> -----Original Message----- > From: [EMAIL PROTECTED] [mailto:amradio- > [EMAIL PROTECTED] On Behalf Of Donald Chester > Sent: Friday, April 21, 2006 7:15 PM > To: amradio@mailman.qth.net > Subject: RE: [AMRadio] antenna tuners > > >From: "Gary Schafer" <[EMAIL PROTECTED]> > > > > Another factor causing loss with a high SWR is dielectric losses at > the > > > recurring high rf voltage points along the line. At low impedances, > it > >is > > > the resistive loss in the wire, and at high impedances, it is > dielectric > > > losses that combine to cause signal loss. But SWR is much, much less > > > critical than most hams have been led to believe. > > > >It is my understanding that at HF only resistive loss comes into play. > >Dielectric loss isn't a problem until you get into vhf. > > I would say it depends on what kind of balanced line you use. If it is > well > insulated, real open wire line, with ceramic or low-loss plastic > spreaders, > there is probably negligible dielectric loss at hf or even lower vhf. But > if it is solid dielectric feedline, or even that pseudo-open wire line > stuff > that is basically heavy duty TV lead-in with square holes punched in the > dielectric, I suspect there would be dielectric losses even at hf, and > that > they would increase with substantial SWR. > > The same goes for solid dielectric or foam type coax. > > However, for moderate SWR's, the loss is much less serious than most hams > have been led to believe. > > Don k4kyv
Actually the dielectric losses don't have much effect until high vhf and into UHF. Changing the dielectric material in coax from a solid to air dielectric where there is very little dielectric material, makes no significant difference in loss at HF. But the reason the loss goes down with air dielectric is because the center conductor is made larger and has less resistance loss. The center conductor has to be made larger to maintain the same impedance line. I think I read somewhere that the open wire line with the holes punched in the dielectric was no better as far as loss goes than if the holes were not there. But punching the holes allows for a little higher impedance line by lowering the capacitance so that lowers the loss. But the presence of less dielectric material itself had no effect on loss. Real open wire line will usually have less loss than the TV style line with the solid or punched dielectric between the wires because real open wire line will have a higher impedance than the other stuff. Usually the TV style line even if advertised as 600 ohm line is lower impedance. The punched hole stuff I think is advertised as 450 ohm line but turns out to be lower than that. 73 Gary K4FMX
