Dish reflector
 

Art Unwin wrote:
What if one put a diode in that ground line?

Then it would no longer be linear to RF. Seems
to me, it would generate some industrial grade
harmonics.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

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"Art Unwin" wrote in message
...

What if one put a diode in that ground line?

you thought you had noise before?? try putting in the diode and see what you
get!

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On Apr 19, 10:09*am, Richard Fry wrote:
On Apr 19, 8:09*am, Cecil Moore wrote: To make matters even worse: I had a similar problem
with drooping 1/4WL radials DC insulated from the tower.
The drooping radials coupled RF into the tower and
turned it into a radiator which screwed, oops, I
mean skewed the radiation pattern upwards.

________________

Some designs use drooping radials to reduce the vertical angle of the
peak radiation launched by the monopole section.

But that is a conclusion made for an infinite distance, with
consideration of the propagation environment on the intrinsic pattern
launched by the monopole, and the height of the monopole + its
elevated radials above the earth.

The link below leads to paste-up of NEC screens showing the
performance of a monopole driven against four 1/4-wave, essentially
horizontal radials. *The entire system is isolated from earth ground.

The driving impedance, the elevation pattern shape, and the peak gain
are close to "textbook" values for a 1/4-wave monopole driven against
a perfect ground plane.

A form of this design is being used with good success in the AM
broadcast industry -- where using a conventional, buried-radial ground
system is impractical due to rocky terrain.

The groundwave performance of these systems shows that their intrinsic
gain is maximum in the horizontal plane, and very close to the
theoretical value of 5.15 dBi.

http://i62.photobucket.com/albums/h8...WithElevatedRa...

RF

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Roy Lewallen wrote:
I measured current, which as everyone with a Novice or higher grade
license should know is the rate of flow of charge(*). The charge flows
in one direction during each half cycle, and in the other during the
other half cycle, resulting in current which is positive for half the
cycle and negative for the other.

Actually, electrons in a wire are slow-moving particles
and tend to oscillate back and forth at RF frequencies
rather than "flowing".

But what is being discussed here is the total current
reported by EZNEC. Is EZNEC wrong when it indicates
1 degree of current phase shift in 30 degrees of
length in a dipole antenna?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

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Cecil Moore wrote:
Roy Lewallen wrote:
I measured current, which as everyone with a Novice or higher grade
license should know is the rate of flow of charge(*). The charge flows
in one direction during each half cycle, and in the other during the
other half cycle, resulting in current which is positive for half the
cycle and negative for the other.

Actually, electrons in a wire are slow-moving particles
and tend to oscillate back and forth at RF frequencies
rather than "flowing".

But what is being discussed here is the total current
reported by EZNEC. Is EZNEC wrong when it indicates
1 degree of current phase shift in 30 degrees of
length in a dipole antenna?

What 30 degrees? There aren't any "30 degrees of length"
in a loading coil, and there doesn't have to be.
Cecil, repeating your fantasies over and over again don't
make them true.
73,
Tom Donaly, KA6RUH

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Tom Donaly wrote:
Cecil Moore wrote:
But what is being discussed here is the total current
reported by EZNEC. Is EZNEC wrong when it indicates
1 degree of current phase shift in 30 degrees of
length in a dipole antenna?

What 30 degrees? There aren't any "30 degrees of length"
in a loading coil, and there doesn't have to be.

I'm glad you asked, Tom. There was no mention of
a loading coil. I am talking about a simple 1/2WL
wire dipole with current probes at the x=1/3 and
y=2/3 points as illustrated. Why I am doing that
will become obvious.

------------------------fp-------x-------y--------

This is a center-fed 1/2WL dipole with current probes
installed at points 'x' and 'y'. The 1/2WL dipole is
known to be 180 degrees long. Half of a 1/2WL dipole
is 1/4WL, i.e. 90 degrees long. From the feedpoint
to point 'x' is 30 degrees. From point 'x' to point
'y' is 30 degrees. From point 'y' to the end of the
dipole is 30 degrees.

This 1/2WL dipole in EZNEC uses two wires of 90 segments
each, i.e. each segment equals one degree of dipole.
Point 'x' is at segment 30 and point 'y' is at segment
60 in Wire No. 2 on the right side of the dipole above.

Here are the results directly from EZNEC:

Source 1 Current = 1 A. at 0.0 deg.

Wire No. 2:
Segment Conn Magnitude (A.) Phase (Deg.)
30 'x' .87634 -1.49
60 'y' .52573 -2.43
90 Open .01185 -3.12

The phase of the current changes by 1.06 degrees between
point 'x' and point 'y' which is 30 degrees of antenna
*WIRE* (not loading coil). How can the phase of that current
possibly be used to determine the delay through the wire
which we know is related to the speed of light in the wire
medium? The delay through 30 degrees of wire at 4 MHz
would be about 20 nanoseconds.

In the 1/2WL wire dipole above, the phase of the current
in each 90 degrees of wire changes by 3.12 degrees.

If Roy performs the measurements, he will correctly report
a negligible phase shift in the current between point 'x'
and point 'y' (just as he did for the loading coil).

Following his previous loading coil logic, he will report
that the delay through 30 degrees of wire dipole is not
20 nS at 4 MHz as would be expected but is instead closer
to zero, maybe one or two nanoseconds. We all know that
report would be false. One cannot use a current with
essentially unchanging phase to calculate delay through
a wire (or through a loading coil).

If Roy cannot accurately measure the delay through
30 degrees of wire, why does anyone suppose Roy can
accurately measure the delay through a loading coil
using the phase of that same total current on a standing
wave antenna?

Note that the true phase information is contained in the
amplitude, not the phase, just as Gene Fuller said. If we
take the ARCCOSine of the magnitudes above, we obtain:

Source, ARCCOS(1.0) = 0 degrees
Seg 30, ARCCOS(0.87634) = 29 degrees
Seg 60, ARCCOS(0.52573) = 58 degrees
Seg 90, ARCCOS(0.01185) = 89 degrees

Incidentally, I told all of this to Roy 5 years ago,
Jan 2004, according to Google. He plonked me.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Dish reflector
 

Cecil Moore wrote:
Tom Donaly wrote:
Cecil Moore wrote:
But what is being discussed here is the total current
reported by EZNEC. Is EZNEC wrong when it indicates
1 degree of current phase shift in 30 degrees of
length in a dipole antenna?

What 30 degrees? There aren't any "30 degrees of length"
in a loading coil, and there doesn't have to be.

I'm glad you asked, Tom. There was no mention of
a loading coil. I am talking about a simple 1/2WL
wire dipole with current probes at the x=1/3 and
y=2/3 points as illustrated. Why I am doing that
will become obvious.

