Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #191   Report Post  
Old November 4th 03, 10:43 PM
Roy Lewallen
 
Posts: n/a
Default

I'm not sure why anyone would think that you can treat an antenna, or a
loading coil of significant length, as a lumped element and expect to
get anything resembling accurate results. Who in the world is proposing
such a thing? Or is this something to be attributed to the "old wives"
and "gurus", so we can then show how much smarter we are by pointing out
how stupid it is?

Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?

Yes, a solenoid produces a local (near) field in the direction of its
axis. The far field that remains depends on the size and aspect ratio of
the solenoid. Hence, we have solenoidal antennas that radiate primarily
axially and those which radiate primarily radially. It's not clear to me
how this bears on the topic.

Jim Kelley wrote:

Jim Kelley wrote:

What if you draw a two terminal black box around the middle few feet of
a 1/4 wave vertical? What makes the sum of the currents at both ends
become equal to zero?



Sorry to be obtuse, Roy. The point is only that antenna circuits
obviously present a problem to the assumption that such two terminal
black boxes will necessarily have equal currents at both terminals.

A solenoid should produce a field in the direction of its axis, should
it not?

73, Jim AC6XG


  #192   Report Post  
Old November 4th 03, 11:16 PM
Jim Kelley
 
Posts: n/a
Default

Roy Lewallen wrote:

Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?


The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG
  #193   Report Post  
Old November 4th 03, 11:47 PM
Roy Lewallen
 
Posts: n/a
Default

Sigh.

I give up. It's time for me to get back to work. Have fun, folks.

Roy Lewallen, W7EL

Jim Kelley wrote:
Roy Lewallen wrote:


Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?



The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG


  #194   Report Post  
Old November 5th 03, 12:37 AM
Roy Lewallen
 
Posts: n/a
Default

No, I will make one more comment. After a bit of reflection, I think
this might be at the core of some people's problem in envisioning a
lumped inductor.

When a current flows into an inductor, it doesn't go round and round and
round the turns, taking its time to get to the other end. An inductor
wound with 100 feet of wire behaves nothing like a 100 foot wire. Why?
It's because when the current begins flowing, it creates a magnetic
field. This field couples to, or links with, the other turns. The
portion of the field from one turn that links with the others is the
measurable quantity called the coefficient of coupling. For a good HF
toroid, it's commonly 99% or better; solenoids are lower, and vary with
aspect ratio. The field from the input turn creates a voltage all along
the wire in the other turns which, in turn, produce an output current
(presuming there's a load to sustain current flow). Consequently, the
current at the input appears nearly instantaneously at the output. Those
who are physics oriented can have lots of fun, I'm sure, debating just
how long it takes. The field travels at near the speed of light, but the
ability of the current to change rapidly is limited by other factors.

So please flush your minds of the image of current whirling around the
coil, turn by turn, wending its way from one end to the other. It
doesn't work at all like that. The coupling of fields from turn to turn
or region to region is what brings about the property of inductance in
the first place.

Radiation is another issue, and provides a path for current, via
displacement current, to free space. (I can see it now in Weekly World
News: WORLD FILLING WITH COULOMBS! DISASTER LOOMS!) For a component to
fit the lumped element model, radiation has to be negligible. And, for
the same reason, it can't be allowed to interact with external fields as
a receiver, either.

This is very fundamental stuff. You can find a lot more about the topic
in any elementary circuit analysis or physics text. If you don't believe
what you read there, just killfile my postings -- you won't believe me,
either, and reading what I post will be a waste of time for both of us.

Real inductors, of course, are neither zero length nor do they have a
perfect coefficient of coupling. And they do radiate. The essence of
engineering is to understand the principles well enough to realize which
imperfections are important enough to affect the outcome in a particular
situation. We simplify the problem by putting aside the inconsequential
effects, but don't oversimplify by ignoring factors that are important
for the job at hand. Those who insist on using only the simplest model
for all applications will often get invalid results. And those who use
only the most complex model for all applications (as is often done in
computer circuit modeling), often lose track of what's really going on
-- they become good analysts but poor designers. I've seen people
capable of only those approaches struggle, and fail, to become competent
design engineers.

And with that, I'm outta here. Hope my postings have been helpful.

Roy Lewallen wrote:

Sigh.

I give up. It's time for me to get back to work. Have fun, folks.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:


Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?




The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG




  #195   Report Post  
Old November 5th 03, 01:37 AM
Ian White, G3SEK
 
Posts: n/a
Default

Roy Lewallen wrote:

no, you can't put a box around anything having any length and expect
the current in to equal the current out. And why should this be
surprising to anyone?

