[Antennas] Car body as a ground plane ??

Sun May 1 15:47:58 EDT 2005

> When a 1/4wl vertical is used on a vehicle, we typically think of the
> car's roof panel (or trunk panel) as the 'ground plane' - completing
> the rf circuit. With a mobile radio installed in the car, the
> 'return path' for the signal is through the car's body and frame and
> then back to the transmitter via either the power return (negative
> side) and/or through a separate ground strap from the car's frame to
> the case of the radio.
>UM....not really.........
The vehicle makes up half the antenna. The 'return path' is not needed. 
You can look at a 1/4 wave vertical as a dipole with the whip as one half, and the vehicle as 
the other. At 2mtrs, it is an off center fed dipole. It is more complex than that, but is the 
basic idea.
> Now.. let's consider this same setup with an HT.
> The HT is connected to a mag mount 1/4wl antenna stuck to the top of
> the car. However, the HT is independent of the vehicle's electrical
> system. How does the 'return' from the ground plane get back through
> the radio to complete the rf circuit ?
> --or--
> In this case, is the antenna system a 1/4wl vertical effectively
> without any ground plane (and therefore inefficient) ?
>In the case of a mag mount, the coax shield is coupled to the vehicle body to form the 
ground plane. I do think there is a loss of efficiency, but not a large amount.
Let me look at the 2 examples you showed above and see how bad I can foul up the theory 
while giving what I understand is occurring.........
With a bolted on antenna, bonded well to the vehicle, the radio and antenna share a 
common ground system. RF will flow through both the coax as well as the radio ground lead 
and any conductive mounting hardware. In most cases, this is no problem, but can cause 
problems in some of the vehicle systems if the RF power is large enough. The amount of 
RF power on your antenna and the vehicle should be roughly equal.
In the case of a mag mount, with an installed radio, will be somewhat similar to the above. 
Since the mag mount is coupled to the vehicle, there should be roughly the same circuit 
present.
In the case of an HT that is not installed in the vehicle and not powered from the vehicle, it 
gets a little different. Your RF must flow through the coax....there are no other paths for it to 
take. Your antenna and vehicle may have different levels of RF power present as some RF 
on the braid may be returned toward the radio....should not cause a high SWR or damage 
the radio if you use an exact length of coax (off the top of my head, multiples of 1/2 wave). 
Your vehicle in this case is no longer a ground plane, but part of your antenna....to a far 
greater extent than the above cases. Generally, operating an HT in a vehicle with a mag 
mount will result in a greater chance of RF affecting other systems in your vehicle...I have 
to attach a ground lead to my HT if I am going to use it in my van, or the ABS gets fouled 
up......
This is an over simplification of what I think I understand of what is going on in a vehicle 
antenna system.....Hopefully, I have not fouled it up too bad...and someone will point out the 
severe errors.
Mel
N7OKL