------------------------fp-------x-------y--------

This is a center-fed 1/2WL dipole with current probes
installed at points 'x' and 'y'. The 1/2WL dipole is
known to be 180 degrees long. Half of a 1/2WL dipole
is 1/4WL, i.e. 90 degrees long. From the feedpoint
to point 'x' is 30 degrees. From point 'x' to point
'y' is 30 degrees. From point 'y' to the end of the
dipole is 30 degrees.

This 1/2WL dipole in EZNEC uses two wires of 90 segments
each, i.e. each segment equals one degree of dipole.
Point 'x' is at segment 30 and point 'y' is at segment
60 in Wire No. 2 on the right side of the dipole above.

Here are the results directly from EZNEC:

Source 1 Current = 1 A. at 0.0 deg.

Wire No. 2:
Segment Conn Magnitude (A.) Phase (Deg.)
30 'x' .87634 -1.49
60 'y' .52573 -2.43
90 Open .01185 -3.12

The phase of the current changes by 1.06 degrees between
point 'x' and point 'y' which is 30 degrees of antenna
*WIRE* (not loading coil). How can the phase of that current
possibly be used to determine the delay through the wire
which we know is related to the speed of light in the wire
medium? The delay through 30 degrees of wire at 4 MHz
would be about 20 nanoseconds.

In the 1/2WL wire dipole above, the phase of the current
in each 90 degrees of wire changes by 3.12 degrees.

If Roy performs the measurements, he will correctly report
a negligible phase shift in the current between point 'x'
and point 'y' (just as he did for the loading coil).

Following his previous loading coil logic, he will report
that the delay through 30 degrees of wire dipole is not
20 nS at 4 MHz as would be expected but is instead closer
to zero, maybe one or two nanoseconds. We all know that
report would be false. One cannot use a current with
essentially unchanging phase to calculate delay through
a wire (or through a loading coil).

If Roy cannot accurately measure the delay through
30 degrees of wire, why does anyone suppose Roy can
accurately measure the delay through a loading coil
using the phase of that same total current on a standing
wave antenna?

Note that the true phase information is contained in the
amplitude, not the phase, just as Gene Fuller said. If we
take the ARCCOSine of the magnitudes above, we obtain:

Source, ARCCOS(1.0) = 0 degrees
Seg 30, ARCCOS(0.87634) = 29 degrees
Seg 60, ARCCOS(0.52573) = 58 degrees
Seg 90, ARCCOS(0.01185) = 89 degrees

Incidentally, I told all of this to Roy 5 years ago,
Jan 2004, according to Google. He plonked me.

I don't blame him for plonking you. You're saying that because you
fantasized that Roy would make a mistake that Roy would never make,
that he also made the same mistake when measuring the delay through a
coil. Cecil, a length of antenna is not a coil. A coil is not an
antenna. Declaring that coils are antennas and vice versa doesn't make
them so. You don't really know what the delay through your bugcatcher
coil is. If you substituted a real transmission line for your coil,
you could make the degree length - within limits - whatever you wanted
it to be just by changing the Z0 of the transmission line.

So here's your logic: Because EZNEC reports a amall angular difference
at the ends of your half-wave antenna in current, and because Roy
measured a small difference in delay through a coil, there must be a
larger real delay across the coil due to the analogy with the half wave
antenna. You're assuming, without proof, that the coil behaves as a
piece of straight wire, therefore the coil behaves as a piece of
straight wire. Nice logic. You have a lot in common with Art.
73,
Tom Donaly, KA6RUH

Dish reflector
 

On Apr 20, 2:29*pm, "Tom Donaly" wrote:
Cecil Moore wrote:
Tom Donaly wrote:
Cecil Moore wrote:
But what is being discussed here is the total current
reported by EZNEC. Is EZNEC wrong when it indicates
1 degree of current phase shift in 30 degrees of
length in a dipole antenna?

What 30 degrees? There aren't any "30 degrees of length"
in a loading coil, and there doesn't have to be.

I'm glad you asked, Tom. There was no mention of
a loading coil. I am talking about a simple 1/2WL
wire dipole with current probes at the x=1/3 and
y=2/3 points as illustrated. Why I am doing that
will become obvious.

------------------------fp-------x-------y--------

This is a center-fed 1/2WL dipole with current probes
installed at points 'x' and 'y'. The 1/2WL dipole is
known to be 180 degrees long. Half of a 1/2WL dipole
is 1/4WL, i.e. 90 degrees long. From the feedpoint
to point 'x' is 30 degrees. From point 'x' to point
'y' is 30 degrees. From point 'y' to the end of the
dipole is 30 degrees.

This 1/2WL dipole in EZNEC uses two wires of 90 segments
each, i.e. each segment equals one degree of dipole.
Point 'x' is at segment 30 and point 'y' is at segment
60 in Wire No. 2 on the right side of the dipole above.

Here are the results directly from EZNEC:

Source 1 * * *Current = 1 A. at 0.0 deg.

Wire No. 2:
Segment *Conn * * *Magnitude (A.) *Phase (Deg.)
30 * * * 'x' * * * *.87634 * * * * *-1.49
60 * * * 'y' * * * *.52573 * * * * *-2.43
90 * * * Open * * * .01185 * * * * *-3.12

The phase of the current changes by 1.06 degrees between
point 'x' and point 'y' which is 30 degrees of antenna
*WIRE* (not loading coil). How can the phase of that current
possibly be used to determine the delay through the wire
which we know is related to the speed of light in the wire
medium? The delay through 30 degrees of wire at 4 MHz
would be about 20 nanoseconds.

In the 1/2WL wire dipole above, the phase of the current
in each 90 degrees of wire changes by 3.12 degrees.

If Roy performs the measurements, he will correctly report
a negligible phase shift in the current between point 'x'
and point 'y' (just as he did for the loading coil).

Following his previous loading coil logic, he will report
that the delay through 30 degrees of wire dipole is not
20 nS at 4 MHz as would be expected but is instead closer
to zero, maybe one or two nanoseconds. We all know that
report would be false. One cannot use a current with
essentially unchanging phase to calculate delay through
a wire (or through a loading coil).

If Roy cannot accurately measure the delay through
30 degrees of wire, why does anyone suppose Roy can
accurately measure the delay through a loading coil
using the phase of that same total current on a standing
wave antenna?

Note that the true phase information is contained in the
amplitude, not the phase, just as Gene Fuller said. If we
take the ARCCOSine of the magnitudes above, we obtain:

Source, ARCCOS(1.0) * * = *0 degrees
Seg 30, ARCCOS(0.87634) = 29 degrees
Seg 60, ARCCOS(0.52573) = 58 degrees
Seg 90, ARCCOS(0.01185) = 89 degrees

Incidentally, I told all of this to Roy 5 years ago,
Jan 2004, according to Google. He plonked me.

I don't blame him for plonking you. You're saying that because you
fantasized that Roy would make a mistake that Roy would never make,
that he also made the same mistake when measuring the delay through a
coil. Cecil, a length of antenna is not a coil. A coil is not an
antenna. Declaring that coils are antennas and vice versa doesn't make
them so. You don't really know what the delay through your bugcatcher
coil is. If you substituted a real transmission line for your coil,
you could make the degree length - within limits - whatever you wanted
it to be just by changing the Z0 of the transmission line.

