Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #221   Report Post  
Old November 5th 03, 11:16 PM
Cecil Moore
 
Posts: n/a
Default

Ian White, G3SEK wrote:
Where you're going astray (I suspect) is in believing that the coil
literally "replaces" the 45deg of antenna that was lost when we halved
the height. It doesn't - the loading coil drastically changes the
current distribution.


No, I do NOT claim that the coil literally replaces the 45 degree of
antenna that was lost. The feedpoint impedance drops which indicates
that it is not a literal replacement. The decrease in the feedpoint
impedance indicates that the antenna is not radiating as much energy
in one cycle as a full 1/4WL vertical since the reflected current has
increased. This is consistent with field strength tests.

Since we have assumed a lumped loading coil, the current at the top of
the coil is the same 0.9A. But now the current distribution in the top
22.5deg is very different from the full-size case: it tapers much more
sharply, from 0.9A down to zero at the very top.


I propose that it cannot do that. Seems to me, the impedance looking toward
the end of the antenna is approximately the same for a six foot whip Vs the
base of a six foot whip mounted atop a coil. I don't see any reason to suspect
otherwise. Please provide me a reason.

the feed impedance of the
loaded antenna is much lower than that of the full-size;


.... which proves my point above. That indicates that it is not as efficient
a radiator as a full size 1/4WL vertical. In fact, the less efficient it
becomes as a radiator, the lower the feedpoint impedance. This fits in
exactly with the fact that a good 75m mobile antenna has a ~12 ohm
feedpoint impedance.

To sum up, shortening and loading the antenna creates so many important
differences that you're misleading *yourself* if you say that the
loading coil simply "replaces" the missing length of antenna.


I never said that so I assume that is your straw man. If that is not
a straw man, I apologize but it walks and talks like a straw man.

1. The diagram for current distribution with center loading in 'Low Band
DXing' is based on the same incorrect assumption that loading coil
somehow fully "replaces" the missing 45deg of antenna.


Please prove that a shortened antenna that is electrically 90 degrees
doesn't replace the missing number of degrees in the physical antenna.

2. I haven't thought about an answer to Cecil's problems of what happens
to the "missing" 45deg, and what happens to the forward and reflected
waves of voltage and current.


That's more than obvious, Ian. :-) How about thinking about those nagging
questions and providing us an answer? This is what happens when one ignores
the basics. The steady-state shortcut strikes again.

My only point is that a correct solution can *not* involve a difference
in the currents at the two ends of an idealized lumped inductor. Such a
difference simply cannot be... so the true solution will be that bit
harder for Cecil to find.


Since there is no such thing in reality as an idealized lumped inductor,
there is no real solution to the problem. Please allow me to suggest that
you guys are just engaging in mental masturbation. Was it good for you? :-)
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----
  #222   Report Post  
Old November 5th 03, 11:23 PM
Cecil Moore
 
Posts: n/a
Default

Mark Keith wrote:

Cecil Moore wrote in message ...
The round trip current phase shift in an electrical 1/4WL vertical must
total 360 degrees. If the coil doesn't perform part of that phase shift,
what does?


What if it does though? If the coil is still a fairly small portion of
the overall length, I don't see the change as severe. Heck, if current
is supposed to be the same on each end of the coil, it should be the
same going in both directions.. Seems to me it would pretty much
equal out in the overall scheme of things.


May I suggest that you think about the problem for awhile longer? If the
current doesn't undergo a 360 degree phase shift in its round trip, that
will *decrease* the feedpoint current. But we know the feedpoint current
*increases* when a loading coil is installed. This is a clear indication
that the reflected current is still in phase with the forward current.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----
  #223   Report Post  
Old November 5th 03, 11:28 PM
Cecil Moore
 
Posts: n/a
Default

Yuri Blanarovich wrote:
I am not as dumb as Reg portrays me.


