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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 |
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