So here's your logic: Because EZNEC reports a amall angular difference
at the ends of your half-wave antenna in current, and because Roy
measured a small difference in delay through a coil, there must be a
larger real delay across the coil due to the analogy with the half wave
antenna. You're assuming, without proof, that the coil behaves as a
piece of straight wire, therefore the coil behaves as a piece of
straight wire. Nice logic. You have a lot in common with Art.
73,
Tom Donaly, KA6RUH

Well Tom I am not part of this debate but to say a coil is not a
radiator is silly It must radiate as does a helix antenna. The only
difference is how much slower the helix forces the charge to delay as
in "slow wave." Thus the coil act as a radiator where you must
multiply it by a velocity factor. After all, a "tesla" style coil will
display a resonance with the wire used much longer than a straight
wire length and like a helix will radiate.
Kraus states that for a helix one should not use wire shorter than two
wavelength
which I suspect is a substitute calculation for the VF change from a
straight radiator.

Ar

Dish reflector
 

Tom Donaly wrote:
I don't blame him for plonking you. You're saying that because you
fantasized that Roy would make a mistake that Roy would never make,
that he also made the same mistake when measuring the delay through a
coil.

Sorry Tom, that is a diversion. The subject is NOT the delay
through a coil. The present subject is the delay through a
straight wire which is well understood. Please deal with the
topic at hand. If you refuse, we will know that you are not
sincere as far as technical facts are concerned.

Please ask Roy to prove that the current on a standing wave
antenna can be used to measure the delay through a straight
piece of wire that is x degrees long. If so, exactly how is
it done?

Roy is NOT omniscient. He definitely made the mistake but
like most gurus, refuses to admit it. You want to sweep the
mistake under the rug through diversions but I won't allow
you to do that. Once you and Roy admit that the current on
a standing wave antenna cannot be used to calculate delay,
everything else will become clear.

Please feel free to contact Roy by private email to resolve
the issue. Roy has, so far, simply stuck his head in the
sandbags and refused to respond. I'm sure he would have
advised you to plonk me instead of engaging me, for fear
of being proved wrong.

Yet, he admitted years ago that the phase of current in a
standing wave antenna varies by a very small amount. He
is presently trying to have his cake and eat it too. In
the process, he (and you as a supporter) are hoodwinking
the unwashed masses. Shame on all of you.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Dish reflector
 

Art Unwin wrote:
Kraus states that for a helix one should not use wire shorter than two
wavelength which I suspect is a substitute calculation for the VF
change from a straight radiator.

Because of adjacent coil coupling, it takes
more wire to achieve the phase shift effect
of a straight wire.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Dish reflector
 

On Apr 20, 4:07*pm, Cecil Moore wrote:
Art Unwin wrote:
Kraus states that for a helix one should not use wire shorter than two
wavelength which I suspect is a substitute calculation for the VF
change from a straight radiator.

Because of adjacent coil coupling, it takes
more wire to achieve the phase shift effect
of a straight wire.
--
73, Cecil, IEEE, OOTC, *http://www.w5dxp.com

Which is what I have always maintained, lumped loads are not included
in Maxwell's
equations .
Art

Dish reflector
 

"Art Unwin" wrote in message
...
Which is what I have always maintained, lumped loads are not included
in Maxwell's equations .

CORRECT! he got one right! must be dumb luck.

you are absolutely correct, there are no terms in maxwell's equations
representing lumped loads. HOWEVER, you can use maxwell's equations to
derive the fields that explain how lumped elements work.

Dish reflector
 

On Apr 20, 6:06*pm, "Dave" wrote:
"Art Unwin" wrote in message

...

Which is what I have always maintained, lumped loads are not included
in Maxwell's equations .

CORRECT! *he got one right! *must be dumb luck.

you are absolutely correct, there are no terms in maxwell's equations
representing lumped loads. *HOWEVER, you can use maxwell's equations to
derive the fields that explain how lumped elements work.

At the expense of efficiency per unit length

Dish reflector
 

Cecil Moore wrote:
Tom Donaly wrote:
I don't blame him for plonking you. You're saying that because you
fantasized that Roy would make a mistake that Roy would never make,
that he also made the same mistake when measuring the delay through a
coil.

Sorry Tom, that is a diversion. The subject is NOT the delay
through a coil. The present subject is the delay through a
straight wire which is well understood. Please deal with the
topic at hand. If you refuse, we will know that you are not
sincere as far as technical facts are concerned.

Please ask Roy to prove that the current on a standing wave
antenna can be used to measure the delay through a straight
piece of wire that is x degrees long. If so, exactly how is
it done?

Roy is NOT omniscient. He definitely made the mistake but
like most gurus, refuses to admit it. You want to sweep the
mistake under the rug through diversions but I won't allow
you to do that. Once you and Roy admit that the current on
a standing wave antenna cannot be used to calculate delay,
everything else will become clear.

Please feel free to contact Roy by private email to resolve
the issue. Roy has, so far, simply stuck his head in the
sandbags and refused to respond. I'm sure he would have
advised you to plonk me instead of engaging me, for fear
of being proved wrong.

Yet, he admitted years ago that the phase of current in a
standing wave antenna varies by a very small amount. He
is presently trying to have his cake and eat it too. In
the process, he (and you as a supporter) are hoodwinking
the unwashed masses. Shame on all of you.

No, it's not a diversion. You're making up things in your head.
The original controversy involved a claim by you that the coil in
a short, mobile antenna made up for the degrees lost in said
shortened antenna. You were wrong. Now you've changed the subject to
a half wave dipole, attributing to Roy a position he would never take.
That's an old, stupid trick a woman might use in a domestic argument,
but it won't work here. I know you have a pathological need to
win every argument (you ought to talk that over with your analyst) but
that's no reason anyone should waste time agreeing with you.
73,
Tom Donaly, KA6RUH

Dish reflector
 

Cecil Moore wrote:

Roy is NOT omniscient. He definitely made the mistake but
like most gurus, refuses to admit it. You want to sweep the
mistake under the rug through diversions but I won't allow
you to do that.

Roy has, so far, simply stuck his head in the
sandbags and refused to respond.

He
is presently trying to have his cake and eat it too. In
the process, he (and you as a supporter) are hoodwinking
the unwashed masses. Shame on all of you.

