question about wire antenna and tuner
 

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Hi, I am learning about antennas, and and wondering about how antenna
tuners work. I've read you can use anything as an antenna as long as you
have a tuner. Well, If I put up a wire dipole, and then use a tuner,
what is the best length of wire to use? If I use an 80 meter dipole with
a tuner, is that better than using a 10 meter dipole with a tuner?


Jim
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question about wire antenna and tuner
 

"James Barrett" wrote in message
. ..
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Hi, I am learning about antennas, and and wondering about how antenna
tuners work. I've read you can use anything as an antenna as long as you
have a tuner. Well, If I put up a wire dipole, and then use a tuner,
what is the best length of wire to use? If I use an 80 meter dipole with
a tuner, is that better than using a 10 meter dipole with a tuner?


Google Antenna tuners

JERD

question about wire antenna and tuner
 

"JERD" wrote in message
...

"James Barrett" wrote in message
. ..
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Hi, I am learning about antennas, and and wondering about how antenna
tuners work. I've read you can use anything as an antenna as long as you
have a tuner. Well, If I put up a wire dipole, and then use a tuner,
what is the best length of wire to use? If I use an 80 meter dipole with
a tuner, is that better than using a 10 meter dipole with a tuner?


Google Antenna tuners

JERD


http://www.arrl.org/tis/info/Ant-tuner-op.html

http://www.hamuniverse.com/tuner.html

Lamont

question about wire antenna and tuner
 

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JERD wrote:
"James Barrett" wrote in message
. ..
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Hi, I am learning about antennas, and and wondering about how antenna
tuners work. I've read you can use anything as an antenna as long as you
have a tuner. Well, If I put up a wire dipole, and then use a tuner,
what is the best length of wire to use? If I use an 80 meter dipole with
a tuner, is that better than using a 10 meter dipole with a tuner?


Google Antenna tuners

JERD



I honestly did not think of that. I googled for different variations of
"antenna". I found a web site hamuniverse, and found a very good
description of how an antenna tuner works. So a tuner is really there to
protect the transmitter. You still need the wire cut to frequency if you
want to maximize efficiency and minimize the SWR in the wire. Of course
I am generalizing here. I need the arrl antennabook. ;-)

http://www.hamuniverse.com/tuner.html

Thanks!!

Jim

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question about wire antenna and tuner
 

James Barrett wrote:
So a tuner is really there to protect the transmitter.

It does that by not allowing reflected energy to
reach the transmitter and redistributing the
reflected energy back toward the antenna as
part of the forward wave. Thus a transmitter can
be sourcing 100 watts while the forward power
on the transmission line is 200 watts.

You still need the wire cut to frequency if you
want to maximize efficiency and minimize the SWR in the wire.

Efficiency can also be maximized by choosing a
near-lossless transmission line. In that case,
SWR doesn't necessarily need to be minimized.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

"Cecil Moore" wrote in message
...
James Barrett wrote:
So a tuner is really there to protect the transmitter.

It does that by not allowing reflected energy to
reach the transmitter and redistributing the
reflected energy back toward the antenna as
part of the forward wave. Thus a transmitter can
be sourcing 100 watts while the forward power
on the transmission line is 200 watts.

What it really is is an impedance matching network. You adjust the antenna
tuner so that the transmitter sees 50 Ohms. If your SWR meter is calibrated
for 50 Ohms, that means an SWR of 1:1 (between the tuner and the radio).

You still need the wire cut to frequency if you
want to maximize efficiency and minimize the SWR in the wire.

Efficiency can also be maximized by choosing a
near-lossless transmission line. In that case,
SWR doesn't necessarily need to be minimized.
--
You mean the SWR doesn't have to be minimized on the transmission line. If
you don't have a tuner and run a 600 Ohm transmission line into the
transmitter, it will be happy at an SWR of 12:1 if the impedance the
transmitter sees is 50 Ohms. On the other hand, with 600 Ohm line, and an
SWR of 1:1 the transmitter will barf.

The longer wire is usually better than the short. Also, the ARRL recommends
a length that is not resonant on any band to make the job of the antenna
tuner easier. Wires around 100 feet are often used.

Tam/WB2TT

73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Tam/WB2TT wrote:
"Cecil Moore" wrote in message
It does that by not allowing reflected energy to
reach the transmitter and redistributing the
reflected energy back toward the antenna as
part of the forward wave. Thus a transmitter can
be sourcing 100 watts while the forward power
on the transmission line is 200 watts.

What it really is is an impedance matching network. You adjust the antenna
tuner so that the transmitter sees 50 Ohms. If your SWR meter is calibrated
for 50 Ohms, that means an SWR of 1:1 (between the tuner and the radio).

Yes, and that is a Z0-match to 50 ohms. What happens
at a Z0-match is wave cancellation of reflected waves
through destructive interference which redistributes
the reflected energy back toward the antenna in the
form of constructive interference energy that joins
the forward wave.

micro.magnet.fsu.edu/primer/java/scienceopticsu/interference/waveinteractions/index.html

"... when two waves of equal amplitude and wavelength that are
180-degrees ... out of phase with each other meet, they are not actually
annihilated, ... All of the photon energy present in these waves must
somehow be recovered or redistributed in a new direction, according to
the law of energy conservation ... Instead, upon meeting, the photons
are redistributed to regions that permit constructive interference, so
the effect should be considered as a redistribution of light waves and
photon energy rather than the spontaneous construction or destruction of
light."

