"Cecil Moore" wrote in message
...
Dale Parfitt wrote:
I have repeatedly asked you to supply us with your original quote from
W8JI's page concerning a straight radiator is the best.

Quoting W8JI's web page:
"How do we make a small antenna as efficient as possible?"
"... we make the antenna as large and straight as possible in a line.
We don't fold, bend, zigzag, or curve the antenna especially in the
high current areas."

I don't know what the fuss is all about. Transmission line
currents don't radiate (much) because they are out of phase.

Random folding of an antenna more often than not introduces
transmission line currents into the antenna itself - not
good for radiation purposes.

Transmission line currents cause destructive interference -
that's good for transferring power from one place to another
but not good for radiating RF.

There are certain special-case antennas where folding occurs
without introducing destructive interference, e.g. a 1/2WL
folded dipole or a full-wave quad where the wires are a
large enough percentage of a wavelength apart so they don't
cause near-field destructive interference.

Hint: RF radiation cannot be understood without understanding
constructive and destructive interference in the near, medium,
and far fields.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Thank you Cecil,
That's all I was looking for.

Dale W4OP

Dale Parfitt wrote:
Thank you Cecil,
That's all I was looking for.

You're welcome and I agree with 95% of what W8JI says.
(For instance, he is mistaken about the delay through
a 100T, 10TPI, 2" diameter 75m loading coil.)

Some may or may not understand why random folding
of antenna radiators tends to change the radiating
conductors into non-radiating conductors. (The same
effect is at work in loading coils.)

When two conductors are carrying differential coherent
currents with no common-mode current, there is negligible
radiation when the two conductors are parallel to
each other and the spacing is a very small fraction of
a wavelength. It's called a transmission line and most
of the losses at HF are I^2*R. Usually, one of the goals
of a transmission line is not to radiate. Transmission line
fields tend to cancel in the near field due to destructive
interference.

A single straight wire in free space is a very efficient
radiator because interference occurs mostly in the far
field. Fold it back upon itself and unless the
second conductor is positioned perfectly, there will exist
differential currents between the two conductors which
will tend to cancel the radiation - leaving mostly I^2*R
losses at HF.

Small folded/loaded antennas tend to cancel the radiating
fields. The only other avenue for a lot of the energy is
conversion to heat.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Cecil Moore wrote:
Dale Parfitt wrote:
Thank you Cecil,
That's all I was looking for.

You're welcome and I agree with 95% of what W8JI says.
(For instance, he is mistaken about the delay through
a 100T, 10TPI, 2" diameter 75m loading coil.)

Some may or may not understand why random folding
of antenna radiators tends to change the radiating
conductors into non-radiating conductors. (The same
effect is at work in loading coils.)

Any relation to the loosely wrapped "coils" of shortened verticals like
bug catchers?

- 73 de Mike N3LI -

Michael Coslo wrote:
Any relation to the loosely wrapped "coils" of shortened verticals like
bug catchers?

Take a look at the geometry. Assuming that the current on one
side of a turn on the coil is equal to the current on the
opposite side of the coil but traveling in the opposite direction,
one can see why those two currents per turn resemble transmission
line currents (differential) rather than antenna currents (common-
mode). (A 10" coil wound with 1/2WL of wire radiates roughly the
same amount of energy as a 10" straight wire.)

Physically large air-core loading coils can be modeled as a
transmission line with a Z0 and VF (delay).

http://hamwaves.com/antennas/inductance.html

The delay through the coil can be calculated by knowing the
Beta = _____ rad/m "Axial propagation factor of n=0 sheath
helix waveguide mode at the design frequency"

The VF of W8JI's 100T, 10TPI, 2" dia test coil calculates
out to be ~0.03 resulting in a ~25 nS (~37 deg) delay
through the coil at 4 MHz.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Cecil Moore wrote:
Michael Coslo wrote:
Any relation to the loosely wrapped "coils" of shortened verticals
like bug catchers?

Ack! I meant to write Hamsticks, not bug catchers! Sum daze I am in a daze!

