Cecil Moore wrote:
Ed wrote:
Actually, significance is based on the frequency of operation, as
you just indicated. Since the original poster was talking about an
aluminum jacketed heliax, I assumed the pertinent frequencies to be
at least VHF, if not higher; which would make the difference between
the foam dielectric RG-8 and solid dielectric RG-8 signifacant!

I'm moving to a new QTH and have only kept up
with this thread sporadically. I have now gathered
that the point is that it's not the foam per se
that has the largest effect, but the larger center
conductor required to bring the impedance back
to 50 ohms.

From the designer's point of view, it was the other way around: centre
conductor first, dielectric constant second.

The boss says: "We want a lower-loss coax, in the same outline as RG213
and still 50 ohms."

Starting from RG213, the first thing the designer does is increase the
diameter of the centre conductor, because that's where most of the
losses come from. He now has a lower-loss solid polyethylene cable that
will fit an RG213 connector body, but has an impedance of around 40
ohms.

Consider the fact that the 9913 center
conductor is #10 while the RG-213 center conductor
is #12.

Just so.

To bring the impedance back up to 50 ohms, the designer then has to
reduce the dielectric constant, by using either foam dielectric or a
semi-airspaced construction such as 9913. The losses do reduce a little
more in the second step, but not much.

As I said yesterday, the third step is that Marketing gets hold of it...
and that's where it all turns into foam.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek