Having had unwelcome visitors last night I now need to replace the cable from the car stereo to the aerial (or more probably to a connection point in the cable as I am sure that it will not be one complete run).
What type of cable do I need? Is it the 50 or 75 ohm coax?
Thanks
-- Adam
adamwadsworth@(REMOVETHIS)blueyonder.co.uk
That be 75!
Sparks...
Car aerial cable is not normal coax, its very low capacitance.
Dave
The correct physical size for the fittings. The impedance is nominally 75 ohms but isn't at all critical. Whether you use 50 or
75 is of little practical significance.
Think you'll have problems finding the correct stuff. IIRC, it's actually
300 ohms. 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. Is a complete aerial for your car that expensive?
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.
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.
That lot went over my head at some speed I can tell you! Frankly I'd buy another arial.
Drew
Well, if you reckon a car aerial will work ok on all broadcast stations with the cable replaced with ordinary 50 or 75 ohm cable one, you go ahead. I'm saying it won't. You could well be right about it being 150 ohm rather than 300 - I wasn't sure hence the IIRC. But I *am* sure it ain't 75.
But I think you'll find in practice that neither is an exact match - receivers have either a manual or auto way of matching the aerial and cable exactly, and if an extension is introduced this has to be of a particular capacitance.
Is a complete aerial for your car that expensive?
Not an option. The aerial itself is fine. The motorised aerial at the back of the car connects to the cable (I know that the cable connects to the aerial and is not a complete unit as I swapped my aerial to the motor version from a friends car after he had tested his airbags). The biggest problem is that the coax is part of the wiring loom (or certainly enclosed with the loom inside some sticky foam) and the inner core of the coax is now missing.
-- Adam
adamwadsworth@(REMOVETHIS)blueyonder.co.uk
So if I was to buy a bog standard car aerial with cable and then cannibalise this cable for my needs, do you think it will work? Now you mention it I do remember the car radios where you had to tune into a (was it a weak) MW station and tweak a pot to match the aerial to the tuner.
-- Adam
adamwadsworth@(REMOVETHIS)blueyonder.co.uk
Go and get a length of TV Coax from a a local shop and use that. If you buy an aerial with a cable most will be 75ohm cable anyway (especially motorised aerials).
It won't make a blind bit of difference. I'll measure the difference later this week and post the results.
I am - there is no commercially available 300 ohm co-ax.
Well measuring a bit of supposedly "150 ohm" cable I happen to have lying around I've come up with a figure of 110 ohms. This cable has an insulator which looks awfully like solid polyethylene so assuming the same permissivity would give an impedance of about 75 ohms.
Neither is any form of match at all.
No - it really doesn't matter - it isn't the same as sticking a capacitor of that value across the output - it's a ladder network and the fundamental capacitance is less important than the mismatch (which isn't that important anyway).
characteristic impedence of 240R. It also has a dense braid screen. By all means buy TV co-ax but (a) you will get too much loss especially on LW/MW and (b) unacceptably high interference. What is more whilst you may be able to get it to fit the radio aerial plug it won't fit the plug now used for the aerial end in most vehicles.
The original requester should buy a radio-end plug from their local car radio or electronics shop and fit it to the damaged cable, then use a ready-made short extension to make it reach the radio.
Where on earth do you get that from?
Cable rated at 300MHz will give high loss at 1MHz? How?
Where from?
can I suggest posting this to uk.tech.digital-tv ?
Martin Warby
uk.rec.sheds would be a closer match.
Don't they object to off topic postings?
An ICE group might be a better choice.
Not a pot, a capacitor. ON MW as Peter Parry said, there is considerable mismatch, and the aerial 'looks' capacitative. The variable cap tunes in the RF stage to more or less use teh cable resonance to tne the frnt end.
On VHF the antenna is nearly matched as a quarter wave whip, and the cable will be more or less non-reactive.
there seems to be a fair few OT topics (theres one running about motorway speed signs at the moment.Was thinking it might not be so OT as there are quite a few people (like say Bill)in there for whom aerial installation is thier business and know there stuff about coax cable.
Martin Warby
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