Possibly because radio amateurs are not taught well about what a "lumped
component" is. All lumped components are defined as having zero (or
negligible) physical dimensions relative to the wavelength at the
operating frequency. Similarly, lumped networks are defined as having
zero (or negligible) lead lengths relative to the operating wavelength.

In this idealized case, the current into and out of the two terminals of
a lumped inductor is always exactly the same. This remains true even if
that component is embedded into an antenna where current variations
along the length of the conductor do exist.

Because all practical components and networks have some finite physical
size, lumped-component behaviour is never absolutely perfect. In
principle, any real component must also show some "antenna-like"
behaviour, which does allow some variation of current between its
terminals... but in practice this effect is usually very small indeed.
For example, for physically small components the lumped-component
approximation works well in circuit simulations at frequencies up to
several GHz. (You may have to simulate each component as a small network
in order to account accurately for self-capacitance, self-inductance and
loss resistance, but these are still networks of idealized lumped
components.)


Yes, a solenoid produces a local (near) field in the direction of its
axis. The far field that remains depends on the size and aspect ratio
of the solenoid. Hence, we have solenoidal antennas that radiate
primarily axially and those which radiate primarily radially. It's not
clear to me how this bears on the topic.


Quite a lot, I think. At one extreme, a loading coil may be so small
that it behaves as a near-perfect lumped inductor. Such an inductor will
not radiate, and will have almost zero difference in current between its
two terminals. Those two properties - lack of radiation and no
difference in terminal currents - are locked together.

At the other extreme, you may have a long, skinny loading coil that has
significant antenna-like properties, radiating at right-angles to its
length like a "rubber-duck" (more formally known as a normal-mode
helix). In this case the coil does form part of the radiating structure,
so you do expect to see a variation in current along the length of the
coil, and hence a difference between the currents at its two ends. Once
again, the two properties of radiation and current variation are locked
together.

This brings us back to the question of practical loading coils, and how
much radiation (and therefore current variation along the length) we can
expect. I haven't ever tried to work it out, but my guess is that a
fairly short "square" coil that has been optimized for high Q is not
going to radiate much, and that we therefore shouldn't expect a large
difference in current between its two ends.

Let's see now... a 3.5MHz loading coil that is as much as 10 inches long
would scale down to 0.010 inches at 3.5GHz... at that frequency I'd
expect to be able to model such a tiny inductor very accurately as a
small network of lumped components with no radiating properties.

On the other hand, people who mistakenly believe that even an ideal
lumped inductor can have a difference between the currents at its two
terminals are rather unlikely to be convinced.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek


  #196   Report Post  
Old November 5th 03, 02:24 AM
Wes Stewart
 
Posts: n/a
Default

On Tue, 04 Nov 2003 15:37:11 -0800, Roy Lewallen
wrote:

[lots of good stuff snipped]
|
|And with that, I'm outta here. Hope my postings have been helpful.

Thanks, Roy. I'm surprised you stuck around this long. Your posts
are always helpful.

73 Wes N7WS

  #197   Report Post  
Old November 5th 03, 02:26 AM
Jim Kelley
 
Posts: n/a
Default

Roy,

Let me first apologize for having put the burr under your saddle. This
is not my intent. The intent is to try to stimulate some additional
thinking on the subject, and perhaps apply some other things we also
know about these devices. Your contributions here are obviously
invaluable to the entire group. Of course they are helpful. Having
recognized that, I hope there are no subjects which are off limits to
debate - or opinions that are deemed beyond reproach. Yuri has
introduced an interesting subject; one that appears to have two
disparate points of view.

At this point, in my view, the side asserting that there is no
difference in current from one end of an inductor to the other hasn't
defended its position as well as the other side. The point you make for
the torroid is well taken I think. Flux from this type of coil is well
confined to the core of the inductor.

But torroidal coils are by design, a unique case. I don't think the
same case can be made for the helix, or a loopstick type coil for
example. These coils do radiate quite well along their axis if nowhere
else, and might therefore be expected to behave in a fashion not unlike
other radiators, i.e. impedance and hence, current, would vary with
position. Since air core coils are ubiquitous in antenna construction,
I don't think it's unreasonable to discuss their performance, and
consider the findings Yuri has presented as being both reasonable and
viable.

73, Jim AC6XG


Roy Lewallen wrote:

No, I will make one more comment. After a bit of reflection, I think
this might be at the core of some people's problem in envisioning a
lumped inductor.

