Wes,

As a starter, look at this site:

http://www.cbtricks.com/~ab7if/coax/coax.htm

When a transmission line is terminated in it's characteristic impedance there is
no voltage or current reflection from the line. The electromagnetic fields
continue to flow into the termination as if the line were infinitely long. When
a mismatch of impedance occurs, reflected waves will be produced and they will
interact with the incident waves. The total voltage and current on the line are
no longer the result of a single traveling wave from the source to the load.
Instead, it is the algebraic sum of two waves traveling in opposite directions.
This interaction results in what is known as standing waves. The waves remain in
fixed positions along the line while they vary in amplitude and polarity. A wave
of any shape can be transmitted along the line without any change of waveshape
or magnitude. Looking at the gif below, we see a line driven with a sine wave
generator, terminated with a short circuit to maximize the reflection.

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.

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.

Dan

Wes Stewart wrote:
On Sat, 27 Aug 2005 16:53:39 -0700, dansawyeror
wrote:


Let's take the case of a 50 Ohm line and some mismatched antenna. The result is
a combination other then 50 Ohm with most likely a zero complex component.


Surely you don't believe this do you? It is -much- more likely that
the impedance is reactive than not. At one (fundamental) frequency
the reactance is zero. At every other frequency it is reactive.


All a
tuner does is match 50 Ohm at the radio to the complex impedance presented to it
at the source of the line.


Isn't that enough?


That the only place with 50 Ohms and zero inductance in the line - antenna
system. The combination of cable and antenna presents something other then R =
50 ohms 0 reactance and the the transmission line see discontinuities. The
result is it radiates.


Oh dear me.

On Sun, 28 Aug 2005 09:38:52 -0700, 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.

Hi Dan,

As true as that may be, the results run the gamut from trivial to
considerable as has been already discussed in this thread.

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.

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. It is THAT radiator that both causes a higher SWR AND
radiation that is not a normal condition for an otherwise tuned
antenna. Given that the length of the line's external conductive
path, and its termination is largely undefined (unless you take great
care to both measure and characterize such issues), the occurrence of
mismatch and radiation is highly variable. Thus, anecdotal accounts
of antennas being poor or good when they are driven by a simple coax
are suspect (barring the reporter also supplying the conditions of the
external path).

To eliminate the effects of this third path, a choke is added to the
drive point. The purpose of the choke is to add impedance to this
path to reduce Common Mode current. Common Mode current is the
current that flows due to an unbalanced system (the unanticipated
third radiator does that in spades). It flows in two wire
transmission lines too when the unbalance occurs for other reasons
(and those are plentiful as well).

73's
Richard Clark, KB7QHC

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

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.
--
73, Cecil http://www.qsl.net/w5dxp


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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.

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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Cecil Moore wrote:

SNIPPED

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.

Yep!! Love that high VSWR! Tuned feeders for 50+ years and they still
work fine.

On Sun, 28 Aug 2005 09:38:52 -0700, dansawyeror
wrote:

Wes,

As a starter, look at this site:

http://www.cbtricks.com/~ab7if/coax/coax.htm

When a transmission line is terminated in it's characteristic impedance there is
no voltage or current reflection from the line. The electromagnetic fields
continue to flow into the termination as if the line were infinitely long. When
a mismatch of impedance occurs, reflected waves will be produced and they will
interact with the incident waves. The total voltage and current on the line are
no longer the result of a single traveling wave from the source to the load.
Instead, it is the algebraic sum of two waves traveling in opposite directions.
This interaction results in what is known as standing waves. The waves remain in
fixed positions along the line while they vary in amplitude and polarity. A wave
of any shape can be transmitted along the line without any change of waveshape
or magnitude. Looking at the gif below, we see a line driven with a sine wave
generator, terminated with a short circuit to maximize the reflection.

I am certainly not the sharpest guy in this forum, however, I have
been a ham for almost 47 years and I've been working with antennas
from the beginning. I retired after 33+ years in the aerospace
business where a good deal of my work involved rf design,
measurements, failure analysis and the writing of specifications for
rf/microwave devices and assemblies. I regret having to even bring
this up, but it seems that you're willing accept something written on
a CB radio site as fact while ignoring anything you hear in this forum
from professionals in the field. Why is that?


My first claim is a tuner at the source does not materially improve what is
happening in the coax.

Please show me the place where I claimed anything different.

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.

Please show me the place where I claimed anything different.


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.

This I've already proved to be wrong by example. Let me try one last
time. Since antennas are reciprocal, you don't even need a
transmitter. Connect any length of coax to your receiver input.
Terminate the far end with a dummy load (50 ohm resistor). Tune around
the bands. What do you hear? Nothing, if the coax is any good and the
receiver and dummy load are well shielded. So I suppose this proves
one of your claims, a matched line doesn't radiate.

Remove the dummy load and replace it with a short circuit. What do
you hear now on this totally mismatched line that by your reckoning
should radiate like crazy?

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