The reason that the transmitter is protected is that
the Z0-match *causes* that redistribution of the reflected
energy back toward the antenna. Protecting the transmitter
is a side-effect of tuning the entire antenna *system*
to resonance.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Cecil Moore wrote:
Tam/WB2TT wrote:

What it really is is an impedance matching network. You adjust the
antenna tuner so that the transmitter sees 50 Ohms. If your SWR meter
is calibrated for 50 Ohms, that means an SWR of 1:1 (between the tuner
and the radio).


Yes, and that is a Z0-match to 50 ohms. What happens
at a Z0-match is wave cancellation of reflected waves
through destructive interference which redistributes
the reflected energy back toward the antenna in the
form of constructive interference energy that joins
the forward wave.

Correct, except for the part about destructive interference
redistributing reflected energy. Please note the absence of any such
claim in the cited (or any other) reference.

micro.magnet.fsu.edu/primer/java/scienceopticsu/interference/waveinteractions/index.html


"... when two waves of equal amplitude and wavelength that are
180-degrees ... out of phase with each other meet, they are not actually
annihilated, ... All of the photon energy present in these waves must
somehow be recovered or redistributed in a new direction, according to
the law of energy conservation ... Instead, upon meeting, the photons
are redistributed to regions that permit constructive interference, so
the effect should be considered as a redistribution of light waves and
photon energy rather than the spontaneous construction or destruction of
light."

One addendum:
"Therefore, simple diagrams, such as the one illustrated in Figure 1,
should only be considered as tools that assist with the calculation of
light energy traveling in a specific direction."

The same holds true for the simplified explanation provided by the site.

The reason that the transmitter is protected is that
the Z0-match *causes* that redistribution of the reflected
energy back toward the antenna.

More to the point, the Z-match reflects energy back toward the antenna.

73, ac6xg

question about wire antenna and tuner
 

Jim Kelley wrote:
Correct, except for the part about destructive interference
redistributing reflected energy. Please note the absence of any such
claim in the cited (or any other) reference.

Please note the presence of constructive interference in
the cited reference: "... the photons are redistributed to
regions that permit constructive interference ...", which
implies an equal magnitude of destructive interference
elsewhere in order to avoid violating the conservation of
energy principle. It is akin to the gain of an antenna.
Constructive interference in one direction is matched
by an equal magnitude of destructive interference in
another direction.

More to the point, the Z-match reflects energy back toward the antenna.

Yes, as "constructive interference" energy which requires
destructive interference elsewhere to balance the
energy equation. Since there are only two directions
available in a transmission line, any constructive
interference toward the load must be balanced by an
equal magnitude of destructive interference toward the
source.

Quoting Reflections II, by Walter Maxwell, page 4-3:
"The destructive wave interference between these two
complementary waves causes a complete cancellation of
energy flow in the direction toward the generator.
Conversely, the constructive wave interference produces
an energy maximum in the direction toward the load,
resulting from the sum of the two reflected waves
and the source wave."

For a non-reflective thin film coating, these two
reflected waves are known as the internal reflection
and the external reflection.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Cecil Moore wrote:
Jim Kelley wrote:

Correct, except for the part about destructive interference
redistributing reflected energy. Please note the absence of any such
claim in the cited (or any other) reference.


Please note the presence of constructive interference in
the cited reference: "... the photons are redistributed to
regions that permit constructive interference ...", which
implies an equal magnitude of destructive interference
elsewhere in order to avoid violating the conservation of
energy principle. It is akin to the gain of an antenna.
Constructive interference in one direction is matched
by an equal magnitude of destructive interference in
another direction.

More to the point, the Z-match reflects energy back toward the antenna.


Yes, as "constructive interference" energy which requires
destructive interference elsewhere to balance the
energy equation. Since there are only two directions
available in a transmission line, any constructive
interference toward the load must be balanced by an
equal magnitude of destructive interference toward the
source.

Quoting Reflections II, by Walter Maxwell, page 4-3:
"The destructive wave interference between these two
complementary waves causes a complete cancellation of
energy flow in the direction toward the generator.
Conversely, the constructive wave interference produces
an energy maximum in the direction toward the load,
resulting from the sum of the two reflected waves
and the source wave."

For a non-reflective thin film coating, these two
reflected waves are known as the internal reflection
and the external reflection.

Physical objects redistribute energy. Interference simply describes
its spacial distribution.

73, ac6xg

question about wire antenna and tuner
 

Jim Kelley wrote:
Physical objects redistribute energy.

If you consider a Z01 to Z02 impedance discontinuity
to be a "physical object" then I agree.

Interference simply describes its spacial distribution.

Interference, the noun event, is somewhere in the chain
of *events* between the reflections caused by the impedance
discontinuity and the spacial (re)distribution of energy.
Interference, the noun event, is a subset of superposition,
i.e. not all superposition results in interference.