Anybody get the feeling that Cecil and Art might be the same guy? :-)

http://www.8notes.com/school/riffs/c..._and_hardy.asp

ac6xg

Dish reflector
 

Art Unwin wrote:
I made a helical end fed antenna that is inside a cone shaped
reflector
The reflector is made from 1/2" mesh steel with an aluminum foil liner
and connected to the braid of the feed coax. No baluns are used, just
direct connections.
I was surprised to hear signals from the rear!
I thought that a dish reflector prevented such signals getting to the
receiver. So what can be wrong with the reflector or can signals get
reflected back from the frontal area? Antenna is at a 40 foot height
Any ideas as to what the fault could be?
Regards
Art
I have no experience with dishes thus the question Note, the helical
antenna does not protrude beyond the dish envelope.
Art

What's the relative size of "reflector" and helix? (i.e. is the
reflector in the near field of the helix, in which case, you could
easily have waves propagating along the surface of the reflector)

Dish reflector
 

On Apr 20, 7:16*pm, Jim Kelley wrote:
Cecil Moore wrote:

Roy is NOT omniscient. He definitely made the mistake but
like most gurus, refuses to admit it. You want to sweep the
mistake under the rug through diversions but I won't allow
you to do that.
Roy has, so far, simply stuck his head in the
sandbags and refused to respond.
He
is presently trying to have his cake and eat it too. In
the process, he (and you as a supporter) are hoodwinking
the unwashed masses. Shame on all of you.

Anybody get the feeling that Cecil and Art might be the same guy? *:-)

http://www.8notes.com/school/riffs/c..._and_hardy.asp

ac6xg

Very good but he rides a motor bike and I don't ride those vehicles of
death.
Cecil is correct tho, The lumped circuit represents the radiator
length times the velocity factor plus a fudge factor for being a
lumped load.
Art

Dish reflector
 

On Apr 20, 7:28*pm, Jim Lux wrote:
Art Unwin wrote:
I made a helical end fed antenna that is inside a cone shaped
reflector
The reflector is made from 1/2" mesh steel with an aluminum foil liner
and connected to the braid of the feed coax. No baluns are used, just
direct connections.
*I was surprised to hear signals from the rear!
*I thought that a dish reflector prevented such signals getting to the
receiver. So what can be wrong with the reflector or can signals get
reflected back from the frontal area? Antenna is at a 40 foot height
Any ideas as to what the fault could be?
Regards
Art
I have no experience with dishes thus the question Note, the helical
antenna does not protrude beyond the dish envelope.
Art

What's the relative size of "reflector" and helix? *(i.e. is the
reflector in the near field of the helix, in which case, you could
easily have waves propagating along the surface of the reflector)

The helix is four foot long and a foot diameter. The base of the
reflector is 1.5 feet
with a 45 degree angle. I have had the helix 0.5 feet shorter and 0.5
feet longer with similar results.On re examination of the antenn I now
see that the ground lead of the radiator is connected to the inside of
the reflector at a half way point and the coax
ground is connected at the base of the reflector. I think I will
change that ground connection to a common point.
Regards
Art

Loading coils: was Dish reflector
 

Tom Donaly wrote:

I'm going to break my reply up into two pieces. First I
will address the actual number of degrees occupied by
a loading coil.

No, it's not a diversion. You're making up things in your head.
The original controversy involved a claim by you that the coil in
a short, mobile antenna made up for the degrees lost in said
shortened antenna.

Sorry Tom, that is a false statement. Please stop misquoting
me. The coil occupies some number of degrees but not nearly
enough to make up for all of the "lost" degrees which are not
lost at all as I have demonstrated in the past and will do so
again here. Following is a *resonant open-circuit 1/4WL stub*
that is electrically 90 degrees long yet it is only physically
38 degrees long.

Z1
---19 deg 450 ohm feedline---+---19 deg 50 ohm feedline---open
-j145

The 450 ohm feedline occupies 19 degrees of the stub. The 50
ohm feedline occupies 19 degrees of the stub. The stub is
physically 38 degrees long total. It needs another 52 degrees
to make it electrically 1/4WL long and resonant. The "lost"
52 degrees is *not lost at all* and occurs abruptly at the
junction point '+'. Call the impedance at that point Z1. The
52 degrees of phase shift occurs between Z1/450 and Z1/50.
Microsmith says that Z1 = -j145.

Z1/450 = -j145/450 = -j0.3222

Z1/50 = -j145/50 = -j2.9

Take a look at the number of degrees between -j0.3222 and
-j2.9 on a Smith Chart. Surprise! There is the "lost" 52
degrees. Those degrees are not lost at all and are just
a fact of physics concerning phase shifts at an impedance
discontinuity.

Now if we multiply the stub impedances by 10, we have
a reasonable facsimile of a resonant base-loaded monopole.

19 deg coil
///////////////-----19 deg ~500 ohm stinger-----open
Z0= ~4500 ohms
VF= ~0.02

The loading coil occupies 19 degrees and the stinger
occupies 19 degrees. There is a 52 degree phase shift
at the coil to stinger junction. There are no "lost"
degrees. 19+52+19 = 90 degrees.

There were (are) two sides to the argument.

1. The coil furnishes the "lost" degrees.
FALSE!
The coil furnishes some number of degrees but not
nearly enough to make up for the phase shift at
the coil/stinger junction.

2. The coil supplies almost zero degrees.
FALSE!
The phase shift at the coil/stinger junction is not
enough to account for the "lost" degrees. The magnitude
of that phase shift is easily calculated on a Smith Chart.

Please skip the ad hominem attacks and use the laws
of physics and mathematics to prove me wrong.
--
73, Cecil, w5dxp.com

Dish reflector
 

Tom Donaly wrote:
You were wrong. Now you've changed the subject to
a half wave dipole, attributing to Roy a position he would never take.

On the contrary, Roy described his procedure in detail. I
then applied Roy's exact procedure to a 1/2WL dipole to see
if the procedure is valid for finding the delay through a
straight wire. Just as I suspected, the change in the phase
of the current is mostly unrelated to the number of degrees
in the antenna wire. Therefore, Roy's procedure is invalid and
cannot be used to measure the delay through a loading coil.

He made very accurate, very meaningless measurements - as did
w8ji. The primary current on a monopole, loaded or not, is of
the form I = Imax*cos(kx)*cos(wt). The amplitude is solely
a function of kx. The phase is solely a function of wt.
At any instant of time, the phase is the same all up and down
the wire including through the loading coil. Roy once verified
that is what EZNEC reports.

So the question remains: How did Roy use the current on a
standing-wave antenna, which doesn't change phase relative to
any other point on the entire antenna, to calculate the delay
through a loading coil or through a wire? The phase at the
bottom of the coil and the phase at the top of the coil are
always the same no matter what the delay through the coil.
Those phases are the same as the feedpoint phase and the phase
close to the tip top of the antenna within a very few degrees.

That's an old, stupid trick a woman might use in a domestic argument,
but it won't work here. I know you have a pathological need to
win every argument (you ought to talk that over with your analyst) but
that's no reason anyone should waste time agreeing with you.

Hurling ad hominem attacks will not help you in a technical
argument, Tom. Please use electronic theory and mathematics
to prove me wrong.

