"Walter Maxwell" wrote in message
...

"tjs" wrote in message ...
I have a 40meter band xmit/recv antenna up with good swr and works great.
Cost me nothing but my time, and a plastic center insulator to strain
releive the zip cord (rather than tie a knot in the cord at the center.
Just
rip down 33 ft of zip cord, tie a knot or use the insulator, and cut the
feedlinne section to a integral half wave long (20, 40, 60etc meters
long).
If cut to a half wave (use a dip meter) the swr of the dipole will be
translated unaltered to the radio end of your feedline and 70 ohms is OK
for
a swr of 1.4 and the xcvr will not care usually.

Great emergency antenna.

Why do you think the SWR of the dipole will be unaltered at the radio end
of the feedline? You are apparently ignoring the loss in the line that
makes the SWR at the radio end less than that at the dipole terminals.

If the zip cord had zero loss the SWR would be the same everywhere along
the line, only the terminal impedance at the radio end would be the same
as at the dipole.

Without knowing the vf (velocity factor) of the zip cord how do you
determine that the length is a half wave?

And last, why would you want the length to be a half wave?

Walt, W2DU

I think he was correct about the half-wave length of feedline:
according to the ARRL Antenna Book - 17th edition copyright 1994 - page
24-12 in chapter 24, under the Heading "Special Cases" and under the
sub-heading "The Half-WaveLength Line", it pretty clearly states that
regardless of z, it will be the same on both ends of a half-wave line. and
sections having such length can be added or removed without changing the
load Z. (as long as loss is negligible)

Also - You don't need to know the VF if you use a dip meter (or MFJ 259) -
And he would want the length to be half-wave so as to be able to ignore the
characteristic impedence of the zip line and deal directly with the
impedence of the dipole directly.