Interference, the adjective, as in the "interference pattern",
describes the results of interference, the noun event. The
interference pattern describes the spacial distribution.

Maybe your answer to the following question will shed
some light on what you think is our disagreement.

In the RF fields in free space surrounding a dipole,
exactly where are the physical objects that redistribute
the energy? As I understand it, each segment is considered
to be a unit radiator and the radiation interference pattern
is the calculated result of the interference events in free
space in the far field.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

As a rule, more wire is "said" to be better. Theoretically, you can make a
different case but just try transmitting at 160m with a tuner and 80m
dipole. At that MF frequency, your tuner will try to reflect so much energy
that you will probably hear the crackling and may see a corona, no matter if
it is rated at 3KW and your are transmitting at 100W.
snip
======================================
It all depends what feeder you use.
When using twin feeder and a matching unit to suit both such a feeder
and the traditional asymmetric 50 Ohms output of any
transmitter/receiver , an 80m dipole,or a dipole of any length for that
matter, can be readily used on 160 m provided that half the dipole + the
length of the feeder is roughly an odd number of quarter wavelengths at
the operating frequency (assuming the velocity factor of the feeder is 1)
In this situation the current in the twin feeder at the matching unit is
relatively high ,hence the impedance is low ,because the voltage is
relatively low (hence no crackling/corona problems.

Cecil Moore's web site shows how he adds lengths of twin feeder to suit
the different bands.


Frank GM0CSZ / KN6WH

question about wire antenna and tuner
 

Highland Ham wrote:
Cecil Moore's web site shows how he adds lengths of twin feeder to suit
the different bands.

If the SWR on 450 ohm ladder-line is between 4.5:1 and
18:1, the impedance at a current maximum point will be
resistive between 25 ohms and 100 ohms for an SWR on
50 ohm coax of 2:1 or less with no tuner. See the Smith
Chart at: http://www.w5dxp.com/smith.htm
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

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Cecil Moore wrote:
Highland Ham wrote:
Cecil Moore's web site shows how he adds lengths of twin feeder to
suit the different bands.

If the SWR on 450 ohm ladder-line is between 4.5:1 and
18:1, the impedance at a current maximum point will be
resistive between 25 ohms and 100 ohms for an SWR on
50 ohm coax of 2:1 or less with no tuner. See the Smith
Chart at: http://www.w5dxp.com/smith.htm

This thread has taken on a life of its own since I posted my first
question. I really don't understand any of this. I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case. I need to learn all
about SWR and impedance in regards to Antennas, from start to finish. Is
there an easy-to-read tutorial out there for a beginner like me? Maybe
Ham Radio for Dummies has something about SWR and antennas?




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question about wire antenna and tuner
 

James Barrett wrote:

This thread has taken on a life of its own since I posted my first
question. I really don't understand any of this. I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case. I need to learn all
about SWR and impedance in regards to Antennas, from start to finish. Is
there an easy-to-read tutorial out there for a beginner like me? Maybe
Ham Radio for Dummies has something about SWR and antennas?

You're not alone. SWR and transmission lines are widely misunderstood,
even, I'm sad to say, among many engineers working with RF. Because of
the widely held misconceptions about these topics, writers of some of
the tutorials and discussions which have made it to print have also
fallen victim to mistaken concepts. I'd be especially leery of
explanations found on the web. While surely there are some which are
entirely correct and very well done, I'm certain there are many others
which aren't.

I recommend the _ARRL Antenna Book_ as a good place to start.

Roy Lewallen, W7EL

question about wire antenna and tuner
 

James Barrett wrote in
:

This thread has taken on a life of its own since I posted my first
question. I really don't understand any of this. I thought that I
wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case. I need to learn all
about SWR and impedance in regards to Antennas, from start to finish.
Is
there an easy-to-read tutorial out there for a beginner like me? Maybe
Ham Radio for Dummies has something about SWR and antennas?

Hi James,

Little wonder.

You asked a few questions:

1. Hi, I am learning about antennas, and and wondering about how antenna
tuners work. I've read you can use anything as an antenna as long as you
have a tuner.

2. Well, If I put up a wire dipole, and then use a tuner, what is the
best length of wire to use?

3. If I use an 80 meter dipole with a tuner, is that better than using a
10 meter dipole with a tuner?

My offering is:

1. That is a very simple statement, and for instance does not address
efficiency or a host of other issues (eg EMR safety). It is a restatement
of the popular ham maxim that "anything works" or the "any antenna is
better than no antenna".

2. The elements of an antenna system have a complex interaction, and
system performance can only be understood when the entire system is
analysed as a system. That means you have to start at the element level
and gain an understanding of those and then how they interact in a
system. Another popular ham maxim is "bigger is always better", it is
easy to say, but is doesn't apply in practice and is usually stated to
mask a lack of fundamental understanding.

3. You are a bit more specific, but not specific enough to answer
definitively. A half wave dipole fed with a balun and a reasonable length
of appropriate coax is an antenna system that takes only moderate
knowledge to design, fabricate, install and set to work with a high level
of confidence that it is working reasonably efficiently. You may even
wish to use an ATU (which is essentially an impedance transforming
network) for small optimisation of the load impedance seen by the
transmitter.

