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

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

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:
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."

Wes, W7WS wrote:
"--it`s Fractenna come back to haunt us."

Wes is very funny. Happy Halloween!

Dr. Cohen knows the difference between balance and matching, so I don`t
think it`s him.

Best regards, Richard Harrison, KB5WZI

Let opprobrious dogs lie... :)

"Wes Stewart" wrote in message
...

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