- 73 de Mike N3LI -

On Sep 4, 7:04*am, Cecil Moore wrote:
Dale Parfitt wrote:
Thank you Cecil,
That's all I was looking for.

You're welcome and I agree with 95% of what W8JI says.
(For instance, he is mistaken about the delay through
a 100T, 10TPI, 2" diameter 75m loading coil.)

Some may or may not understand why random folding
of antenna radiators tends to change the radiating
conductors into non-radiating conductors. (The same
effect is at work in loading coils.)

When two conductors are carrying differential coherent
currents with no common-mode current, there is negligible
radiation when the two conductors are parallel to
each other and the spacing is a very small fraction of
a wavelength. It's called a transmission line and most
of the losses at HF are I^2*R. Usually, one of the goals
of a transmission line is not to radiate. Transmission line
fields tend to cancel in the near field due to destructive
interference.

A single straight wire in free space is a very efficient
radiator because interference occurs mostly in the far
field. Fold it back upon itself and unless the
second conductor is positioned perfectly, there will exist
differential currents between the two conductors which
will tend to cancel the radiation - leaving mostly I^2*R
losses at HF.

Small folded/loaded antennas tend to cancel the radiating
fields. The only other avenue for a lot of the energy is
conversion to heat.
--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

Cecil I must respectively disagree. Your arguement is based on the
presence of common mode current. When there is a state of equilibrium
there is no vector that represents common mode. Since the radiator is
a full WL that represents a period it is of closed circuit form. In
such a case any radiator bend is accompanied by a bend that is equal
and opposite per Newtons laws. The moment you introduce common mode
currents you have strayed from the concepts of equilibrium, where all
forces are accounted for. Maxwells laws are based on the position that
all forces involved are accounted for where the summation of such
equals zero.
Regards

Art Unwin wrote:
Since the radiator is
a full WL that represents a period it is of closed circuit form.

My comments were about a one-wavelength straight
wire dipole in free space. The reflections from
the ends are what result in that large resonant
resistance at the center.

--Vf
Open --If
----------------------fp----------------------
Circuit Vr--
Ir--

Zfp - feedpoint impedance, Vf - forward voltage,
Vr - reflected voltage, If - forward current,
Ir - reflected current

Zfp = (Vf+Vr)/(If-Ir) = thousands of ohms

However, if we fold the 1WL dipole into a circular
1WL loop it is still a standing-wave antenna but
the phase of the reflections is reversed.

Zfp = (Vf-Vr)/(If+Ir) = ~100 ohms.

Where are those reflections coming from in a
circular 1 WL loop? Why is the phase of the
reflections reversed?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

On Sep 4, 12:53*pm, Cecil Moore wrote:
Art Unwin wrote:
Since the radiator is
a full WL that represents a period it is of closed circuit form.

My comments were about a one-wavelength straight
wire dipole in free space. The reflections from
the ends are what result in that large resonant
resistance at the center.

* * * * * * * * * --Vf
Open * * * * * * --If
----------------------fp----------------------
Circuit * * * * *Vr--
* * * * * * * * * Ir--

Zfp - feedpoint impedance, Vf - forward voltage,
Vr - reflected voltage, If - forward current,
Ir - reflected current

Zfp = (Vf+Vr)/(If-Ir) = thousands of ohms

However, if we fold the 1WL dipole into a circular
1WL loop it is still a standing-wave antenna but
the phase of the reflections is reversed.

Zfp = (Vf-Vr)/(If+Ir) = ~100 ohms.

Where are those reflections coming from in a
circular 1 WL loop? Why is the phase of the
reflections reversed?
--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

Equilibrium means equilibrium thus there are no reflections. Actions
have an equal and opposite reaction. What are you going to draw upon
for an equalizing vector?

Art Unwin wrote:
Equilibrium means equilibrium thus there are no reflections.

No reflections on a standing-wave antenna?
Where do the standing waves come from?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

On Sep 4, 3:00*pm, Cecil Moore wrote:
Art Unwin wrote:
Equilibrium means equilibrium thus there are no reflections.

No reflections on a standing-wave antenna?
Where do the standing waves come from?
--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

There are no standing waves either