Will Rogers said, "Be sure you are right and then go on ahead."
I'm sure I am right.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Dish reflector
 

Jim Kelley wrote:
Anybody get the feeling that Cecil and Art might be the same guy? :-)

A humorous diversion instead of a technical argument -
usually the sign that one realizes that one is wrong.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Loading coils: was Dish reflector
 

Cecil Moore wrote:

For anyone interested in an in-depth look at the
subject of loading coils, here is an excellent
reference.

http://www.g3ynh.info/zdocs/magnetics/part_1.html

"When modeling and using inductive devices, it is
important to be aware that the concept of lumped
inductance is only strictly applicable at low
frequencies."

"In the high-frequency region, it is no longer possible
to treat the coil as though its reactance is purely
inductive; the reason being that a wave emerging from
the coil is now significantly delayed, and therefore
has a phase which differs from its phase on entry."
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Loading coils: was Dish reflector
 

Cecil Moore wrote:
Tom Donaly wrote:

I'm going to break my reply up into two pieces. First I
will address the actual number of degrees occupied by
a loading coil.

No, it's not a diversion. You're making up things in your head.
The original controversy involved a claim by you that the coil in
a short, mobile antenna made up for the degrees lost in said
shortened antenna.

Sorry Tom, that is a false statement. Please stop misquoting
me. The coil occupies some number of degrees but not nearly
enough to make up for all of the "lost" degrees which are not
lost at all as I have demonstrated in the past and will do so
again here. Following is a *resonant open-circuit 1/4WL stub*
that is electrically 90 degrees long yet it is only physically
38 degrees long.

Z1
---19 deg 450 ohm feedline---+---19 deg 50 ohm feedline---open
-j145

The 450 ohm feedline occupies 19 degrees of the stub. The 50
ohm feedline occupies 19 degrees of the stub. The stub is
physically 38 degrees long total. It needs another 52 degrees
to make it electrically 1/4WL long and resonant. The "lost"
52 degrees is *not lost at all* and occurs abruptly at the
junction point '+'. Call the impedance at that point Z1. The
52 degrees of phase shift occurs between Z1/450 and Z1/50.
Microsmith says that Z1 = -j145.

Z1/450 = -j145/450 = -j0.3222

Z1/50 = -j145/50 = -j2.9

Take a look at the number of degrees between -j0.3222 and
-j2.9 on a Smith Chart. Surprise! There is the "lost" 52
degrees. Those degrees are not lost at all and are just
a fact of physics concerning phase shifts at an impedance
discontinuity.

Now if we multiply the stub impedances by 10, we have
a reasonable facsimile of a resonant base-loaded monopole.

19 deg coil
///////////////-----19 deg ~500 ohm stinger-----open
Z0= ~4500 ohms
VF= ~0.02

The loading coil occupies 19 degrees and the stinger
occupies 19 degrees. There is a 52 degree phase shift
at the coil to stinger junction. There are no "lost"
degrees. 19+52+19 = 90 degrees.

There were (are) two sides to the argument.

1. The coil furnishes the "lost" degrees.
FALSE!
The coil furnishes some number of degrees but not
nearly enough to make up for the phase shift at
the coil/stinger junction.

2. The coil supplies almost zero degrees.
FALSE!
The phase shift at the coil/stinger junction is not
enough to account for the "lost" degrees. The magnitude
of that phase shift is easily calculated on a Smith Chart.

Please skip the ad hominem attacks and use the laws
of physics and mathematics to prove me wrong.
--
73, Cecil, w5dxp.com

I don't have to prove you wrong, Cecil, you have to prove yourself
right since you came up with this novel way of explaining antenna
behavior. A false analogy won't prove you right, in any case. Anyway,
this has all been chewed over before, and you've already used your hick
style argumentative techniques to little avail. It's too bad some
amateurs take you seriously enough to believe this garbage. They'd do
a lot better, and know a lot more if they'd learn the techniques and
mathematics found in innumerable books on the subject.
73,
Tom Donaly, KA6RUH

Dish reflector
 

Cecil Moore wrote:
Jim Kelley wrote:
Anybody get the feeling that Cecil and Art might be the same guy? :-)

A humorous diversion instead of a technical argument -
usually the sign that one realizes that one is wrong.

In this case it's a sign that you are wrong - and humorless. :-)

ac6xg

Loading coils: was Dish reflector
 

Tom Donaly wrote:
I don't have to prove you wrong, Cecil, you have to prove yourself
right since you came up with this novel way of explaining antenna
behavior.

I have offered a proof with which I detect no technical
problems and nobody has offered any valid technical argument
against what I have presented. My argument is not novel
and is based on sound physics as presented by the technical
references I have provided.

What I find difficult to understand is the sandbagging
going on in defense of an old wives' tale.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Dish reflector
 

Jim Kelley wrote:
In this case it's a sign that you are wrong -

Jim, please feel free to offer some valid technical
proof that what I have presented is wrong. Most of
what I have presented is from my college textbook,
"Fields and Waves ...", by Ramo and Whinnery.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Loading coils: was Dish reflector
 

Cecil Moore wrote:

What I find difficult to understand is the sandbagging
going on in defense of an old wives' tale.

Your description of the phenomenon is exactly that. Your claims about
standing wave current are unadulterated bull crap. Your understanding
of wave phenomena is significantly flawed in certain respects. You
refuse to recognize where you have erred, and you fend off criticism by
making ludicrous accusations of other people. With all due respect your
behavior is absolutely pathological, which unfortunately, tend to negate
the value in any valid arguments you might otherwise make.

Although some people do occasionally attempt to correct you where you
have made a mistake (others have given up trying), they are not 'out to
get you'. Try to keep it all real and in perspective, OM.

jk ac6xg

Loading coils: was Dish reflector
 

Jim Kelley wrote:
Your claims about
standing wave current are unadulterated bull crap.

You are certainly free to produce the physics and
mathematics to prove your assertion. Where is it?

I have provided equations and references. Please
tell me exactly which ones you dispute so I can
quote them.

Although some people do occasionally attempt to correct you where you
have made a mistake ...

The only mistakes of which I have been accused
are poor choices of words to which I plead guilty.
Nobody has accused me of invalid equations.

What you are experiencing is the dumbing down of
technical people where the lumped circuit model
and "mashed potatoes" model of energy in a transmission
line has taken over.

The equation for standing waves has been quoted
from "Optics", by Hecht; "... Optics", by Born and
Wolf, "Fields and Waves ...", by Ramo and Whinnery,
"Antennas ...", by Kraus, and "Antenna Theory", by
Balanis.

I strongly suspect you are capable of understanding
those references.

The following two equations are equivalent and are
the equations for pure standing wave current as
exists as the primary current on standing wave
antennas.

