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.

Significance is in the eye of the beholder. At 400 MHz,
RG-8 foam seems to have a loss advantage over ordinary
RG-8 of ~2 dB per 100 feet. At 10 MHz, it is ~0.2 dB.
Is that significant?


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!


Ed