On Tue, 30 Aug 2005 20:05:05 -0700, dansawyeror
wrote:


I have tried several parameters and have gotten results from a very low level
with a close match to over 90% of power radiated in the feedline when the dipole
resonance is far from the transmit frequency.

We may use this model for a couple of things. It is available and it predicts
radiation. That gives the opportunity to create a test. I for one are willing to
experiment.

Dan

Will you please describe the details of your experiment where you
claim 90% of the power is radiated from the feedline? How did you make
this measurement?

Walt, W2DU

I have to call BS on this one Dan!

On Tue, 30 Aug 2005 20:05:05 -0700, dansawyeror
wrote:


I have tried several parameters and have gotten results from a very low
level
with a close match to over 90% of power radiated in the feedline when the
dipole
resonance is far from the transmit frequency.

We may use this model for a couple of things. It is available and it
predicts
radiation. That gives the opportunity to create a test. I for one are
willing to
experiment.

Dan

Reg, G4FGQ wrote:
"Perhaps somebody might be prepared to state the power actually radiated
from feedlines in watts."

Someone has. A feedline is a way to get energy from here to there.
Usually we want to keep all the energy on or close to the line as it
travels and lose as little as possible to radiation and conversion to
heat.

Terman gives an approximate formula credited to Sterba and Feldman for
radiation from a 2-wire nonresonant line, provided that the lengrh is at
least 20X the spacing and the spacing is less than 0.1 wavelength:

Radiated power/Isquared=160(pi D/lambda)squared.

D/lambda is the spacing in wavelengths

I is the rms line current.

Terman notes that the parallel line radiates 4X the power that a doublet
of length equal to the line spacing would, providing that the line and
doublet currents are equal.

Terman provides a figure to be used to adjust the estimated radiation
upwards for longer feedlines (up to 5 wavelengths), and for greater
heights (up to 0.5 wavelength above the earth).

See 1943 "Radio Engineers` Handbook", page 194.

Best regards, Richard Harrison, KB5WZI

Dan wrote:
"We may use this model for a couple of things."

Dan attached a posting from Reg, G4FGQ.

Reg`s description of the "Centre-Fed Dipole - Radiation from Coaxial
Feedline" tells it as it is.

About 45 years ago, I (Richard Harrison) worked for a company exploring
for oil in the Chaco Jungle of Bolivia. Communications were by HF radio
at all sites in the jungle and at offices in Cochabamba and La Paz.
Radios were Collins 30K-5 transmitters and 51-J receivers at all
locations. All antennas were center-fed 1/2-wave dipoles up only about
1/4-wave due to economics and generally were broadside to some favored
direction. The feedline was also about 1/4-wavelength for convenience,
but the antennas loaded, took a lot of power and the 30-K finals dipped
OK. Soil at the radio sites was mostly sand and didn`t really ground
anything very much. The length of the coax presented a high impedance to
the outside shield of the coax cable at the dipole, whatever it might
have been.

At 1/4-wave antenna height, directionality was small and whatever the
coax radiated, it likely filled-in any nulls.

All stations could clearly hear all others all day, so the operators
were satisfied. Much of the radiation was straight up so we likely
bebefitted from near vertical incidence propagation. The transmitters
shook the aether with their power, but the 51-J has a nice dial but not
much else. Nevertheless, everything worked.

My previous employer had given away all its 51-J`s, they were so sorry,
and replaced them with Hammarland SP-600`s. I didn`t take a 51-J, I was
using one of the Super Pro`s.
My experience is anecdotal.

Best regards, Richard Harrison, KB5WZI

"Cecil Moore" wrote in message
...
dansawyeror wrote:
Below is a link to a site that claims to model coax radiation from a
dipole.

http://www.smeter.net/feeding/feedpowr.php

Please note that the third wire to ground creates the unbalance
that causes feedline radiation. You seem to be confusing cause
and effect. The cause of the feedline radiation is the existence
of that third wire, not SWR. All it proves is that feedline
radiation is caused by that third wire path which unbalances
the source currents.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil

How wrong would it be to say that the *coax* part of the line doesnt
radiate at all?
I see this as a situation where the *outer shield* of a transmission line
is conducting current that radiates.
It seems that a "balanced" antenna that is comprised of a single
conductor and a L shaped conductor that includes the outer conductor of the
coax, could be fed with a balanced line for modeling.

Jerry

Jerry Martes wrote:

"Cecil Moore" wrote in message
...

dansawyeror wrote:

Below is a link to a site that claims to model coax radiation from a
dipole.

http://www.smeter.net/feeding/feedpowr.php

Please note that the third wire to ground creates the unbalance
that causes feedline radiation. You seem to be confusing cause
and effect. The cause of the feedline radiation is the existence
of that third wire, not SWR. All it proves is that feedline
radiation is caused by that third wire path which unbalances
the source currents.
--
73, Cecil http://www.qsl.net/w5dxp


Cecil

How wrong would it be to say that the *coax* part of the line doesnt
radiate at all?
I see this as a situation where the *outer shield* of a transmission line
is conducting current that radiates.
It seems that a "balanced" antenna that is comprised of a single
conductor and a L shaped conductor that includes the outer conductor of the
coax, could be fed with a balanced line for modeling.

Jerry


What I was taught is that in a properly installed antenna system the
coax will not radiate. If the antenna is not properly matched to the
coax you get current flow along the outside of the coax shield.

Dave WD9BDZ

dansawyeror wrote:
If it is true it proves that a driving a "bad" load can cause the coax
feedline to radiate a significant portion of the feed energy.

You need to define "bad load". A "bad load" for unbalanced line
is a balanced load, no matter what the impedance. A "bad load"
for balanced line is an unbalanced load, no matter what the
impedance.

The third wire used in the aforementioned software is designed
to unbalance the system, no matter what the impedance.

To illustrate a balanced system, a fourth wire needs to be
added in parallel with and about four inches away from the
third wire. Then compare the currents in the third and fourth
wires under conditions of changing loads.

I due intend to perform experiments to measure and verify what is
happening and the model.

Please feel free to experiment but at least a dozen participants
of this newsgroup already know what is happening and have been
trying to tell you.
--
73, Cecil http://www.qsl.net/w5dxp

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On Wed, 31 Aug 2005 16:17:42 -0500, "David G. Nagel"
wrote:

What I was taught is that in a properly installed antenna system the
coax will not radiate. If the antenna is not properly matched to the
coax you get current flow along the outside of the coax shield.

Dave WD9BDZ

Please read:

http://www.w2du.com/r2ch21.pdf

Jerry Martes wrote:
How wrong would it be to say that the *coax* part of the line doesnt
radiate at all?

The physical construction of the coax (ideal version) ensures
that the inside of the coax doesn't radiate because, for
ideal coax, the internal currents are perfectly balanced.
Any unbalance in the currents is forced to the outside shield
by the laws of physics.
--
73, Cecil http://www.qsl.net/w5dxp


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David G. Nagel wrote:
What I was taught is that in a properly installed antenna system the
coax will not radiate. If the antenna is not properly matched to the
coax you get current flow along the outside of the coax shield.

Is "matched" the correct word to use there? A 50 ohm balanced dipole
is perfectly "matched" to 50 ohm coax but the feedline will likely
radiate. A 50 ohm balanced dipole is not "matched" to 600 ohm balanced
line but with proper attention to details the feedline will like not
radiate much.
--
73, Cecil http://www.qsl.net/w5dxp


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