When a current flows into an inductor, it doesn't go round and round and
round the turns, taking its time to get to the other end. An inductor
wound with 100 feet of wire behaves nothing like a 100 foot wire. Why?
It's because when the current begins flowing, it creates a magnetic
field. This field couples to, or links with, the other turns. The
portion of the field from one turn that links with the others is the
measurable quantity called the coefficient of coupling. For a good HF
toroid, it's commonly 99% or better; solenoids are lower, and vary with
aspect ratio. The field from the input turn creates a voltage all along
the wire in the other turns which, in turn, produce an output current
(presuming there's a load to sustain current flow). Consequently, the
current at the input appears nearly instantaneously at the output. Those
who are physics oriented can have lots of fun, I'm sure, debating just
how long it takes. The field travels at near the speed of light, but the
ability of the current to change rapidly is limited by other factors.

So please flush your minds of the image of current whirling around the
coil, turn by turn, wending its way from one end to the other. It
doesn't work at all like that. The coupling of fields from turn to turn
or region to region is what brings about the property of inductance in
the first place.

Radiation is another issue, and provides a path for current, via
displacement current, to free space. (I can see it now in Weekly World
News: WORLD FILLING WITH COULOMBS! DISASTER LOOMS!) For a component to
fit the lumped element model, radiation has to be negligible. And, for
the same reason, it can't be allowed to interact with external fields as
a receiver, either.

This is very fundamental stuff. You can find a lot more about the topic
in any elementary circuit analysis or physics text. If you don't believe
what you read there, just killfile my postings -- you won't believe me,
either, and reading what I post will be a waste of time for both of us.

Real inductors, of course, are neither zero length nor do they have a
perfect coefficient of coupling. And they do radiate. The essence of
engineering is to understand the principles well enough to realize which
imperfections are important enough to affect the outcome in a particular
situation. We simplify the problem by putting aside the inconsequential
effects, but don't oversimplify by ignoring factors that are important
for the job at hand. Those who insist on using only the simplest model
for all applications will often get invalid results. And those who use
only the most complex model for all applications (as is often done in
computer circuit modeling), often lose track of what's really going on
-- they become good analysts but poor designers. I've seen people
capable of only those approaches struggle, and fail, to become competent
design engineers.

And with that, I'm outta here. Hope my postings have been helpful.

Roy Lewallen wrote:

Sigh.

I give up. It's time for me to get back to work. Have fun, folks.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:


Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?



The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG



  #198   Report Post  
Old November 5th 03, 02:28 AM
Roy Lewallen
 
Posts: n/a
Default

Thanks for the very cogent and informative posting, as yours always are.

One thing you said really rang a bell, and made me consider something
I'd never thought much about before. If a coil is radiating
significantly, the Q will of course be necessarily poor due to the
energy "lost" by radiation. Yet this "loss" won't be detrimental to the
antenna performance. Tom Rauch has pointed out that loading inductor
loss is very often insignificant compared to ground loss in a typical HF
mobile system. Maybe the presumed "loss" of some coils is not as bad as
it appears in other antenna applications, either.

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
. . .
This brings us back to the question of practical loading coils, and how
much radiation (and therefore current variation along the length) we can
expect. I haven't ever tried to work it out, but my guess is that a
fairly short "square" coil that has been optimized for high Q is not
going to radiate much, and that we therefore shouldn't expect a large
difference in current between its two ends.
. . .


  #199   Report Post  
Old November 5th 03, 03:37 AM
Art Unwin KB9MZ
 
Posts: n/a
Default

Thanks for your down to earth input Roy
I haven't entered this thread because I was
very confused how the group was dealing with
inductance since what is important to me is
the field around it that permits effective,
efficient coupling of different circuits.
When looking for a lossless system ,coupling
by either capacitance or inductance is necessary
together with the all important HIGH Q.
And if one were to dwell on the wire used as a
missing part of a radiator alone and disregarding
the ebb and flow of the enclosing field
just blows my mind.
Interesting that you spoke of lumped loads and
distributed loads in terms of modeling, many people
here would learn a lot by starting of with a T or pi
type matching system e.t.c. all of which use lumped loads,
and then manipulate these same lumped loads
into distributed loads in multiple coupled circuits
in a form of many coupled distributed load ircuits to
produce a radiator that can maintain a constant input
impedance.
After playing with such circuits to form a combination
radiating lossless circuit( reverse of complex circuit resolving)
the idea of missing radiator lengths would quickly
disappear, as it becomes noticable that the energy field equates
to the actual length of the inductance and not to a slinky style stretch.
There again, as a total amateur with respect to electrical
thing a me jigs I could be adding to the mental riots of those who
are partaking in this never ending gymnastics.If so I will now
sneak quietly out of this conference room before Tom arrives and
put everybody in their place as only he can do.
Regards
Art



Roy Lewallen wrote in message ...
No, I will make one more comment. After a bit of reflection, I think
this might be at the core of some people's problem in envisioning a
lumped inductor.

