Antenna design question
 

While looking for a way to get a little more bandwidth out of an 80
meter antenna, I mocked up an antenna in EZNEC that was thick. I plan on
making an antenna with spreaders and run 4 wires on each leg of the dipole.

I emulated this in EZNEC by simply making the wire thickness quite
thick, ranging from 4 inches to a foot.

The interesting thing was that as the thickness increased, the antenna
length decreased for min VSWR.

Is this a real thing?

Antenna design question
 

Yes.
- 'Doc

Antenna design question
 

On Mon, 20 Oct 2008 08:40:35 -0400, Michael Coslo
wrote:

While looking for a way to get a little more bandwidth out of an 80
meter antenna, I mocked up an antenna in EZNEC that was thick. I plan on
making an antenna with spreaders and run 4 wires on each leg of the dipole.

I emulated this in EZNEC by simply making the wire thickness quite
thick, ranging from 4 inches to a foot.

The interesting thing was that as the thickness increased, the antenna
length decreased for min VSWR.

Is this a real thing?

Yep. It's called a "cage dipole". See:
http://www.smeter.net/antennas/wire-cage-dipole.php
with a program to help with the numbers:
http://www.smeter.net/software/dipcage2.exe

Even the ARRL uses them:
http://www.arrl.org/news/stories/2001/05/03/2/

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Antenna design question
 

Yes, Mike, both Jeff and Doc are correct, the thicker dipole will be shorter
than a thin one for resonance at the same frequency..

Walt, W2DU

Antenna design question
 

Michael Coslo wrote:
While looking for a way to get a little more bandwidth out of an 80
meter antenna, I mocked up an antenna in EZNEC that was thick. I plan on
making an antenna with spreaders and run 4 wires on each leg of the
dipole.

I emulated this in EZNEC by simply making the wire thickness quite
thick, ranging from 4 inches to a foot.

The interesting thing was that as the thickness increased, the antenna
length decreased for min VSWR.

Is this a real thing?

yes.

It's often explained as "extra capacitance from the bigger size", but I
think that's not what's really going on.

Antenna design question
 

On Mon, 20 Oct 2008 08:40:35 -0400, Michael Coslo
wrote:

Is this a real thing?

Hi Mike,

Push it further. There is a copy of my EZNEC file at:
http://home.comcast.net/~kb7qhc/ante.../Cage/cage.htm

73's
Richard Clark, KB7QHC

Antenna design question
 

"Jim Lux" wrote in message
...
Michael Coslo wrote:
While looking for a way to get a little more bandwidth out of an 80 meter
antenna, I mocked up an antenna in EZNEC that was thick. I plan on making
an antenna with spreaders and run 4 wires on each leg of the dipole.

I emulated this in EZNEC by simply making the wire thickness quite thick,
ranging from 4 inches to a foot.

The interesting thing was that as the thickness increased, the antenna
length decreased for min VSWR.

Is this a real thing?

yes.

It's often explained as "extra capacitance from the bigger size", but I
think that's not what's really going on.


The effect is clearly shown in Hallen's curves which appear in many of the
standard text books (e.g. Jordan & Balmain, Electromagnetic waves and
radiating systems). There's certainly a likelihood of greater shunt
capacitance at the feed point, but if the limbs are made conical - radiating
geometrically from the feedpoint - then this doesn't apply. In that case,
Schelkunoff might be of some help.

Chris

Antenna design question
 

Jim Lux wrote:
"It`s often explained as "extra capacitance from the bigger size", but I
think that`s not what`s really going on."

Arnold B. Bailey in "TV and Other Receiving Antennas" agrees with Jim.
Bailey writes on page 317:
"We should expect such thin rods to be resonant when their physical
length is slightly less than a free-space half-wave length. When the rod
is thick, the effective velocity along the rod is considerably less than
the free-space velocity, thus reducing the wavelength proportionally."

The above may be grist for Arthur`s mill.

Bailey produces emperical equations (equilibrium?), graphs, and worked
examples for various cross sections.

Best regards, Richard Harrison, KB5WZI

Antenna design question
 

Richard Harrison wrote:
Jim Lux wrote:
"It`s often explained as "extra capacitance from the bigger size", but I
think that`s not what`s really going on."

Arnold B. Bailey in "TV and Other Receiving Antennas" agrees with Jim.
Bailey writes on page 317:
"We should expect such thin rods to be resonant when their physical
length is slightly less than a free-space half-wave length. When the rod
is thick, the effective velocity along the rod is considerably less than
the free-space velocity, thus reducing the wavelength proportionally."


Some might argue, though, that the reason the effective velocity is less
is because the sqrt(1/LC) term is smaller because C is bigger because of
the increased surface area. And that might not be far from the truth
for a restricted subset of antennas.

All of this kind of confusion is trying to make one sort of model (a
transmission line) fit something else (a radiator). Just like the
things that treat the antenna as a lumped RLC.

Antenna design question
 

On Mon, 20 Oct 2008 17:57:03 -0700, Jim Lux
wrote:

All of this kind of confusion is trying to make one sort of model (a
transmission line) fit something else (a radiator).

Hi Jim,

I've seen this kind of assertion made before, generally as a blanket
prohibition/warning/incantation/supplication/condemnation - but never
with any demonstrable problem that wasn't an example of designed-in
failure suited to the argument.

Lest there be any confusion: an antenna IS a transmission line.

The clarity to this confusion starts with the Biconical Dipole. S. A.
Schelkunoff describes it as a "Linear" antenna and used transmission
line math to build the mathematical model for the thin wire dipole in
his classic publication "Theory of antennas of arbitrary size and
shape," Proc. I.R.E., 29, 493, 1941 and S. A. Schelkunoff, "Advanced
Antenna Theory, " John Wiley and Sons, Inc., New York, (1952) I'm
inclined to allow the weight of his work stand until someone tips it -
or can demonstrate I incorrectly read his thesis. Somehow given the
weight of authorities (Ronold King being one) that cite him for this
very reading (specific to the correlation) are abundant, I will await
heavier lifting to tip the math.

Accessible reference work can be found by searching the PTO with his
patent number: 2235506.

73's
Richard Clark, KB7QHC