What Reg needs is some Bella Sera Merlot. Dang, that's good stuff.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----
  #224   Report Post  
Old November 6th 03, 12:09 AM
Ian White, G3SEK
 
Posts: n/a
Default

Jim Kelley wrote:


"Ian White, G3SEK" wrote:
Since we have assumed a lumped loading coil, the current at the top of
the coil is the same 0.9A. But now the current distribution in the top
22.5deg is very different from the full-size case: it tapers much more
sharply, from 0.9A down to zero at the very top.


Right. But the discussion is about whether the lumped loading coil
model is accurate. Any conclusions based upon inaccurate assumptions
would probably also be inaccurate.


The discussion has had to take a step back, because we quickly
discovered that there isn't agreement about the properties of an
idealized lumped inductor.

Without that shared fundamental understanding of what "the lumped model"
is, it's hopeless to discuss whether or not that model is accurate for a
practical coil.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
  #225   Report Post  
Old November 6th 03, 12:21 AM
Ian White, G3SEK
 
Posts: n/a
Default

Cecil Moore wrote:

[More wriggling, followed by]

Please allow me to suggest that
you guys are just engaging in mental masturbation. Was it good for you? :-)


I'm finished with you.


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


  #226   Report Post  
Old November 6th 03, 12:48 AM
Jim Kelley
 
Posts: n/a
Default

"Ian White, G3SEK" wrote:
The discussion has had to take a step back, because we quickly
discovered that there isn't agreement about the properties of an
idealized lumped inductor.


There's been some blustery speculation about that, but I think the issue
is more whether or not an idealized inductor can actually tell us the
whole story. It doesn't seem to.

Without that shared fundamental understanding of what "the lumped model"
is, it's hopeless to discuss whether or not that model is accurate for a
practical coil.


Perhaps so. I haven't seen anyone actually share their fundamental
understanding of the model with the group yet.

73, Jim AC6XG
  #228   Report Post  
Old November 6th 03, 05:19 AM
Cecil Moore
 
Posts: n/a
Default

Jim Kelley wrote:
There's been some blustery speculation about that, but I think the issue
is more whether or not an idealized inductor can actually tell us the
whole story. It doesn't seem to.


If a loading coil has no effect on the current or the electrical
length of the antenna, exactly what effect does it have?
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----
  #229   Report Post  
Old November 6th 03, 05:22 AM
Cecil Moore
 
Posts: n/a
Default

Ian White, G3SEK wrote:

Cecil Moore wrote:
Please allow me to suggest that
you guys are just engaging in mental masturbation. Was it good for
you? :-)


I'm finished with you.


Ian, I apologize if my humor offended you.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----
  #230   Report Post  
Old November 6th 03, 03:56 PM
Richard Harrison
 
Posts: n/a
Default

Cecil, W5DXP wrote:
"Until someone can explain exactly how a lumped inductor causes a
45-degree phase shift in the current, I`m going to assume that a lumped
inductor is incapable of that feat."

There are base loading coils in shielded boxes which don`t radiate but
do have significant inductance which make too-short antennas seem long
enough for impedance matching purposes.

How to get 45-degrees? Make te reactance equal to the total resistance
in the circuit. Pythagoras showed how about 2500 years ago. You have a
1:1 ratio between resistance and reactance which are at right-angles to
each other. Thus, the resultant impedance is the vector sum which is the
square root of 2 times the resistance or reactance which are equal.
Pythagoras said: c sq = a sq + b sq.

c = impedance
a = resistance
b = reactance

Resistance and reactance are at right angles to each other. When they
are equal, their resultant impedance bisects the right-angle of
90-degrees, so the impedance is at 45-degrees.

In a resistance, the voltage drop is instantaneous with the current
through the resistance. In a pure inductance, current lags the voltage
by 90-degrees. In a circuit containing both resistance and inductive
reactance, the current lag is somewhere between 0 and 90-degrees. In
fact, the current makes the same angle with the applied voltage as
impedance in the circuit makes with the resistance because volts and
amps are in-phase in the resistance.

An inductor which radiates brings an extra to the impedance calculation,
its radiation resistance.

Best regards, Richard Harrison, KB5WZI

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 07:00 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