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  #61   Report Post  
Old July 17th 03, 07:57 PM
Roy Lewallen
 
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Yes, he died not long ago, within the last year I believe.

Roy Lewallen, W7EL

Dr. Slick wrote:
Roy Lewallen wrote in message ...

I'd be one of the people arguing. Radiation resistance fits every
definition of resistance. There's no rule that a resistance has to
dissipate power. The late Mr. Carr was quite apparently confusing
resistance with a resistor, a common mistake.



BTW, did Joseph Carr really pass away? Sad, his book is very practical.


Slick



  #62   Report Post  
Old July 17th 03, 08:05 PM
Roy Lewallen
 
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Most simple derivations for the input impedance of a dipole assume it's
infinitely thin. The general problem of a dipole made from wire of
finite diameter is a lot tougher, and is the topic of the papers by the
authors I listed in another recent posting. With EZNEC, you'll find that
the dipole impedance will continue to change as you make the wire
diameter smaller and smaller, until it gets too small for the program to
handle at all.

Roy Lewallen, W7EL

Tarmo Tammaru wrote:
Kraus comes up with Z=73 + j42.5. He then goes on to say that an actual
dipole is made a few % shorter, which yields 65 + j0. When I did an EZNEC
calculation on a 1/2 wave dipole at 3MHz, I did not quite get that. For a
#30 wire in free space I got 76.81 + j43.89 at 3 MHz, and 72.88 + j0.3465
at 2.94 MHz.

I let EZNEC tell me what the wavelength was, and used 1/2 of that for the
length of the dipole.

Tam/WB2TT


  #63   Report Post  
Old July 17th 03, 09:00 PM
Reg Edwards
 
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"Tom Bruhns" wrote
"Reg Edwards" wrote

I didn't say they were unimportant.
I said they served only to add to the
confusion when considering operation of the usual amateur installation

when
the generator internal resistance is unknown.


Indeed, and not only that, the generator (ham transmitter) is commonly
neither a linear system nor time invariant. Also, maximum power
(conjugate-matched load) from a linear generator is generally not the
most efficient case. A great many generators and amplifiers are
distincly NOT designed to deliver power to a matched load, but rather
to deliver power efficiently to a specific load which is mismatched
with respect to the output impedance of the generator/amplifier.

There are times when knowing that a generator is a linear 50 ohm
source (within some small tolerance) is important--I deal with them
all the time in the work I do--but in a typical ham transmitter
application, that's very seldom if ever the case.

====================================
Tom, To add a bit more -

50-ohm generators as used in laboratories (so that measured reflexion loss,
mismatch loss etc, mean something) are effectively constant voltage
generators in series with a 50-ohms resistor, or constant current generators
in shunt with a 50-ohm resistor. They may be followed by an ampifier whose
output impedance is held constant at 50-ohms by some automatic means. None
of these circuits bear much resemblance to a pair of 807's and a tuned tank.

The best that can be said about Rg of the usual HF radio transmitter is that
Rg is indeterminate. IT EVEN VARIES AS THE LOAD IMPEDANCE IS CHANGED which
most of the Guru's contributing to this newsgroup appear to be unaware of or
at least choose to disregard. So what does "adjusting RL to equal Rg" mean?
To use it in a description of feeder + antenna behaviour further propagates
myths, including those surrounding SWR, forward power, reflected power, SWR
meters, etc.

Does Terman ever bother to mention Rg of a Tx PA? If he doesn't it can't
matter very much to him. The ARRL handbook, when numerically designing a
transistor linear HF PA, makes no mention of Rg.
----
Reg, G4FGQ



  #64   Report Post  
Old July 17th 03, 09:05 PM
Richard Harrison
 
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Dr. Slick wrote:
"What`s the definition of "surge impedance" versus regular old
"impedance"?"

Arnold B. Bailey treats this better than anybody I`ve seen. But, there
are many treatments. The regular old impedance of an antenna depends
upon its termination.

Surge impedance of an antenna depends on its conductor`s inductance per
unit length and capacitance per unit length. In the antenna these are
not uniform as they are in a transmission line, and average values have
been found useful.

Best regards, Richard Harrison, KB5WZI

  #65   Report Post  
Old July 17th 03, 09:28 PM
Richard Clark
 
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On Thu, 17 Jul 2003 12:05:19 -0700, Roy Lewallen
wrote:

Most simple derivations for the input impedance of a dipole assume it's
infinitely thin. The general problem of a dipole made from wire of
finite diameter is a lot tougher, and is the topic of the papers by the
authors I listed in another recent posting. With EZNEC, you'll find that
the dipole impedance will continue to change as you make the wire
diameter smaller and smaller, until it gets too small for the program to
handle at all.

Roy Lewallen, W7EL


Hi All,

The derivation of dipole electrical characteristics comes by neither
thin nor thick (cylindrical) elements but through a simpler
(conceptually, not mathematically) work described by S.A. Schelkunoff
in "Advanced Antenna Theory," John Wiley and Sons, 1952.

Schelkunoff approaches the design as merely the extension of the
transmission line and he answers the issue of the antenna (the thin
wire form) being non-linear (the presumed incremental
inductance/capacitance is not constant along the length of the split
transmission line) by simply employing conical structures.

The Biconical Dipole "develops a transverse spherical (TEM) wave
analogous to that on a conventional transmission line" (reference
"Antennas and Radiowave Propagation," Robert E. Collin, McGraw Hill,
1985). "Thus the biconical antenna theory provides a theoretical
basis for assuming a sinusoidal current distribution on thin-wire
antennas."