Whilst the temptation to use the antenna system described at 3 on
multiple bands may be great, and it is done, the outcome is often very
poor. For example, such an antenna designed for 80m (system efficiency
should be greater than 80%) is likely to be well less than 10% efficient
on 40m.

Be ware of simple Rules Of Thumb, there are often a plethora of unstated
assumptions, which when considered make them ROT.

Owen

question about wire antenna and tuner
 

James Barrett wrote:
This thread has taken on a life of its own since I posted my first
question. I really don't understand any of this. I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case. I need to learn all
about SWR and impedance in regards to Antennas, from start to finish. Is
there an easy-to-read tutorial out there for a beginner like me? Maybe
Ham Radio for Dummies has something about SWR and antennas?

"The ARRL Antenna Book" is a pretty good start.
Here's a matched antenna system with an SWR of 9:1
on the ladder-line. Understanding this system will
be a step in the right direction.

50 ohm
XMTR----------1/2WL 450 ohm feedline-------50 ohm antenna
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Stefan Wolfe wrote:

As part of the amateur license exams you will run into complex impedances
and perhaps even a few questions about what a smith chart is. My advice is,
once you learn how to graph complex impedances on a simply x-y plot, all
you need to realize is that a smith chart is a graph just like this except
but x AND Y DIMENSIONS ARE TURNED INTO THEMSELVES AS CIRCLEs OF FINITE
RADIUS rather than as circles of iinfinite radius. . .

Actually, a Smith chart is just an overlay on top of a simple polar
graph of reflection coefficient. (Mathematically, it's a mapping of
impedance values to a reflection coefficient graph.) If you plot the
value of a (complex) reflection coefficient on an ordinary polar graph
having radius = 1, then place a Smith chart over it, you can read from
the Smith chart the impedance that results in that reflection
coefficient (normalized to the transmission line Z0). That's why, for
example, a constant SWR locus is a circle on a Smith chart - it
corresponds to a single magnitude of reflection coefficient.

I second Stefan's comments about the value of a Smith chart. It's an
excellent tool for visualization once you learn how to use it. But first
you need a basic understanding of transmission line principles.

Roy Lewallen, W7EL

question about wire antenna and tuner
 

"James Barrett" wrote in message
news:WN6dnT9x6ZU6I7HanZ2dnUVZ_v-

snip

I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case.

You were mostly right; this is the theoretical ideal, but reality forces
compromises on all of us.

To put it simply, yes, you want the most power to "jump off the antenna"
into space. Whatever doesn't jump off is dissipated (wasted) as heat
somewhere in the system. If too much is reflected back from the antenna and
dissipated within in your transmitter, the transmitter overheats ($$$) or it
reduces power to protect itself and nobody hears you.

A simple, inexpensive antenna that comes close to the ideal will probably
work on only one band. For multiband operation you'd need several of them
($$$) and maybe need multiple poles. ($$$) ... or you could buy a
multiband, combination antenna. ($$$)

The tuner is trickery to deal with the power that didn't jump off. It
allows a compromise antenna -- one not perfectly suited for the intended
purpose -- to be used without overheating the transmitter. (The transmitter
must see a 50-ohm load to stay happy.) Changing the length of the feedline
is another form of trickery to keep the transmitter happy, but it doesn't
improve the antenna, either.

Tuners are very useful and not terribly expensive, which is what makes them
so common in the shack.

I need to learn all
about SWR and impedance in regards to Antennas, from start to finish.

Maybe, but only if you want to know the _why_ of antennas. You can buy and
use lots of great ready-made ham things without understanding exactly how
they work. [Example: My car has fuel injection and electronic ignition; I
only sort of understand how they work and could NOT fix them if I had to. I
don't need to.]

Is there an easy-to-read tutorial out there for a beginner like me?

Somebody said The ARRL Antenna Manual. I agree. At first, you can pick and
choose what to read. As you read more of it, a coherent picture should
emerge. I hope you didn't ditch too much HS math.

"Sal"
(KD6VKW)

question about wire antenna and tuner
 

"Sal M. Onella" wrote in
:

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

Owen

question about wire antenna and tuner
 

Sal M. Onella wrote:
"James Barrett" wrote in message
news:WN6dnT9x6ZU6I7HanZ2dnUVZ_v-

snip

I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case.

You were mostly right; this is the theoretical ideal, but reality forces
compromises on all of us.

To put it simply, yes, you want the most power to "jump off the antenna"
into space. Whatever doesn't jump off is dissipated (wasted) as heat
somewhere in the system. If too much is reflected back from the antenna and
dissipated within in your transmitter, the transmitter overheats ($$$) or it
reduces power to protect itself and nobody hears you. . .

As I said earlier, there's a lot of misinformation floating around.

A high SWR doesn't mean there's "reflected energy" which is going to be
dissipated anywhere, least of all in your transmitter. Except for
transmission line loss (which admittedly will be greater, although
usually insignificantly so, if the SWR is very high -- see the Antenna
Book), all the power leaving the transmitter will arrive at your
antenna. Thinking of waves of energy bouncing back and forth looking for
somewhere to be dissipated will lead you down paths that you won't be
able to reason your way out of. As has been clearly demonstrated on this
newsgroup over and over. Read the Antenna Book and other good texts, and
don't try to make up additional imaginary waves.

