[Elecraft] Another feedline question

[drcuthbert@micron.com]

Below about 1 GHz coaxial cable losses are virtually all conductor loss. =
You can see this in the loss graphs. The transition frequency is where =
the loss rises as the square rt of freq (conductor loss) and begins to =
rise linearly with freq (dielectric loss). Why then does foam dielectric =
coax and air dielectric coax have lower losses even at HF? The center =
conductor, which accounts for >80% of the loss, is larger with a =
low-dielectric constant material.

   Dave WX7G

-----Original Message-----
From: Stuart Rohre [mailto:rohre@arlut.utexas.edu]
Sent: Thursday, December 18, 2003 6:59 PM
To: drcuthbert; pbuckley@woh.rr.com; elecraft@mailman.qth.net
Subject: Re: [Elecraft] Another feedline question


Loss is related to the dielectrics in the RF field, and how lossy is the
dielectric.  True open wire line is almost lossless, as it has mostly =
air
between the wires carrying RF.  Coax lines have the higher losses than =
any
of the parallel lines because of the close spacing of the conductors to =
the
dielectric and it has to pass thru thicker dielectric layer between =
center
wire of coax and the shield.  Maybe I better qualify that comment on =
coax,
as there are air insulated large coaxes in broadcast use that have low =
loss
spacers.  They may well equal some fully insulated parallel lines.  It =
is
the plastic continuously insulated coax I am saying has the greatest =
loss.
-Stuart
K5KVH