Like any transmission line terminated in its own character impedance,
the Biconical Dipole (within limits imposed by size and apex angle)
also presents a wide frequency range exhibiting a constant radiation
resistance (about 160 Ohms across three octaves, by my margin notes).

The easiest validation of this is found in the Discone.
http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm

73's
Richard Clark, KB7QHC


  #66   Report Post  
Old July 17th 03, 09:50 PM
Roy Lewallen
 
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Schelkunoff's method is elegant, and one that lends itself to relatively
simple calculation -- in closed form -- with a computer. However, it
doesn't give results which are in as good agreement with measurements
than some other methods, so some assumptions made in his derivation
aren't completely correct. A good summary of various methods and their
validity appears in David Middleton and Ronold King, "The Thin
Cylindrical Antenna: A Comparison of Theories", _Journal of Applied
Physics_, Vol. 17, April, 1946.

The program for calculation of the "Field Day Special" antenna
(ftp://eznec.com/pub/fdsp~.exe) uses Schelkunoff's method, and it's
perfectly adequate for the purpose.

Of course, these days we can easily do very accurate calculations from
very fundamental equations with a computer using the method of moments
or other methods. There's a very good description of the method of
moments in the second edition of Kraus' _Antennas_. I assume it's also
in the third edition, which I don't yet have.

Roy Lewallen, W7EL

Richard Clark wrote:
On Thu, 17 Jul 2003 12:05:19 -0700, Roy Lewallen
wrote:


Most simple derivations for the input impedance of a dipole assume it's
infinitely thin. The general problem of a dipole made from wire of
finite diameter is a lot tougher, and is the topic of the papers by the
authors I listed in another recent posting. With EZNEC, you'll find that
the dipole impedance will continue to change as you make the wire
diameter smaller and smaller, until it gets too small for the program to
handle at all.

Roy Lewallen, W7EL



Hi All,

The derivation of dipole electrical characteristics comes by neither
thin nor thick (cylindrical) elements but through a simpler
(conceptually, not mathematically) work described by S.A. Schelkunoff
in "Advanced Antenna Theory," John Wiley and Sons, 1952.

Schelkunoff approaches the design as merely the extension of the
transmission line and he answers the issue of the antenna (the thin
wire form) being non-linear (the presumed incremental
inductance/capacitance is not constant along the length of the split
transmission line) by simply employing conical structures.

The Biconical Dipole "develops a transverse spherical (TEM) wave
analogous to that on a conventional transmission line" (reference
"Antennas and Radiowave Propagation," Robert E. Collin, McGraw Hill,
1985). "Thus the biconical antenna theory provides a theoretical
basis for assuming a sinusoidal current distribution on thin-wire
antennas."

Like any transmission line terminated in its own character impedance,
the Biconical Dipole (within limits imposed by size and apex angle)
also presents a wide frequency range exhibiting a constant radiation
resistance (about 160 Ohms across three octaves, by my margin notes).

The easiest validation of this is found in the Discone.
http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm

73's
Richard Clark, KB7QHC


  #67   Report Post  
Old July 17th 03, 10:27 PM
Reg Edwards
 
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I referred to Terman as "him".

It should, of course, have been "HIM". ;o)
---
Reg.


  #68   Report Post  
Old July 17th 03, 10:42 PM
W5DXP
 
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William E. Sabin wrote:
If the transmitter output is 100 W and the reflected power is 3 W, then
the 100 W is the difference between 100+3=103 W (forward power) and 3 W
(reflected power).


If the source is a signal generator equipped with a circulator and
load, the generator is putting out 103 watts, and the circulator
load is dissipating 3 watts, is the generator still only putting
out 100 watts by definition?
--
73, Cecil, W5DXP

  #69   Report Post  
Old July 17th 03, 11:56 PM
Dilon Earl
 
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On Thu, 17 Jul 2003 17:12:33 GMT, Richard Clark
wrote:

On Thu, 17 Jul 2003 17:00:55 GMT, Dilon Earl
wrote:
If I have a 100 watt transmitter and my wattmeter shows 3 watts
reflected. Is 3 watts actually being dissipated in the tank and final
PA?


Hi Dilon,

Does it become 3 watts hotter under the same drive conditions without
the reflected power? You would be surprised how few pundits actually
discuss this in these terms. Of course everyone would be surprised if
anyone attempted to perform this chore.

I like to include this jab at those who rave on about the
impossibility of knowing the internal resistance of a transmitter and
are satisfied to squeak out 100W RF for 250W DC in.

73's
Richard Clark, KB7QHC


Richard;
I'm not sure if it does get 3 watts hotter. I was always under
the impression that operating a transmitter with a high reflected
power was unhealthy for my PA.



  #70   Report Post  
Old July 18th 03, 12:00 AM
Dilon Earl
 
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On Thu, 17 Jul 2003 14:42:37 -0700, W5DXP
wrote:

William E. Sabin wrote:
If the transmitter output is 100 W and the reflected power is 3 W, then
the 100 W is the difference between 100+3=103 W (forward power) and 3 W
(reflected power).


If the source is a signal generator equipped with a circulator and
load, the generator is putting out 103 watts, and the circulator
load is dissipating 3 watts, is the generator still only putting
out 100 watts by definition?


No, you just said it was putting 103 watts.. :-)

Was that the right answer?




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