"Sal" is right about one thing, though. Most transmitters will reduce
output power if the SWR gets too high, which tells the transmitter that
the impedance it's seeing is beyond the range for which it's designed.
(The problem is that various places in the transmitter can encounter
voltages and/or currents too far above design values, or impedances
which might cause instability. It's not because there are waves of
"reflected energy" which dissipate themselves in the transmitter.) So
you do want to keep the SWR measured at the transmitter below that
value. There's no harm in having a very high SWR on the feedline,
however, as long as it has low matched loss.

Roy Lewallen, W7EL

question about wire antenna and tuner
 

Owen Duffy wrote:
"Sal M. Onella" wrote in
:

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

Yes, this misconception will never die. Is it really worth the trouble
continually trying to contradict it?

Roy Lewallen, W7EL

question about wire antenna and tuner
 

Owen Duffy wrote:
"Sal M. Onella" wrote in
:

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

If he said "sometimes overheats", he would be correct.
An SWR of 10:1 certainly *can* cause an over-current condition
in an unprotected transmitter assuming the reflected current
is in phase with the forward current at the transmitter.

However, just as likely is that the reflected voltage is in
phase with the forward voltage at the transmitter and an
over-voltage condition *can* result in punch-through of the
final transistor.

If over-current and over-voltage were not a problem caused
by reflected waves, protection of the finals would not be
necessary.

Note that the impedance seen by the transmitter above is a
*virtual* impedance, not an impedor. Virtual impedances are
only a *result* and not the cause of anything. Virtual impedances
are not the *cause* of over-current or over-voltage conditions.

Anyone who scoffs at virtual opens and virtual shorts being
the *cause* of the re-reflection of reflected energy cannot,
without contradicting himself, turn around and argue that
the virtual impedance seen by a transmitter is the *cause* of
the mismatch. One cannot have it both ways.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Roy Lewallen wrote:
A high SWR doesn't mean there's "reflected energy" which is going to be
dissipated anywhere, least of all in your transmitter.

Most transmitters will reduce
output power if the SWR gets too high, which tells the transmitter that
the impedance it's seeing is beyond the range for which it's designed.

Not readily apparent is the contradiction between these
two statements above which needs to be resolved. Reflected
energy cannot exist without energy, i.e. without ExB watts.
It is the energy in the reflected waves that is the *cause*
of the impedance "seen" by the transmitter. There is *zero*
dissipation in that virtual impedance so it is NOT a
real resistor - it is a dissipationless resistance.

The impedance seen by the transmitter is not a resistor
or inductor or capacitor, but instead is a *virtual*
impedance *caused by* the magnitude and phase of the
reflected wave with respect to the magnitude and phase
of the forward wave. The impedance seen by the
transmitter is:

Z = (Vfor+Vref)/(Ifor+Iref)

where the voltages and currents are phasors, each with
a magnitude and associated phase. There is NO resistor!
There is NO inductor! There is NO capacitor! There is
no power dissipation! Virtual impedances cannot cause
anything.

It is interesting to note that the very people who
support the virtual impedance seen by a transmitter as
being the cause of the conditions there are the same
people who rail loud and long against a virtual short
being able to cause 100% re-reflection. Why does a
virtual impedance cause things to happen only at a
transmitter but nowhere else?

If there's no "reflected energy", the transmitter will
see the characteristic impedance of the transmission
line, e.g. 50 ohms. So the transmitter CANNOT see any
impedance other than Z0 unless reflected energy is the
cause of the deviation away from Z0.

Depending upon the phase of the reflected energy, all
or some or none of the reflected energy may make its
way into the transmitter. The exact magnitude of
joules/sec making its way into the transmitter is:

P = P1 + P2 + 2*SQRT(P1*P2)cos(A)

where 'A' is the phase angle between the E-fields of
EMWave1 and EMwave2 and P1 = E1xB1 and P2 = E2xB2
The last term is known as the "interference term".
If it is negative, it represents destructive
interference. If it is positive, it represents constructive
interference. It should be obvious that 'P' above, can
assume any value between zero and a maximum value so
the amount of reflected energy flowing into the transmitter
can be anything from zero to that maximum value. If the
transmitter is looking into an ideal shorted 1/4WL stub,
the reflected energy flowing into the transmitter will
be zero just as the food-for-thought example demonstrates,
i.e. none of the available power is dissipated in the
transmitter. If the transmitter is looking into an ideal
open-circuit 1/4WL stub, all of the reflected energy will
flow into the transmitter, i.e. all of the available power
will be dissipated in the transmitter. How much depends
upon the relative magnitudes and phases of the forward
and reflected waves.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Roy Lewallen wrote:
Yes, this misconception will never die. Is it really worth the trouble
continually trying to contradict it?

Not if all you do is trade one old-wives tale
for another. One cannot understand these concepts
without understanding the conditions that cause
EM waves to interact.

The conditions that cause interaction between EM
waves are coherency and collinearity (in the same
direction in a transmission line).

