"Peter Parry" wrote in message
news
On Mon, 23 Feb 2004 00:19:20 +0000 (GMT), Dave Plowman
wrote:
Think you'll have problems finding the correct stuff. IIRC, it's actually
300 ohms.
The impedance of a coaxial cable is determined purely by the
dimensional ratios of the cross section of the line. With an air
dielectric 2.3:1 will give you a 50 ohm line. The usual practical
range of coaxial cable impedances is 50-120 ohms with most being 50
or 75ohms. Some car antenna cables claim an impedance of 150 ohms.
cable impedance= 138 Log (D/d) (d=inner diameter, D=outer diameter)
If the space between the conductors is (as it usually is) filled with
an insulator with a dielectric constant greater than unity then the
impedance calculated must be divided by the square root of the
dielectric constant. The usual insulator is polythene with a
dielectric constant of 2.25 so most solid cable has an impedance
about 2/3 of that calculated using the formula above.
Although 300 ohm co-ax cable can be made it isn't used in cars (or
any other commercial application).
Automobile radios and antennae are designed (using the word loosely)
to match 50, 75 or 130/150 ohm antenna systems. Of these the latter
two are the most common. Most car radio suppliers do not supply the
antenna impedance figure they expect, neither do most antenna
suppliers quote nominal impedance figures. A few, Hirschman for
example, quote 130 or 150 ohms but it is neither standard nor
particularly important.
The feedpoint impedance of the antenna varies widely with frequency
and in car applications a large degree of mismatch is accepted as
accurately matching an end fed rod antenna to the feeder over a range
of frequencies in bands from 190kHz to 110 MHz would simply be too
complicated.
In most cases the link margin is adequate to allow for inefficient
antennas and the cable lengths involved are short enough to make
cable losses insignificant.
In order for a cable's characteristic impedance to make any
difference in the way the signal passes through it the cable must be
at least a large fraction of a wavelength long for the particular
frequency it is carrying. In car applications this only occurs at
VHF frequencies. As matching the antenna to the cable is impractical
most manufacturers try to match the radio to the cable. However, not
many radio manufacturers design to a specific input impedance more
accurately than "low".
As the impedance of the cable only becomes important when the cable
is long and the source and load have the same impedance it follows
that with a wide mismatch from the source and short runs the cable
impedance is pretty much inconsequential when used for broadcast
reception in cars.
It has a strange spiral central conductor. It also has to be of
*exactly* the right capacitance to match the tuner correctly, especially
for MF reception.
Capacitance of the cable is immaterial. Typically it is 5-100pf/m
but if the cable is matched at both ends the source "sees" neither
capacitance nor inductance in the feeder. With a mismatch the figure
becomes frequency dependent.
--
Peter Parry.
http://www.wpp.ltd.uk/
How do I repair a dent in the wall behind me?
That lot went over my head at some speed I can tell you!
Frankly I'd buy another arial.
Drew