes up the other half of the antenna with equal and opposite currents flowing between the vertical element and ground plane. If everything is perfect all the current supplied to the antenna will flow equally in both halves of the antenna and there will be no loss. In this case the ground plane doesn't radiate because of equal and opposite currents flowing within the ground plane itself between the center and outer edges.
Place the same antenna close to the earth (dirt) and reduce the size of the ground plane to something insufficient like a car body with a 40m antenna and most of the other half of the antenna is missing. The car now becomes loosely coupled (capacitive) to the earth beneath it and you now have large ground losses and much less current will flow due to the missing antenna half and very lossy earth under the car.
(Ok Warren, the dirt does come into play here, I apologize for 5% of my snappy response.)
All this ground loss is kinda like placing a resistor in line with the ground connection of your coax and limiting RF current to half the antenna causing the antenna to not radiate all the energy fed to it. Smaller car, more half of the antenna missing, more dependent on lossy earth, more ground loss, less radiated power, etc.
This is one reason not to attach an HF mobile antenna to a trailer hitch, its closer to the ground and will incur more ground loss than mounted up higher with some sheet metal beneath it. The car body, albeit small, is a much better conductor than the lossy resistive dirt or asphalt under the car.
One test you can do to see the ground resistance effects on a short loaded HF mobile antenna is check the antenna VSWR at resonance on a high spot on the vehicle with lots of sheet metal under it. Without some matching at the feedpoint the VSWR will be lousy because a short loaded HF antenna has a very low feedpoint impedance, probably well under 20ohms on 40/80m and a poor match to a 50 ohm system. Move the same antenna to the trailer hitch and the VSWR at resonance on 40/80m will get much better. Why? Because the additional ground loss from being closer to the dirt raises the feedpoint impedance and makes it appear to be a better antenna. In reality it has more loss and is less efficient.
Anyway there are a lot of things to consider in an effective HF mobile install and you can turn a decent antenna into a dummy load if you don't take everything into consideration. There are web sites that cover all this and I'm sure will explain it much better than I have here. If I find a good one I'll attach a link.
prcguy
"Contrary to another post, using the vehicle body as a counterpoise is very important for HF performance and the surrounding dirt or asphalt has little to due with anything."
Not to start a squabble or anything, consider the fact that a ground counterpoise be it a radial system or vehicle body, in order to be effective must extend at least 1/4 wave around the radiator. With the possible exception of 10M please explain (to yourself) how a vehicle body makes this possible.
This is rhetorical, I'm not looking for debate but only that you give it some considerable thought.