The interaction of reflected EM waves can result
in zero or maximum reflected power being dissipated
in the transmitter - and anything in between.

The interaction of reflected EM waves at a thin-
film coating on glass can result in zero or
maximum reflected energy and anything in between
depending upon the thickness of the thin-film.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

James Barrett wrote:



This thread has taken on a life of its own since I posted my first
question. I really don't understand any of this. I thought that I wanted
an antenna with zero reflected energy or as close to that as possible.
Now it sounds like that is not always the case. I need to learn all
about SWR and impedance in regards to Antennas, from start to finish. Is
there an easy-to-read tutorial out there for a beginner like me? Maybe
Ham Radio for Dummies has something about SWR and antennas?


I've been a ham since '58, and did 26 years in the navy in various areas
of communications and radar as an operator, technician, and manager. I have
done my best to learn about antennas and have quite a body of knowledge
accumulated, some of it good, some of it fallacious. Some of it remains
a complete mystery to me. There are a great many misconceptions when it
comes to antennas, feeders, SWR, and the like.

If I had paid attention to what some people say about SWR, conjugate
matches, and the like I would still be hesitant about putting up an
antenna. I'd still be looking for the perfect one.

Despite all the myths and misconceptions, many hams are surprised to find
out that regardless of how bad their antennas may seem in theory, in
practice they are getting out.

Put up an antenna, and 'tune' it for minimum SWR, then have fun. You
will find years of fun ahead and they will give you the time required
to learn more about antennas. Do not try to learn all there is -- you
won't get there. Instead, try to learn where the fallacies lie and
try to avoid them.

The hardest part is committing yourself to putting up an antenna which
you know from theory is not perfect. Once you get it up and find that you
can work the world with a few watts and a wet noodle, then you can
take the time to learn, experiment, and find an antenna that meets
most of your needs. Don't go looking for the perfect one -- it does
not exist.

Irv VE6BP (Heating the ionosphere and loving it!)

question about wire antenna and tuner
 

Irv Finkleman wrote:
I'd still be looking for the perfect one.

Antennas are like females - just try them out,
one by one, until you are satisfied. :-)
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

"Owen Duffy" wrote in message
...
"Sal M. Onella" wrote in
:

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

Owen

What did I say wrong?

question about wire antenna and tuner
 

"Roy Lewallen" wrote in message
...
Owen Duffy wrote:
"Sal M. Onella" wrote in
:

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

Yes, this misconception will never die. Is it really worth the trouble
continually trying to contradict it?

Roy Lewallen, W7EL

What did I say wrong?

question about wire antenna and tuner
 

"Cecil Moore" wrote in message
. ..
Roy Lewallen wrote:
Yes, this misconception will never die. Is it really worth the trouble
continually trying to contradict it?

Not if all you do is trade one old-wives tale
for another.

It's hardly an old wives' tale.

I mistakenly put a 2m antenna on my dual band HT and tried to use it for a
short QSO on a nearby 440 repeater. The other ham said I was barely making
the repeater, while my poor HT got so hot that I could barely hold it after
a minute's use.

The antenna was wrong and the heat was real -- whatever the theory behind
it.

question about wire antenna and tuner
 

"Sal M. Onella" wrote in news:ptyXi.1068
:

What did I say wrong?

Hi Sal,

A recurring theme here is the myth that all the energy in reflected waves
on a transmission line from a mismatched antenna is carried all the way
back to the transmitter and necessarily dissipated as heat which is likely
to damage the PA.

It is no doubt an appealling explanation of why a PA may run hotter under
some circumstances, but it does not explain why for instance under some
circumstances, a PA may run cooler on a mismatched load.

Being appealing does not imply correctness of the explanation.

I will sit down now, and await the inevitable stream of anecodotal evidence
that doesn't and cannot support the generality of the statement.

Owen

question about wire antenna and tuner
 

"Sal M. Onella" wrote in
:

I mistakenly put a 2m antenna on my dual band HT and tried to use it
for a short QSO on a nearby 440 repeater. The other ham said I was
barely making the repeater, while my poor HT got so hot that I could
barely hold it after a minute's use.

The antenna was wrong and the heat was real -- whatever the theory
behind it.

Let the anecodotes flow...

Your FM HT is a classic case than can be adequately represented by a steady
state analysis. Your HT was operating into a load that increased its
dissipation, but there would be almost certainly be other mismatched loads
that would decrease its dissipation... but you wouldn't notice the event,
you would likely only remember the times the HT was too hot to handle.

I await the inevitable photon explanation.

Owen

question about wire antenna and tuner
 

Owen Duffy wrote:
"Sal M. Onella" wrote in
:

I mistakenly put a 2m antenna on my dual band HT and tried to use it
for a short QSO on a nearby 440 repeater. The other ham said I was
barely making the repeater, while my poor HT got so hot that I could
barely hold it after a minute's use.

The antenna was wrong and the heat was real -- whatever the theory
behind it.

Let the anecodotes flow...

Your FM HT is a classic case than can be adequately represented by a steady
state analysis. Your HT was operating into a load that increased its
dissipation, but there would be almost certainly be other mismatched loads
that would decrease its dissipation.