I(x,t) = 2(V+/Z0)cos(kx)*cos(wt)

I(x,t) = (V+/Z0)[e^(jwt-kx) - e^(jwt-kx)]

If you cannot look at those equations and see that
the phase is unchanging relative to all points on
the wire, you need to go back to school and
hone your math skills.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Loading coils: was Dish reflector
 

Cecil Moore wrote:
Tom Donaly wrote:

No, it's not a diversion. You're making up things in your head.
The original controversy involved a claim by you that the coil in
a short, mobile antenna made up for the degrees lost in said
shortened antenna.

Sorry Tom, that is a false statement. Please stop misquoting
me.

I have the same recollection as Tom.

The loading coil occupies 19 degrees and the stinger
occupies 19 degrees. There is a 52 degree phase shift
at the coil to stinger junction. There are no "lost"
degrees. 19+52+19 = 90 degrees.

There were (are) two sides to the argument.

1. The coil furnishes the "lost" degrees.
FALSE!
The coil furnishes some number of degrees but not
nearly enough to make up for the phase shift at
the coil/stinger junction.

2. The coil supplies almost zero degrees.
FALSE!
The phase shift at the coil/stinger junction is not
enough to account for the "lost" degrees. The magnitude
of that phase shift is easily calculated on a Smith Chart.

Or, maybe

3. A less than quarter wave antenna is less than 90 degrees long.

ac6xg

TEST
 

TEST

Loading coils: was Dish reflector
 

As a newcomer to the group I'm hesitant to join a discussion which has
been running for almost 200 postings, and where the protagonists
understand the topic in much greater depth than I do. But here
goes ....

My starting assumption is that EZNEC can model a helical inductor
reasonably accurately, with the exception of the increase in AC
resitance caused by proximity effects.

If I take an EZNEC model of a coil - 40 turns #14 wire, 6" diameter,
12" long - I discover it has a characteristic impedance of about 2550
ohms at a self-resonant frequency of around 6.1 MHz. If I use it as
the base loading coil for a short vertical antenna with a 6ft whip
above it, I notice that EZNEC shows a difference in the current at the
top of the coil compared with the bottom of about 0.69:1, and a
resonant frequency of 3.79MHz.

I then look to see which of the various models might reasonably
predict the values observed in the EZNEC modelling.

Clearly, a simple lumped-element inductor doesn't get close. I've read
various web pages and postings which argue qualitatively that things
like "distributed capacitance" might explain some of the observations,
but as yet I've seen no quantitative analysis which attempts to
predict the numbers.

In contrast, I look at the work of Corum & Corum and of G3YNH who
insist that "coils are best regarded as transmission lines", and I get
quantitative results which closely match the EZNEC results. For my
example coil, I get a self resonant frequency of 6.3MHz (cf 6.1MHz),
a characteristic impedance of 2792 ohms (cf 2550 ohms) and an Iout/Iin
ratio of 0.72 (cf 0.69)

Not only that, the transmission line model predicts an inductive
reactance very close to that needed for antenna resonance at 3.79 MHz

I'm a simple soul, and I don't pretend to understand all the maths
involved; I merely observe that the transmission line approach
delivers "hard numbers" that closely match those predicted by EZNEC.
I've yet to see another model get close. So, until I do, I guess I
have to favour the approach of Corum & Corum, G3YNH et al.

If someone can show me similarly accurate results from an approach
based on a lumped-element model, I'd be interested to see them.

Steve G3TXQ

Loading coils: was Dish reflector
 

Jim Kelley wrote:

I have the same recollection as Tom.

If you do, it was from many years ago when I was young
and foolish. :-) For the past 5 years, at least, I have
been telling everyone that both sides of the argument
are wrong as rail-arguments usually are. The facts lie
somewhere in between the two rails.

Or, maybe
3. A less than quarter wave antenna is less than 90 degrees long.

Obviously true for the physical length. Just as obviously
impossible for the electrical length. If you understand
that the feedpoint is purely resistive and
Zfp = (Vfor-Vref)/(Ifor+Iref) then you will understand
that the antenna *must* be electrically an interger
multiple of 90 degrees long.

If you need help with that concept, let me know. If you
are embarrassed to discuss it in public, send me an email.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Loading coils: was Dish reflector
 

steveeh131047 wrote:
As a newcomer to the group I'm hesitant to join a discussion which has
been running for almost 200 postings, and where the protagonists
understand the topic in much greater depth than I do. But here
goes ....

My starting assumption is that EZNEC can model a helical inductor
reasonably accurately, with the exception of the increase in AC
resitance caused by proximity effects.

Yes, that's correct. Fortunately, proximity effect is generally
negligible unless the turn spacing is very close.

If I take an EZNEC model of a coil - 40 turns #14 wire, 6" diameter,
12" long - I discover it has a characteristic impedance of about 2550
ohms at a self-resonant frequency of around 6.1 MHz.

A single conductor doesn't have a characteristic impedance -- it's the
impedance between the two conductors of a transmission line. You can
measure a characteristic impedance between, say, a coil and ground, but
its value depends on the spacing between the two. If the coil is tilted
with respect to the ground, the impedance of this two-conductor system
will change with the position along the coil.

If I use it as
the base loading coil for a short vertical antenna with a 6ft whip
above it, I notice that EZNEC shows a difference in the current at the
top of the coil compared with the bottom of about 0.69:1, and a
resonant frequency of 3.79MHz.

I then look to see which of the various models might reasonably
predict the values observed in the EZNEC modelling.

Clearly, a simple lumped-element inductor doesn't get close. I've read
various web pages and postings which argue qualitatively that things
like "distributed capacitance" might explain some of the observations,
but as yet I've seen no quantitative analysis which attempts to
predict the numbers.

It's difficult or impossible to do with lumped elements. A vertical
loading coil has not only series inductance, but also capacitance to
ground or, in the case of a dipole, to the other half of the dipole.
This capacitance varies along the coil, being greatest at the bottom and
increasing toward the top. (This is the cause of the varying Z0 I
mentioned above.) But there's also a delay associated with the
capacitance which complicates the interaction to the point where you
can't easily model it with lumped elements. And the coil radiates, which
alters its current distribution.

That said, a lumped inductor makes a fairly decent model for a
physically very small (in terms of wavelength) toroidal loading coil,
since it has minimal capacitance to ground and a minimal amount of
radiation. I actually built a vertical, loaded it with one, and made
careful measurements which I posted on this newsgroup several years ago.
Cecil is still complaining about it.

The displacement current flowing through those capacitances, not some
"effective degrees of antenna" phenomenon, is what causes the current
along a solenoidal loading coil to vary. If you reduce the capacitances
to a low value as I did in my measurement, the currents at the ends
become nearly the same, which is what the measurement showed.

