Ian White GM3SEK wrote in
:

Ed wrote:
....
While I have no disagreement with everything else Ian stated, I do
take
some exception to the above comment about foam. Just take a look at
RG-8 for example. With all other aspects of it remaining the same,
there certainly is a significant difference in loss figures when the
dielectric is changed from solid to foam.

Detailed specifications, please?

It is impossible to change only the dielectric and have literally "all
other aspects remaining the same". If you want to keep the same
characteristic impedance, at least one more thing has to be changed -
either the centre conductor diameter or the shield diameter (or
possibly both).



There is a market for lower loss cables that are of similar dimensions to
existing cables like RG213 and RG58. In my experience there are a range
of cables with the same outside conductor dimensions, foam dielectric and
a effectively larger inner conductor. It is often stated that the foam
dielectric give the cable its lower loss, whereas the mechanism at HF is
that for the same sized outer conductor, the lower permittivity of the
foam dielectric requires a larger centre conductor for same Zo.

For example, the k1, k2 factors for a loss=k1*f^0.5+k2*f model for two
dimensionally similar cables a

Belden 8262 (RG58C/U): 1.30e-5, 2.95e-10
Times Microwave LMR195: 1.17e-5, 1.54e-11

k1 is proportional to copper loss, and k2 is proportional to dielectric
loss.

Looking at LMR195, the reduced loss at 10MHz is almost entirely due to
the reduced copper loss.

It is not until about 2GHz that the dielectric loss in RG58 equals the
copper loss.

The message to carry away is that an 'RG8 foam' cable may be manufactured
with the same diameter dielectic and braid, but use a larger inner
conductor. Connector compatibility might be more about compatibiility
with the inner conductor than the connector body.

Owen