The transmitter gets hot because it is operating into an incorrect load
impedance, not the 50-ohm load for which it was designed. As far as the
transmitter is concerned, that is the only problem.

What caused that incorrect load impedance is a totally different topic.

If you measured the impedance of that incorrect antenna, and then
replaced the antenna with a dummy load of the same impedance (a resistor
of the correct value, in series with an inductor/capacitor of the
correct value) then your transmitter will not know the difference. The
same value of load impedance will cause it to behave in exactly the same
way.

There are many different physical types of loads that could present
exactly the same impedance to the transmitter. These include antennas,
dummy loads and various combinations, with or without some length of
transmission line involved. So long as the load impedance presented to
the transmitter is exactly the same in all cases, the transmitter
behaves exactly the same (once it has reached steady state, after the
first few cycles of RF... more about that later).

The amount of power that the transmitter can deliver into that incorrect
load will depend on the transmitter circuit and on the value of the load
impedance - but NOT on the physical type of load.

You can measure the impedance of the load by disconnecting it from the
transmitter and connecting it to an impedance meter. (Seems obvious?
Think again - every time you make an impedance measurement, you are
using the principle that impedances of the same value are
interchangeable with no effect on steady-state operation.) If the load
happens to be an antenna and transmission line, you can use programs
like NEC and established transmission line theory to make an accurate
prediction of the load impedance. If the system happens to include an
ATU, that is just another device that modifies the load impedance
presented to the transmitter.

At that point, you're finished with antennas, transmission lines and
ATUs - once you know the load impedance they present to the transmitter,
everything else depends on the transmitter alone.

In other words, the antenna/transmission-line/ATU system can - and
wherever possible, SHOULD - be cleanly separated from transmitter
design. The separation interface is the output connector at the rear of
the transmitter.

In the huge majority of applications, both amateur and professional, it
IS possible to separate those two topics cleanly and completely. It
seems perverse to tangle them together unnecessarily.


All the above refers to the steady state, where the signal level is
constant; and if a transmission line is involved, the pattern of
standing waves is established and unchanging. For completeness, we now
need to check if anything was different during the few moments after
switch-on, while the steady-state pattern of standing waves was becoming
established. Starting from switch-on, we need to look at each of the
successive reflections and re-reflections along the transmission line,
and see how the steady state came to be.

The first thing to notice is that with the types of signals and lengths
of transmission line that we amateurs use, the steady state is
established within the first few cycles of RF, ie it all happens over
timescales much shorter than the signal's own envelope rise/decay time.
This means it is 'nice to know', but will seldom be of practical
importance.

A detailed analysis of the buildup of reflections along a transmission
line will be forced to consider reflections at the transmitter as well
as at the load - in other words, we have to specify a reflection
coefficient at *both* ends of the line. Chipman's book [1] gives a very
detailed analysis of this, and shows how the addition of voltages over
multiple reflections gives rise to a standing wave. The amplitude of the
standing wave builds up as mathematical series, in which each successive
reflection and re-reflection contribute an additional term. Some terms
add to the total while others subtract, and each successive term makes a
smaller contribution than the one before, so the series will converge
towards a constant value which represents the steady state. It should be
absolutely no surprise that, when summed to an infinite number of terms,
this series produces exactly the same results as the steady-state model
- exactly the same pattern of standing waves, and exactly the same load
impedance presented to the transmitter.

The important conclusion from this more detailed time-dependent analysis
is that re-reflections at the transmitter have NO effect on the final
steady-state pattern of standing waves. The ONLY effect of
re-reflections at the transmitter end was on the time-dependent details
of how that pattern built up, and on the final steady-state signal
levels. The magnitude of the standing waves depends on the transmitter
characteristics (in other words, on the 'signal level') but the shape of
the standing waves and their location along the transmission line
depends only on the line and the load. There are no special cases he
the same conclusion holds for all values of reflection coefficient at
the transmitter end, including 1 and 0.

Thus, even a detailed time-dependent analysis confirms that, once we
have reached the steady state, we can indeed make a clean separation
between the transmitter and its load. And since we can, we should.



[1] R A Chipman, 'Theory and Problems of Transmission Lines, Schaum's
Outline Series', McGraw-Hill. ISBN 0-07-010747-5. (Chipman isn't an
easy read, because he is Mr Meticulous who wants to tell you everything;
but you can rely on him not to cut corners.)


I await the inevitable photon explanation.

None needed. If anyone wishes to introduce additional complications
where none are necessary, then of course they're at liberty to do so.
But when invited to join in, everyone else is at liberty to decline.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

question about wire antenna and tuner
 

Sal M. Onella wrote:

What did I say wrong?


As always, Ian has done a much better job than I could have, so there's
not much point in trying to repeat what he said.

But I did write up a lengthy essay several years ago, in response to the
same insistent rantings about reflecting waves of average power that's
still going on in this newsgroup, and it has some numerical examples
with a very simple circuit which illustrate the problems with what you
said. You can get it at http://eznec.com/misc/Food_for_thought.pdf. A
little past half way down is "Food for thought: Forward and reverse
power". If you're not interested in the math, scroll down a few
paragraphs from there to the table in Courier font. It and the text
below explain how it shows where the source dissipation is higher,
lower, and about the same when the load is matched, for three different
loads all having the same "reverse power". Another entry in the table is
an example where the "reverse power" equals the forward power (an
infinite SWR) yet the source dissipation is zero. People who believe
that "reflected power" ends up heating up the transmitter should take a
careful look at this, and see if they can explain it.