In contrast, I look at the work of Corum & Corum and of G3YNH who
insist that "coils are best regarded as transmission lines", and I get
quantitative results which closely match the EZNEC results. For my
example coil, I get a self resonant frequency of 6.3MHz (cf 6.1MHz),
a characteristic impedance of 2792 ohms (cf 2550 ohms) and an Iout/Iin
ratio of 0.72 (cf 0.69)

Not only that, the transmission line model predicts an inductive
reactance very close to that needed for antenna resonance at 3.79 MHz

You've kind of lost me here, since I can't see how you've replaced a
two-terminal coil with a four-terminal transmission line. And a
transmission line doesn't radiate, so that sometimes-important property
of a solenoidal coil is ignored.

I'm a simple soul, and I don't pretend to understand all the maths
involved; I merely observe that the transmission line approach
delivers "hard numbers" that closely match those predicted by EZNEC.
I've yet to see another model get close. So, until I do, I guess I
have to favour the approach of Corum & Corum, G3YNH et al.

Be sure to test the approach with other configurations, such as longer
and shorter coils, frequencies well away from resonance, etc. to find
the limits of applicability of the approach. Does it correctly predict
the field strength? Efficiency? Bandwidth?

If someone can show me similarly accurate results from an approach
based on a lumped-element model, I'd be interested to see them.

Me, too. The thing which prompted me to add the automated helix
generation feature to EZNEC was the realization that lumped loads so
often did a poor job of simulating solenoidal loading inductors.

Roy Lewallen, W7EL

Dish reflector
 

"Art Unwin" wrote in message
...

At the expense of efficiency per unit length

and what does that refer to?? i don't think i've ever heard of something
with those units... they really don't make any sense.

Loading coils: was Dish reflector
 

Roy Lewallen wrote:
A single conductor doesn't have a characteristic impedance --

On the contrary, that is a false statement. In my
"Electronic Equations Handbook", it gives the
characteristic impedance for a single horizontal
wire about ground. Obviously, ground is the missing
conductor. I believe that equation is also given in
ARRL publications. A horizontal #14 wire 30 feet
above ground has a characteristic impedance very
close to 600 ohms. Since all of our antennas are
located a finite distance from ground, your assertion
seems ridiculous.

I actually built a vertical, loaded it with one, and made
careful measurements which I posted on this newsgroup several years ago.
Cecil is still complaining about it.

Yes, because the current on a standing wave antenna
doesn't change phase through the coil no matter what
the delay through the coil. EZNEC agrees with me.
Here is what EZNEC says about the current through
90 degrees of antenna:

EZNEC+ ver. 4.0
thin-wire 1/4WL vertical 4/21/2009 5:50:11 PM
--------------- CURRENT DATA ---------------
Frequency = 7.29 MHz
Wire No. 1:
Segment Conn Magnitude (A.) Phase (Deg.)
1 Ground 1 0.00
2 .97651 -0.42
3 .93005 -0.83
4 .86159 -1.19
5 .77258 -1.50
6 .66485 -1.78
7 .54059 -2.04
8 .40213 -2.28
9 .25161 -2.50
10 Open .08883 -2.71

How do you explain the fact that the current changes by
less than 3 degrees in 90 degrees of antenna? How can you
possibly measure the delay through a coil, or through a
wire, using a current like that?

The displacement current flowing through those capacitances, not some
"effective degrees of antenna" phenomenon, is what causes the current
along a solenoidal loading coil to vary.

Rhetorical question: Did you know that "displacement current"
is a patch added to the lumped circuit model to try to make
get closer to reality?

You've kind of lost me here, since I can't see how you've replaced a
two-terminal coil with a four-terminal transmission line. And a
transmission line doesn't radiate, so that sometimes-important property
of a solenoidal coil is ignored.

You wouldn't be lost if you knew that a single horizontal
wire above ground is a transmission line.

Me, too. The thing which prompted me to add the automated helix
generation feature to EZNEC was the realization that lumped loads so
often did a poor job of simulating solenoidal loading inductors.

Too bad you don't accept the EZNEC results of that addition
which I have posted on my web page and you have ignored.

P.S. Roy has threatened to refund my purchase price for EZNEC
and declare my copy of EZNEC to be a pirated copy unless I stop
using it to prove him wrong.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Dish reflector
 

On Apr 21, 5:49*pm, "Dave" wrote:
"Art Unwin" wrote in message

...

At the expense of efficiency per unit length

and what does that refer to?? *i don't think i've ever heard of something
with those units... they really don't make any sense.

Somebody changed the subject. I suppose you can start a new one since
this one has been taken away
Art

Loading coils: was Dish reflector
 

Cecil Moore wrote:

Yes, because the current on a standing wave antenna
doesn't change phase through the coil no matter what
the delay through the coil. EZNEC agrees with me.
Here is what EZNEC says about the current through
90 degrees of antenna:

EZNEC+ ver. 4.0
thin-wire 1/4WL vertical 4/21/2009 5:50:11 PM
--------------- CURRENT DATA ---------------
Frequency = 7.29 MHz
Wire No. 1:
Segment Conn Magnitude (A.) Phase (Deg.)
1 Ground 1 0.00
2 .97651 -0.42
3 .93005 -0.83
4 .86159 -1.19
5 .77258 -1.50
6 .66485 -1.78
7 .54059 -2.04
8 .40213 -2.28
9 .25161 -2.50
10 Open .08883 -2.71

How do you explain the fact that the current changes by
less than 3 degrees in 90 degrees of antenna? How can you
possibly measure the delay through a coil, or through a
wire, using a current like that?

Not to intrude, but I thought you were discussing a coil. The above
seems to be about an antenna.

By extension, if an inductor acts the same as an antenna, then a
capacitor also acts like an antenna. QEF.

So I guess that implies that a capacitor isn't much different than an
inductor.

I've misunderstood so much, I think I may just have to end it all.

tom
K0TAR

Loading coils: was Dish reflector
 

Perhaps I could share a few thoughts on the "missing degrees" topic;
and again I apologise as the new boy if this has all been covered
before! I found the following argument helpful when trying to get my
head around some of the issues, and it may help others:

Picture the short, base-loaded, 6ft vertical antenna example I gave
earlier which resonates at 3.79MHz with the coil dimensions I quoted.
The 6ft whip represents an electrical length of about 9 degrees. Now
suppose I remove the 12" long loading coil leaving a 12" vertical gap
in the antenna. At this point I find it much more helpful to think in
terms of a "missing" +j2439 ohms reactance, rather than a "missing" 81
degrees, for reasons we shall see later.

Now I run out a couple of horizontal wires from where the top and
bottom of the coil were connected, and short them at the far end
thereby forming a short-circuit stub. That stub will insert some
"loading inductance" in place of the coil. How long do I need to make
the stub to bring the vertical back to resonance?

Using the simplified stub formula Xl=+jZo.tan(Bl), and assuming for
now that the characteristic impedance is 600 ohms, I find that the
electrical length needed to generate +j2439 is 76 degrees - well short
of any "missing" 81 degrees. And if I increase the characteristic
impedance of the stub to 1200 ohms I only need 64 degrees. The Corum &
Corum formulas tell me that the characteristic impedance of my
original loading coil is 2567 ohms at this frequency, so that only
requires an electrical length of 43 degrees.

