On Mon, 23 Feb 2004 17:22:03 +0000 (GMT), Dave Plowman
wrote:


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.

It won't make a blind bit of difference. I'll measure the difference
later this week and post the results.

You could well be right about it being 150 ohm rather than 300

I am - there is no commercially available 300 ohm co-ax.

I wasn't sure hence the IIRC. But I *am* sure it ain't 75.

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.

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.

But I think you'll find in practice that neither is an exact match -

Neither is any form of match at all.

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.

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).

--
Peter Parry.
http://www.wpp.ltd.uk/