The established transmission line theory that's been well established
for over a century and that Ian, I, and countless others use daily for
solving real problems will, as shown in the example, tell us exactly how
much power is where and why. The "power is absorbed in the load" folks
can point to transmitters that get warm (sometimes) when working into
(some) mismatches. But they can never come up with a coherent reason for
the results shown in the essay table, or equations which will predict
just how much "reflected power" a transmitter will absorb and when. And
the reason is just as Ian said.

Unfortunately, some people, when presented clear evidence that the
concept is wrong, cling desperately to it nonetheless. For those,
explanations and evidence are a waste of time. But hopefully there are a
few readers out there who will see the problems in resolving their
theory with the evidence and redirect their thinking.

Roy Lewallen, W7EL

question about wire antenna and tuner
 

Sal M. Onella wrote:

It's hardly an old wives' tale.

I mistakenly put a 2m antenna on my dual band HT and tried to use it for a
short QSO on a nearby 440 repeater. The other ham said I was barely making
the repeater, while my poor HT got so hot that I could barely hold it after
a minute's use.

The antenna was wrong and the heat was real -- whatever the theory behind
it.

Once a black cat walked across the street in front of me. I had a
wonderful day! The wonderful day was real, whatever the theory behind
it. My basis for crediting the cat is just as valid as yours for
crediting "reflected energy" for the heating. And based on similar logic.

Roy Lewallen, W7EL

question about wire antenna and tuner
 

Roy Lewallen wrote:
But I did write up a lengthy essay several years ago, in response to the
same insistent rantings about reflecting waves of average power that's
still going on in this newsgroup, and it has some numerical examples
with a very simple circuit which illustrate the problems with what you
said. You can get it at http://eznec.com/misc/Food_for_thought.pdf.
===========================
Roy ,Tnx vy much for that ,much appreciated. ( it now sits in my
'Antenna' file)

Frank GM0CSZ / KN6WH

question about wire antenna and tuner
 

Sal M. Onella wrote:
What did I say wrong?

You implied that reflected energy is always dissipated
in the transmitter.

How much of the reflected energy is dissipated in the
source depends upon the interference pattern at the
source. As w7el points out, dissipation in a voltage
source can be reduced to zero by the astute choice of
a special best case of complete constructive
interference in the direction of the load.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Sal M. Onella wrote:
"Cecil Moore" wrote in message
. ..
Roy Lewallen wrote:
Yes, this misconception will never die. Is it really worth the trouble
continually trying to contradict it?
Not if all you do is trade one old-wives tale
for another.

It's hardly an old wives' tale.

1. The original old wives tale is that reflected power
*always* causes the source dissipation to increase,
even when it is a voltage source.

2. The subsequent old wives tale is that reflected
energy always ends up being radiated by the antenna.

Examples of decreased dissipation in the voltage source
in the presence of reflections proves #1 wrong.

Examples of increased dissipation in a current source
in the presence of identical reflections above proves
#2 wrong.

The power density in reflected waves is proportional to
ErefxHref and *CANNOT* be zero. Where that energy goes
obeys the conservation of energy principle. Anyone who
says all energy winds up being radiated by the antenna
is simply ignorant even if portraying himself as a guru.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Owen Duffy wrote:
It is no doubt an appealling explanation of why a PA may run hotter under
some circumstances, but it does not explain why for instance under some
circumstances, a PA may run cooler on a mismatched load.

What does explain it is the amount of destructive vs
constructive interference occurring at the source.
Assuming an unprotected source:

If the constructive interference is toward the
load, the source dissipation will decrease. If the
constructive interference is toward the source,
the source dissipation will increase. The conservation
of energy principle really does work to conserve the
ExH energy in a reflected wave.

Just because a special case results in zero dissipation
in a voltage source does not give us the permission
to make a magical leap of faith to "reflected power
doesn't exist" or "reflected power is *always*
dissipated in the load".
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Owen Duffy wrote:
Your FM HT is a classic case than can be adequately represented by a steady
state analysis. Your HT was operating into a load that increased its
dissipation, but there would be almost certainly be other mismatched loads
that would decrease its dissipation... but you wouldn't notice the event,
you would likely only remember the times the HT was too hot to handle.

If the constructive interference is toward the source,
it will run warm. If the constructive interference is
toward the antenna, the source will run cool. Antenna
tuners cause total destructive interference toward
the source and total constructive interference toward
the load. RF energy obeys the conservation of energy
principle.

I await the inevitable photon explanation.

None needed. Interference patterns work for voltages and
that's all one needs to figure out why the source is too
hot or cool. It's a no-brainer. If the energy is going
into the antenna and not being reflected, that energy
is not being dissipated in the source. If the energy is
not going into the antenna, it doesn't take a rocket
scientist to figure out where it must be going.
--
73, Cecil http://www.w5dxp.com