about radiation resistance of antennas on vehicles

hi everybody!
my self-made circle antennas on my landy rover can't works well, i think the problem is radiation efficiency.
it's well-known that antennas' efficiency depond on radiation resistance .
how to increase radiation resistance of loop antennas on off-road vehicles,especially in the way of antennas figurations.
thanks for any replys.

Using a loop antenna in a mobile vehicle is not very common. Why you don?t use a vertical monopole using for ground the body of the car?
The dissipative resistance in the loop, is depended by the loop circumference, the conductor thickness, and by the frequency.
To get the best transmission loop antenna the circumference shall be approximately 1λ.

I don't think vertical whip is OK, you cant hear voice from the place 80KM away,but
semi-loop works well!

All small (compared to a wavelength) antennas are very inefficient for transmitting because the radiation resistance is low and the ohmic loss is high.

They work well for receiving because the input impedance of the RF amplifier is much higher than the radiation and loss resistance and so the voltage divider effect does not reduce the signal much.