Hi Iain

Without going into a lot of transmission line theory.... grin

For the purposes of this post, coax is the same as "screened cable". I'd
suggest you use coax though (RG58 will do) as its impedance doesnt vary
along its length and it is a known value... unlike plain screened cable.

When you setup a radio in a car the antenna attachment on the radio end
is normally a coaxial connector. The centre conductor goes into the
"live" part of the box and the screen goes to the metal chassis of the
radio itself. At the antenna end the centre coax conductor connects to
the whip itself and the screen is connected to the car body at the point
where the whip starts. The whip is of course insulated from the car body.

In a correctly installed and tuned installation (usually the whip
length) the coax doesnt pickup any noise.

You dont need to connect the radio end screen to the car body. In your
case you might be able to use a nearby metal screw on the radio. The
last alternative maybe to use the negative power line where it enters
the radio. The idea is to get the screen connection as close to the
receiver input ground as possible. The more exposed centre conductor
there is at the radio end the less efficient the system is.

Given the frequency you have mentioned a "good" whip length will be
around 570mm. There are of course a number of unknowns in setting up
this system so dont try playing with the whip length too much.

Cheers Bob VK2YQA

zentace wrote:
Gidday Bob

Thanks very much for replying to my questions.
I take it then, that if you connect a screened cable between the
aerial and the receiver, the length of the cable itself does not act
as an aerial because the screening prevents this?
So if the aerial is only one "core" where do I connect the screen? I
know I could connect it at the aerial end to the car body (earth) but
what about at the receiver end? Does it have to connect at all at that
end as long as the screen is connected to the car body?
By the way, the car itself will be stationary and switched off so
hopefully I won't have the problem of interference from the car
electrics.
Also the transmitter is apparently fixed frequency 100-125MHz and the
receiver switched to this.
I'll experiment and see!
Let me know if you have any thoughts about the screen.

Thanks again

Iain (Scotland)