dansawyeror wrote:
Since you are talking about 50 Ohms I assume you are talking about a
transmission line. If that is the case you should definitely match the
feedline to antenna at the antenna feed point.

Why is it definite? What is the loss in 50 ft. of LMR-400 at the
frequency of interest when the SWR is 50/8 = 6.25:1?

Any attempt to match the
feedline with a tuner in the shack only turns the whole feedline into
part of the antenna system.

Simply not true if the currents remain differentially balanced.
SWR doesn't cause feedline radiation.
--
73, Cecil http://www.qsl.net/w5dxp


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

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.

Dan

Cecil Moore wrote:
dansawyeror wrote:

Since you are talking about 50 Ohms I assume you are talking about a
transmission line. If that is the case you should definitely match the
feedline to antenna at the antenna feed point.


Why is it definite? What is the loss in 50 ft. of LMR-400 at the
frequency of interest when the SWR is 50/8 = 6.25:1?

Any attempt to match the feedline with a tuner in the shack only turns
the whole feedline into part of the antenna system.


Simply not true if the currents remain differentially balanced.
SWR doesn't cause feedline radiation.

dansawyeror wrote:
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.

If the currents are balanced, a 50 ohm transmission line seeing
something other than a 50 ohm load does NOT cause it to radiate.
If a 50 ohm unbalanced transmission line sees a 50 ohm balanced
load and common-mode currents flow on the outside of the coax,
it will usually result in radiation from the feedline. Simply
knowing the magnitude of the feedpoint impedance doesn't tell us
anything about feedline radiation.
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" wrote in message
...
dansawyeror wrote:
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.

If the currents are balanced, a 50 ohm transmission line seeing
something other than a 50 ohm load does NOT cause it to radiate.
If a 50 ohm unbalanced transmission line sees a 50 ohm balanced
load and common-mode currents flow on the outside of the coax,
it will usually result in radiation from the feedline. Simply
knowing the magnitude of the feedpoint impedance doesn't tell us
anything about feedline radiation.
--
73, Cecil http://www.qsl.net/w5dxp

Even grossly mismatched open wire transmission line does not radiate
significantly. For example a 66 ft length of 3" spaced, open wire line,
shorted at one end, radiates only 4 - 5% of the input power. 95% is
dissipated in the conductor losses.

73,

Frank

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.

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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