Richard,

Simply create a model of a coax feed to a poorly tuned dipole. It is quite easy
to create a situation where a significant portion or the power is radiated from
the coax feedline.

It is really quite simple: Coax is designed as an unbalanced transmission line
from a source of a known impedance (nominally 50 Ohms for most common coax) to a
similar known destination impedance. Under those circumstances there in trivial
leakage.

However in the case where the 'antenna' at the other end is poorly matched it is
possible to have significant radiation from the coax. Again look at a poorly
tuned dipole.

The whole idea of coax is to use it the way it was intended. Then it works well.

Dan

Richard Harrison wrote:
Somone wrote:
"My second claim is when the mismatch condition at the coax destination,
i.e. antenna that may result in significant radiation from the coax
itself."

Responses already show this is untrue.

Radiation from the external coax surface comes from launching a signal
on that surface. Good coax does not let signals penetrate its shield.

A mismatch between a transmission line and its attached antenna affects
both transmitting and receiving from the antenna, but does not launch
signals on the outside of the coax.

A mismatched transmitting antenna does not accept all available power
incident upon it and reflects a portion back toward the sender depending
on how bad the mismatch is.

A mismatched receiving antenna has a source resistance (radiation
resistance) and may also have reactance. A conjugate match is needed for
maximum power transfer to the feedline. The mismatched antenna will
either not extract all the power available to it in the passing wave or
else reradiate more than 50%, (with full extraction, the minimum
possible reradiation is with a perfectly matched antenna). Consider a
short circuit across the antenna feedpoint. 100% of the energy extracted
by the antenna is reradiated. Consider an open circuit at the antenna
feedpoint. Little if any power is extracted from the wave sweeping the
receiving antenna.

The most power is received by a receiving antenna when its radiation
resistance is matched to the Zo of the feedline. In this case, 50% is
the best possible received carrier power in the receiver input. Nobody
tells the antenna it is a receiving antenna. It is a conductor carrying
a current, never mind where it came from, so it is going to radiate.
When matched resistances are involved in source (radiation resistance)
and load (Zo matched), the power is split 50-50 between source and load.
The radiation resistance, is the source resistance for the receiver
load, and it represents the reradiation from the reeiving atenna.

50% of the received power accepted by the load is the best possible
performance. Mismatch means less. Either less power accepted by the
antenna or more power reradiated by by the antenna.

A transmatch can make the feedline appear as a matching load at the
antenna junction for receiving. If matched for both transmitting and
receiving, all available power will be transmitted and received.

Best regards, Richard Harrison, KB5WZI

Cecil,

To this point I do not know where the radiation comes from. I suspect it is from
the antenna. The easiest case is to model a poorly balanced dipole directly feed
by coax. It radiates significant power back down the shield.

After the response of the majority of posters to this concept I plan to find out
more.

Dan

Cecil Moore wrote:
dansawyeror wrote:

My first claim is a tuner at the source does not materially improve
what is happening in the coax. That is a tuner does not recreate the
condition above where the coax is functioning as a properly matched
and terminated transmission line. All the tuner does is match the
impedance at the coax source back to some known, usually 50 Ohm, value.


No matter what the voltages and currents are, if they are balanced,
the transmission line won't radiate (much). If the SWR is 100:1 and
the currents are balanced, the transmission won't radiate (much). If
the SWR is 1:1 and the currents are unbalanced, the feedline is likely
to radiate.

My second claim is when the mismatch condition at the coax
destination, i.e. antenna that may result in significant radiation
from the coax itself.


Please understand it is not impedance mismatches that cause radiation
from the feedline. It is unbalance in the feedline currents that causes
feedline radiation. Current imbalance and impedance mismatches are not
necessarily related.

Current imbalance in a matched system can cause feedline radiation.

Impedance mismatches can exist with negligible feedline radiation.

Richard,

Thank you for this well constructed reply.


This mismatch could arise for any number of reasons, and not all
contribute to radiation from the coax. Wes has already demonstrated a
deliberate mismatch at the end of a cable that exhibits absolutely no
radiation from the coax. This is because he has contrived to contain
the fields from emerging and coupling to the outside of the coax
shield. You should be aware that the shield does support currents on
the inside and outside that are wholly unaffected by each other -
except at the drive point where the two conduction paths are joined.

When you drive a dipole with a coax, the exterior conductive path of
the shield (a separate circuit from the interior conductive path of
that same shield) is in parallel with one arm of the dipole. This
means you have a third radiator that has a length and termination that
is undefined.

In the case where a mistuned dipole is being driven directly from coax there is
radiation from the coax feed. This can only happen from current in the shield.
Is this what you are referring to in the second paragraph?