So, for me, the "missing degrees" question is not really about missing
degrees; rather, it's about a missing inductive reactance which can be
provided by transmission line structures with a wide range of
electrical lengths depending on their characteristic impedance. The
"constant" is the reactance, not the electrical length.

I also find this picture helpful because I can visualize that,
although there must be forward and return waves on the stub, the net
current I would observe is a standing wave whose phase doesn't change
along the length of the stub. Incidentally, taking 43 degrees as the
length of my loading coil I would expect to see a change in current
amplitude along the length of the stub of cos(43); that's 0.73 -
pretty close to the 0.69 observed in the EZNEC model between the ends
of the coil.

Finally, I ask what the transmission line characteristic impedance
would need to be for its length to be exactly the "missing" 81
degrees? Answer: 2349/atan(81)=273 ohms. Isn't that in the right ball
park for the characteristic impedance of a single straight piece of
wire - in fact the piece of wire that's needed to turn the 6ft whip
into a full quarter-wave vertical?

And finally, finally, to Roy: I struggle with the "mental gymnastics"
needed to move from the simple stub model outlined above, to one where
the "transmission line" is a single wire, not two wires, and "in-line"
with the antenna elements. If you read the Curum & Corum paper I'm
sure it will be clearer to you than to me! But until I can understand
it better, I content myself with this thought: if we removed 56ft of
wire from our full-sized quarter-wave vertical to leave just the 6ft
whip, we'd be happy to analyse this 56ft straight piece of wire using
a transmission line approach (including considering forward &
reflected waves, and the resultant standing wave along it), and to
ascribe to it an equivalent inductive reactance. I don't understand
why I (we?) find it intellectually any more difficult to take the same
approach with a piece of wire once it is wound into a helix.

Regards,
Steve G3TXQ

Loading coils: was Dish reflector
 

steveeh131047 wrote:

Steve, congratulations on your QST article.

Now I run out a couple of horizontal wires from where the top and
bottom of the coil were connected, and short them at the far end
thereby forming a short-circuit stub. That stub will insert some
"loading inductance" in place of the coil. How long do I need to make
the stub to bring the vertical back to resonance?

I would also ask the questions: How much delay is there through
a series stub? What is the phase shift through the stub measured
by using the current on this standing-wave antenna? See below.

I also find this picture helpful because I can visualize that,
although there must be forward and return waves on the stub, the net
current I would observe is a standing wave whose phase doesn't change
along the length of the stub.

Someone is likely to point out that if one uses a current probe
to observe the current, it looks like a sine wave, i.e. its
phase is obviously changing with time. The point is that the
phase changes very little with length.

What we must be careful to say is that the phase doesn't change,
RELATIVE TO THE SOURCE PHASE, along the length of the stub. Here's
what EZNEC says about the phase in a 1/4WL open-circuit stub.

EZNEC+ ver. 4.0
1/4WL open stub in free space 4/22/2009 7:08:09 AM
--------------- CURRENT DATA ---------------
Wire No. 2:
Segment Conn Magnitude (A.) Phase (Deg.)
1 W1E1 .99665 -0.25
2 .97169 -0.67
3 .92292 -1.01
4 .85155 -1.30
5 .75929 -1.53
6 .64841 -1.72
7 .52163 -1.86
8 .38205 -1.96
9 .23309 -2.03
10 Open .07839 -2.07

Only 2 degrees of current phase shift in 90 degrees of stub.
How can that current be used to calculate delay through the
stub?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Loading coils: was Dish reflector
 

steveeh131047 wrote:
. . .
And finally, finally, to Roy: I struggle with the "mental gymnastics"
needed to move from the simple stub model outlined above, to one where
the "transmission line" is a single wire, not two wires, and "in-line"
with the antenna elements. If you read the Curum & Corum paper I'm
sure it will be clearer to you than to me! But until I can understand
it better, I content myself with this thought: if we removed 56ft of
wire from our full-sized quarter-wave vertical to leave just the 6ft
whip, we'd be happy to analyse this 56ft straight piece of wire using
a transmission line approach (including considering forward &
reflected waves, and the resultant standing wave along it), and to
ascribe to it an equivalent inductive reactance. I don't understand
why I (we?) find it intellectually any more difficult to take the same
approach with a piece of wire once it is wound into a helix.

Regards,
Steve G3TXQ

The similarities between an antenna and transmission line have been
known for a very long time and described in a number of papers. (See for
example Boyer, "The Antenna-Transmission Line Analog", _Ham Radio_,
April and May 1977, and Schelkunoff, "Theory of Antennas of Arbitrary
Size and Shape", _Proc. of the I.R.E., Sept. 1941.) It's a useful
conceptualization tool but, like comparing electricity to water in a
pipe, has its limitations. If you look at the transmission line
properties of a vertical, you see that the two conductors (the antenna
and ground plane) get farther and farther apart as the distance from the
feedpoint increases. This behaves like a transmission line whose
impedance increases with distance from the feedpoint and, in fact, a TDR
response shows just this characteristic. It's open circuited at the end,
so it behaves pretty much like an open circuited transmission line,
resulting in the same reflections and resulting standing waves you see
on a real antenna. One difficulty is accounting for the radiation, which
adds resistance to the feedpoint. I've never seen an attempt at
simulating it with distributed resistance, which I don't think would
work except over a narrow frequency range. Boyer deals with this by
simply adding a resistance at the model feedpoint, noting that the
resistance doesn't change very rapidly with frequency. So this is one
inherent shortcoming of the transmission line analog. As long as you
incorporate the increasing Z0 with distance from the feedpoint and the
limitations of the resistive part, the model does reasonably well in
predicting the feedpoint characteristics of simple antennas. But one
shortcoming of many antenna transmission line analogies is the attempt
to assign a single "average" or "effective" characteristic impedance to
the antenna, rather than the actual varying value. This is where a lot
of care has to be taken to assure that the model is valid in the regime
where it's being used.

There's no reason you can't also include a loading coil in the
transmission line model, and Boyer devotes much of the second part of
his article to doing just that. A solenoidal coil raises the
characteristic impedance of the length of "line" it occupies, because of
the increase in L/C ratio in that section. The traveling wave delay in
that section of the transmission line also increases due to the
increased LC product. (L and C are per unit length in both cases.) But
don't forget the C which is an essential part of this analysis, and
don't forget that the C is decreasing from the bottom to the top of the
coil, resulting in an increasing characteristic impedance. A very short
coil like a toroid will raise the Z0 only for a very short distance, so
behaves differently from a long solenoidal coil.

Models or analogs can be very useful in gaining insight about how things
work. You have to remain vigilant, though, that you don't extend the
analogy beyond it realm of validity.

Roy Lewallen, W7EL