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  #131   Report Post  
Old July 19th 03, 09:19 PM
W5DXP
 
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Roy Lewallen wrote:
The "reflected power" is not dissipated in the source, matched or not.


If reflected energy could not cause over-current/over-voltage
problems in the source there would be no need for protection
circuitry. I once burned up a pair of ARC-5 1625's because my
feedline came unsoldered at the antenna. If there had not been
any reflected power, it would not have happened.
--
73, Cecil http://www.qsl.net/w5dxp



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  #132   Report Post  
Old July 19th 03, 09:25 PM
W5DXP
 
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Dilon Earl wrote:
You can indeed get 200 watts
to (incident upon) the antenna with a 100 watt transmitter. Trouble
is, the antenna only accepts half of that power.

Where does the other 100 watts go?


It is reflected back toward the source. It causes standing waves
and additional losses in the transmission line.

For some reason I need a circulator on my SB-401.


Only if you allow reflected energy to reach your SB-401.


How can I stop it from reaching my SB-401?


Does the SB-401 have an adjustable Pi-net output? If so,
you can adjust it for a Zg-match which will keep reflected
energy from being incident upon the SB-401 amp. If not, you
can use an external antenna tuner.
--
73, Cecil http://www.qsl.net/w5dxp



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  #133   Report Post  
Old July 19th 03, 10:16 PM
Ian White, G3SEK
 
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W5DXP wrote:
Tarmo Tammaru wrote:
Thanks for the lowdown. I am saving the message. I looked through the Moto
RF book last night and noticed some specs spell out by what they mean by
output impedance (conjugate of load), ...


Hmmmmm, the load is in control of the amp's load line.


In part, of course it is.

The standing bias controls one fixed point on the line.

The load, transformed backward through the output matching network,
controls the slope of the line.

Finally, the drive power controls how far the instantaneous operating
point moves along the line.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
  #134   Report Post  
Old July 20th 03, 12:57 AM
Tarmo Tammaru
 
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The stuff I did point to the actual impedance, not the SWR that does the bad
thing. For instance, a load of 25 Ohms greatly increases the internal
dissipation; a load of 100 Ohms greatly reduces it. If you go too high with
the impedance, you start to saturate the hell out of the device, and other
things happen.

Did you melt the plates on the 1625s or burn out the grids? Your feedline
might have opened an odd multiple of 1/4 wave from the radio.

Tam/WB2TT
"W5DXP" wrote in message
...
Roy Lewallen wrote:
The "reflected power" is not dissipated in the source, matched or not.


If reflected energy could not cause over-current/over-voltage
problems in the source there would be no need for protection
circuitry. I once burned up a pair of ARC-5 1625's because my
feedline came unsoldered at the antenna. If there had not been
any reflected power, it would not have happened.
--
73, Cecil http://www.qsl.net/w5dxp



  #135   Report Post  
Old July 20th 03, 02:24 AM
Reg Edwards
 
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The haggling, for years, about congugate matching, SWR, forward & reflected
power, silly virtuallty, etc, has gone on long enough.

Only G3SEK, amongst the small minority of the radio population who
contribute to these walls, has a grasp of what it's all about. There are
obviously others who are too busy to waste their time on newsgroups.

But who am I to judge?

INTERNAL IMPEDANCE OF RF POWER AMPLIFIERS

Programs TRIODE1 and TETRODE1 assist with the classical design of tube
(valve) power amplifiers. They are very closely related to each other. A key
design feature, the cathode current operating angle, is an input parameter.

Apart from the drive and input-circuits, they also apply to their lower
power transistor equivalents.

For the benefit of those who may insist on knowing, these two programs are
based on the clssical theoretical analysis by americans Everitt, 1932 (who
was probably not original at that relatively late stage of the thermionic
tube). He was followed by Terman in a more practical but more uncertain
manner in the 1940's. No doubt there have been others. None of them, aware
of their weaknesses, would have wished to be worshipped as little tin Gods.

The basics havn't changed since Ohm, Ampere, Voltaire and Heaviside.

