What impedance would an end-feed infinitely long wire in free space have?
I'm about to put up 60 ft of wire 30 ft above the ground, and pipe it into
my living room. At first I intended it for 'DC to Light' receive only with
my new IC-R3 toy. But now I'm wondering if I could transmit into it on 2M
and 440. (I know this sounds chincy, but it's bound to get out better than a
rubber duck.) I figure at VHF and UHF 60 ft of wire would look like a
beverage antenna, except this wire is going to be many, many wavelengths
above the ground. So what does a wire many, many wavelengths long, many,
many wavelengths high look like? Would I need a balun?

What would be a good reference be for questions like this? The books I have
here and Google never seem to have the info I'm looking for.

--
Antennas - Last Refuse of the Homebrewer
Bob Donahue
ND9B

On Sun, 3 Sep 2006 09:51:21 -0500, Bob D. wrote:

--
Antennas - Last Refuse of the Homebrewer

Dude, you need a dictionary.

And, you need to fix your sig delimiter.

Bob D. wrote:
What impedance would an end-feed infinitely long wire in free space have?

Since you're not in free space, I can tell you that the
characteristic impedance of a horizontal wire over ground is:

Z0 = 138*log(4D/d)

where 'D' is height above ground and 'd' is wire diameter.
The Z0 of #14 wire at 30 ft. is very close to 600 ohms.
Since it is a log function, the Z0 doesn't change very
fast as 'D' and 'd' are varied.

An infinitely long wire would be a traveling wave antenna
with a feedpoint impedance of Z0. However, your 60 ft. long
wire is a standing-wave antenna and the feedpoint impedance
depends upon the phase of the reflected wave.

What are you planning to use as a counterpoise?
--
73, Cecil http://www.qsl.net/w5dxp

Regarding your very long wire antenna.

Radiation pattern depends on the number of wavelengths that the wire is
long. When it is very long, also the propagations speed inside the wire
is going to count (some insulation, does slow down the propagation
speed of the traveling wave a bit, reducing the far field radiation).
Also the straightness of the line is important with very large
Le/lambda.

As propagation speed is equal to c, the radiation pattern will be a
cone (with side lobes), the longer the wire with respect to wavelength,
the narrower the cone will be (so a high directivity). But I believe it
is not a useful antenne, the radiation pattern of such a line is not
that good and maybe you don't want directivity. When you really want
to experiment with long wires at VHF/UHF, a non-linear expanding two
wire structure, fed from a balun would give better results. That gives
a better radiation pattern (with side lobes). If you would prefer a
V-structure, you have to adapt the angle between the wires depending on
frequency to get highest directivity (high frequency requires small
angle).

The impedance of the single wire antenna shown at the antenna terminal
depends on how you feed it. When you feed it from a gradually expanding
coaxial structure (launcher), you don't need a balun and get wide band
behavior. The "return" conductor is the coaxial structure. When you
insulate the wire it becomes a Goubau line (with less radiation).

The impedance of the wire itself is more difficult. The standard
formulas are only valid for cross sections much smaller than the
wavelength. In your case Mother Earth is not the return conductor.

Your wire is not terminated, so you may expect a reverse traveling wave
that cause a cone like radiation pattern in the opposite direction (and
of cause reflection into the transmitter).

When you search for traveling wave antenna or surface wave antenna you
will find useful information.

Best Regards,

Wim
PA3DJS

On Sun, 3 Sep 2006 09:51:21 -0500, "Bob D."
wrote:
So what does a wire many, many wavelengths long, many,
many wavelengths high look like?

End-fire.