When a current flows into an inductor, it doesn't go round and round and
round the turns, taking its time to get to the other end. An inductor
wound with 100 feet of wire behaves nothing like a 100 foot wire. Why?
It's because when the current begins flowing, it creates a magnetic
field. This field couples to, or links with, the other turns. The
portion of the field from one turn that links with the others is the
measurable quantity called the coefficient of coupling. For a good HF
toroid, it's commonly 99% or better; solenoids are lower, and vary with
aspect ratio. The field from the input turn creates a voltage all along
the wire in the other turns which, in turn, produce an output current
(presuming there's a load to sustain current flow). Consequently, the
current at the input appears nearly instantaneously at the output. Those
who are physics oriented can have lots of fun, I'm sure, debating just
how long it takes. The field travels at near the speed of light, but the
ability of the current to change rapidly is limited by other factors.

So please flush your minds of the image of current whirling around the
coil, turn by turn, wending its way from one end to the other. It
doesn't work at all like that. The coupling of fields from turn to turn
or region to region is what brings about the property of inductance in
the first place.

Radiation is another issue, and provides a path for current, via
displacement current, to free space. (I can see it now in Weekly World
News: WORLD FILLING WITH COULOMBS! DISASTER LOOMS!) For a component to
fit the lumped element model, radiation has to be negligible. And, for
the same reason, it can't be allowed to interact with external fields as
a receiver, either.

This is very fundamental stuff. You can find a lot more about the topic
in any elementary circuit analysis or physics text. If you don't believe
what you read there, just killfile my postings -- you won't believe me,
either, and reading what I post will be a waste of time for both of us.

Real inductors, of course, are neither zero length nor do they have a
perfect coefficient of coupling. And they do radiate. The essence of
engineering is to understand the principles well enough to realize which
imperfections are important enough to affect the outcome in a particular
situation. We simplify the problem by putting aside the inconsequential
effects, but don't oversimplify by ignoring factors that are important
for the job at hand. Those who insist on using only the simplest model
for all applications will often get invalid results. And those who use
only the most complex model for all applications (as is often done in
computer circuit modeling), often lose track of what's really going on
-- they become good analysts but poor designers. I've seen people
capable of only those approaches struggle, and fail, to become competent
design engineers.

And with that, I'm outta here. Hope my postings have been helpful.

Roy Lewallen wrote:

Sigh.

I give up. It's time for me to get back to work. Have fun, folks.

Roy Lewallen, W7EL

Jim Kelley wrote:

Roy Lewallen wrote:


Gee whiz, golly, yes, representing an antenna as a two terminal black
box with zero size presents a problem. And no, you can't put a box
around anything having any length and expect the current in to equal the
current out. And why should this be surprising to anyone?



The wire comprising an inductor has length. The inductor radiates.
The inductor has two terminals with different currents at each. What
was it you said about Coulombs again?

73, Jim AC6XG



  #200   Report Post  
Old November 5th 03, 03:48 AM
Art Unwin KB9MZ
 
Posts: n/a
Default

If reactance can be seen as a "{missing" part
of a radiator how should we view what a
capacitor represents? Grin
Art


Cecil Moore wrote in message ...
w4jle wrote:
Current through a coil in an antenna.

If we feed an antenna at the current point, the current decreases as the
voltage increases along the antenna element from feed point to end..

That being said, a coil replacing a segment of an antenna (in order to
physically shorten it) will exhibit the same properties (relating to
currents) as the segment it replaced.


Yep, if the feedpoint impedances are the same and both are lossless,
that has to be true.

Here's a repeat of a diagram I drew earlier.

-----y----------x-----FP-----x----------y----- 1/2WL dipole

-----coil-----FP-----coil----- loaded dipole

Assume the physical length of the loaded dipole is 1/4WL.

Each coil replaces the section between 'x' and 'y'. The currents
at 'x' and 'y' are quite different, being 1/8WL apart.

Consider an 8 foot center-loaded 75m mobile antenna. 87% of the
electrical length of the antenna is in the coil.

Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Inverted ground plane antenna: compared with normal GP and low dipole. Serge Stroobandt, ON4BAA Antenna 8 February 24th 11 11:22 PM
Smith Chart Quiz Radio913 Antenna 315 October 21st 03 05:31 AM
QST Article: An Easy to Build, Dual-Band Collinear Antenna Serge Stroobandt, ON4BAA Antenna 12 October 16th 03 07:44 PM
Eznec modeling loading coils? Roy Lewallen Antenna 11 August 18th 03 02:40 AM


All times are GMT +1. The time now is 09:18 PM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017