Anyone responding to an enquirer, who feels in need of extra support in his
reply, who refers back to the ancients merely displays his inability to
provide a logical explanation and a lack of underlying understanding.
There's nothing wrong, of course, in a lack of understanding except in
propagating it. (Most enquirers do not have the ancient books or easy access
to the books anyway)

Neither of this pair of programs require a congugate match between internal
impedance and the load. There's nothing magical about 50 ohms. It could be
any value as any appropriately designed SWR meter will assume. I have a
75-ohm model. All is based on assumptions. The only absolute value is SWR
itself which does not depend on Zo or Z load but merely on their ratio.

The only way to determine dynamic internal impedance of a PA is to calculate
it AFTER THE AMPLIFIER HAS BEEN COMPLETELY DESIGNED. It is then too late to
have any effect on design.

Just to satisfy curiosity the dynamic internal resistance of an amplifier is
a calculated output quantity of program TETRODE1. It is of course of no
practical value. In the program it is referred to as the Source Resistance
when looking back into the 50-ohm output socket.
----
=======================
Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================




  #136   Report Post  
Old July 20th 03, 04:01 AM
W5DXP
 
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Tarmo Tammaru wrote:
Did you melt the plates on the 1625s or burn out the grids?


Melted the plates and smoked the parasitic suppressors. I don't
know how long the feedline was.
--
73, Cecil http://www.qsl.net/w5dxp



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  #137   Report Post  
Old July 20th 03, 07:47 AM
Ian White, G3SEK
 
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W5DXP wrote:
Ian White, G3SEK wrote:
The load, transformed backward through the output matching network,
controls the slope of the line.


Which is a V/I ratio - which is an impedance. Could this be the impedance
"seen" by the reflected waves transformed through the output matching network?


Specifically, precisely NOT... as apparently you well know (except in
the special case of class A).


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
  #138   Report Post  
Old July 20th 03, 03:01 PM
W5DXP
 
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Ian White, G3SEK wrote:
W5DXP wrote:
If reflected energy could not cause over-current/over-voltage
problems in the source there would be no need for protection
circuitry. I once burned up a pair of ARC-5 1625's because my
feedline came unsoldered at the antenna. If there had not been
any reflected power, it would not have happened.


Transmitters are damaged *only* by excessive heat dissipation, voltage
or current, all caused by the wrong value of load impedance. Reflections
don't come into it.


Reflections are the *CAUSE* of the wrong value of load impedance. The load
impedance on a transmitter when reflections are allowed to reach the
transmitter is (Vfor+Vref)/(Ifor+Iref). Please note that half the terms
in the impedance equation are reflected terms. Please note that without
the reflected terms, the transmitter would probably be perfectly happy
with the "right" value of impedance at (Vfor/Ifor).

Think what would have happened if you had measured the impedance at the
TX end of your o/c transmission line (very high or very low, depending
on the length) and replaced it with a resistor and inductor/capacitor
giving the same value of R +/- jX.

There's no transmission line, so no traveling waves of anything, and no
reflections - just a transmitter with a very wrong value of load
impedance. The 1625s would have burned up just the same.


Yes they would, but in that case reflections are not the cause of the
impedance. In the first case, reflections are the *CAUSE* of the
impedance that burned up the transmitter. Without the reflections, the
transmitter would see (Vfor/Ifor) as the "right" impedance. With the
reflections, the transmitter sees (Vfor+Vref)/(Ifor+Iref) as the "wrong"
impedance.
--
73, Cecil http://www.qsl.net/w5dxp



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  #139   Report Post  
Old July 20th 03, 03:11 PM
W5DXP
 
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Ian White, G3SEK wrote:

W5DXP wrote:

Ian White, G3SEK wrote:

The load, transformed backward through the output matching network,
controls the slope of the line.


Which is a V/I ratio - which is an impedance. Could this be the impedance
"seen" by the reflected waves transformed through the output matching
network?


Specifically, precisely NOT... as apparently you well know (except in
the special case of class A).


Class-A and combined Class-AB are both linear. The impedance "seen" by
the reflected waves transformed through the output matching network IS
linear. The output matching network acts like a pendulum or flywheel.
--
73, Cecil http://www.qsl.net/w5dxp



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  #140   Report Post  
Old July 20th 03, 03:52 PM
Art Unwin KB9MZ
 
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Slick


Dr Slick,
Way back in this thread you alluded to antennas as being
transformers. The more I think about that statement the more I see
it as being fact. Anything that involved coupling which all
antennas do can be drawn as a transformer !
Since the thread migrated all over the place did you feel that
the group agreed with that position?
Regards
Art


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