Thanks,
Dan

On Mon, 29 Aug 2005 21:12:58 -0700, dansawyeror
wrote:

When you drive a dipole with a coax, the exterior conductive path of
the shield (a separate circuit from the interior conductive path of
that same shield) is in parallel with one arm of the dipole. This
means you have a third radiator that has a length and termination that
is undefined.

In the case where a mistuned dipole is being driven directly from coax there is
radiation from the coax feed. This can only happen from current in the shield.
Is this what you are referring to in the second paragraph?

Hi Dan,

Hmmm, The dipole is mistuned by the third conductor, the coax's
shield's exterior; otherwise, the dipole would be suitably matched
(this is the presumption, of course).

The source of the current on the coax's shield's exterior comes from
the excitation voltage seen across the dipole drive point (to which
the shield is common to one of the arms). The arm of the dipole that
is not attached to the shield, sees both its opposite arm, and the
undefined length of the shield's exterior path. This additional load
both unbalances, and mismatches. It is the unbalance that gives rise
to the Common Mode current, the mismatch simply comes for free.

Of course, you could fall into the condition where the dipole would
not normally be tuned, but through luck and happenstance, the addition
of the third radiator creates a match - this is strictly opportunistic
and sometimes the source for glowing reports of an otherwise horrible
antenna design. And this is the genesis of favorable accolades for
many of the mythic antennas that go by initials: CFA, EH, and so on
down the line. The "inventors" have simply contrived to tune the
driveline to their "inventions." Their aversion to discussing
driveline isolation is a hallmark of their "science." Their
insistence that choking the driveline is unnecessary or an impediment
to the design's utility, is further evidence of a generous thumb on
the scale of proof.

The addition of the choke gives its Z to snub this Common Mode
current. As both interior paths (that of the line's center wire, and
the interior of the shield) driving the dipole pass through the same
loops, their magnetic fields are unperturbed and see no additional
impedance. However, the "return" path of the shield exterior sees
these loops alone, and thus the Z is inserted into series with it.

If you think in terms of the W2DU style BalUn, the interior
current/magnetic lines both transit THROUGH the beads, whereas the
exterior shield current/magnetic lines CUT the beads - hence the
choking action is more apparent in this configuration.

73's
Richard Clark, KB7QHC

Dan wrote:
"Simply create a model of a coax feed to a poorly tuned dipole."

Dan has something there. The "ARRL Antenna Book" has an explanation of
"Commonn-Mode Transmission Line Currents" on page 26-16 in my 19th
edition. Launch of coax radiation is shown in Fig 24 on the same page.
This picture may be worth 1000 words.

Best regards, Richard Harrison, KB5WZI

On Tue, 30 Aug 2005 02:15:47 -0500, (Richard
Harrison) wrote:

Dan wrote:
"Simply create a model of a coax feed to a poorly tuned dipole."

Dan has something there. The "ARRL Antenna Book" has an explanation of
"Commonn-Mode Transmission Line Currents" on page 26-16 in my 19th
edition. Launch of coax radiation is shown in Fig 24 on the same page.
This picture may be worth 1000 words.

Best regards, Richard Harrison, KB5WZI

Walt's excellent explaination on this subject can be found he

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

73,
Danny, K6MHE

email: k6mheatarrldotnet
http://users.adelphia.net/~k6mhe/

dansawyeror wrote:
To this point I do not know where the radiation comes from. I suspect it
is from the antenna. The easiest case is to model a poorly balanced
dipole directly feed by coax. It radiates significant power back down
the shield.

Actually, a perfectly balanced dipole can also radiate from the coax
shield because coax is unbalanced. Take a look at www.w2du.com and
www.eznec.com for information on baluns and chokes which tend to
reduce feedline radiation.

It appears to me that you may be confusing "balance" with "matching".
Those words have very different definitions.

A balanced 50 ohm antenna fed with coax and having an SWR of 1:1 is
matched but unbalanced and is likely to radiate from the feedline.

A balanced 50 ohm antenna fed with 600 ohm balanced line and having
an SWR of 12:1 is unmatched but balanced and is not likely to radiate
much from the feedline.
--
73, Cecil http://www.qsl.net/w5dxp


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dansawyeror wrote:
Simply create a model of a coax feed to a poorly tuned dipole.

Please define, "poorly tuned", "mismatched", and "unbalanced".
--
73, Cecil http://www.qsl.net/w5dxp


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On Mon, 29 Aug 2005 20:55:08 -0700, dansawyeror
wrote:




I think I've got this guy figured out; it's Fractenna come back to
haunt us.

On Tue, 30 Aug 2005 07:13:14 -0700, Wes Stewart
wrote:

On Mon, 29 Aug 2005 20:55:08 -0700, dansawyeror
wrote:




I think I've got this guy figured out; it's Fractenna come back to
haunt us.

No Wes, I think it's a new guy getting his kicks from reading that new
technical best seller, "Good Buddy for Dummies."