[Elecraft] RE: zip cord as antenna wire

Thu Mar 25 11:29:01 2004

Actually zip cord is a wonderful feed line... If you tie the conductors
together and use it as a single-wire feed line that radiates! <G>. So it
makes good 'antenna' wire too.=20

Insulation on a wire has virtually no effect on its efficiency as an
antenna. Remember, what's happening in an antenna is totally different =
from
what happens in wires carrying currents, even RF currents, elsewhere in =
your
rig. In an antenna the wire is long enough so that currents flowing in =
the
wire set up an electromagnetic field in the space around the wire. =
That's
something totally different from, say, current in the wire inducing a
current in another wire by magnetic coupling or a voltage on a wire =
inducing
a voltage in another conductor by electrostatic action.=20

The only thing that insulation will do on an antenna is to cause the
electromagnetic wave to flow ever so SLIGHTLY slower. That means that =
the
wire will be ever so SLIGHTLY shorter to be 'self resonant' as a 1/2 =
wave or
1/4 wave at some frequency. But the difference is far to little to be of =
any
practical concern anywhere in the HF spectrum, even when cutting a =
resonant
antenna to length. Yes, it will attenuate the electromagnetic wave =
slightly,
just as ions in the atmosphere or the of the earth do as they slow down =
the
wave (the electromagnetic wave moves at the speed of light only in free
space, remember?). But the attenuation is completely insignificant at =
HF. (I
keep saying HF because if you get up into the GHz range, things are
different. But then, your insulated ZIP wire would have insulation an
appreciable wavelength thick! It'd be like having an 80 meter half wave =
with
zip insulation 30 feet or so thick around it!).=20

The whole issue over Zip cord as a transmission line is when it's used =
as a
two-wire transmission line. In that case there are points where there is
significant RF voltage between the wires, and that zip insulation is not =
a
very good RF insulation, so you have what are effectively partial =
'shorts'
between the conductors at points all along the wire.=20

Ron AC7AC

-----Original Message-----
From: [email protected] =
[mailto:[email protected]]
On Behalf Of Peter Wollan
Sent: Thursday, March 25, 2004 6:21 AM
To: [email protected]
Subject: [Elecraft] RE: zip cord as antenna wire

Guy Ollinger wrote:
"For the most part, common zip cord insulation has ABSOLUTELY AWFUL
dielectric losses."

I had known that this means that zip cord makes bad transmission line. =
Does
it also imply that it makes bad antenna wire?  That is, does an unzipped
cord, single wire, radiate much worse than wires with other kinds of
insulation, when used in a tuned dipole?  Or a wire tossed in a tree =
plus a
ground radial?

Zip cord is so cheap ....

	Peter N8MHD