Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Old December 31st 15, 02:33 PM posted to rec.radio.amateur.homebrew
external usenet poster
 
First recorded activity by RadioBanter: Dec 2015
Posts: 2
Default Where does the power go

On Wednesday, November 2, 1994 at 6:52:22 PM UTC-5, Guy Fletcher wrote:
I have followed the thread on reflected power in a transmission line
with interest and curiosity, hoping to learn something.
I have also thought deeply about the question on buses and in the shower!
For any new readers who didn't follow the earlier discussions,
the question is about reflected power from a mismatched load (antenna)
driven through a transmission line from a high power tube amplifier,
though it applies to any power level and any amplifier type.
What happens to the reflected power?
Is it dissipated in the amplifier? Where?
Or is it reflected again at the possible mismatch looking back
into the amplifier from the line?
The power will all go to the load if the source and the transmission line had no resistive components
I believe the problem lies in the question being wrongly posed,
though it seems a perfectly reasonable question to ask.
A transmission line is a wondrous invention which, if infinitely
long to avoid reflections, has the property that the input
voltage and forward current are in phase and have a definite
ratio - the characteristic impedance, usually 50 ohms for coax.
The infinite line can then be terminated at any point by 50 ohms
without the input seeing any difference - the line is matched.

Please forgive the long and well-known introduction;
I am getting to the point!

If the load is not 50 ohms (and real), a forward pulse is partly
reflected and returns to the input, where it may be reflected
again depending on the load it sees looking back into the source.
The speed on the line is about 1ns/foot in air, and about 2/3
of that in solid line, so for any reasonable line the round trip
time is sub-microsecond. Unless our transmitter is modulated
at a frequency above 1MHz, so that the amplitude can vary
significantly in less than a microsecond, the returning wave
will see the original amplitude still coming out, but several
cycles later depending on the line length. This applies to keying
too unless you want to splatter over the whole band (or HF spectrum).

So we are realistically talking about a steady key-down state of
the amplifier, except perhaps for UHF TV signals.

In a mismatched line (or a matched one) you can open the line at
any point and measure the signal. The ONLY things you can measure
are the voltage, the current, and their phase relationship.
At the load end, this defines the complex load impedance,
and if this were the power frequency of 50 or 60 Hz, we would
mutter about power factor to calculate the absorbed power,
but we would NOT talk about reflected power back down the wires.

A mismatched transmission line has the property that the apparent
load impedance varies along the line, and we have a standing wave.
The apparent impedance is complex except at the nodes and antinodes
where it is real but smaller (50 / SWR) at a node, or larger
(50 x SWR) at an antinode.

We find it MATHEMATICALLY convenient to analyze this standing wave
in terms of a forward wave and a refected wave, though the definition
of SWR refers only to the voltage ratio at antinode and node, and
doesn't need the formal idea of 2 opposing waves.

We have even designed an instrument to measure SWR, which we tend
to think of as measuring forward and reflected power because we
have put a scale on it marked in this way! What it actually
measures is the in-phase (forward) component of current,
and the out-of-phase (reverse) component, into a presumed
50-ohm load.

There is only one power flow in the line, and that is forward
(hopefully). The voltage, current, and phase difference at
any point in the line define that power flow (in either
direction depending on the phase). Amazingly the magnitude
and direction of the power flow are the same everywhere on
the line apart from losses.

So at the line input a fast pulse always sees 50 ohms, whatever
the distant load, but a steady sine wave sees a complex load
calculated in the usual way.

The tube amplifier is usually designed to deliver its rated
power into 50 ohms real. It will probably deliver more into
a lower load (which may damage it), and it will certainly
deliver less into a higher real load. It will also deliver
less into a complex load unless you tune out the imaginary part.
The efficiency of the amplifier depends on the load. If into
a certain (complex) mismatched load its efficiency is 70%,
then 30% of the dc input power is lost as heat, mostly in
the plate of a tube amplifier.

That 30% was never delivered as power down the line in the
first place, so the question of what happens when it comes
back down the line never arises.

Nor does the output impedance of the amplifier matter -
it may be 50 ohms, or it may be much less as with most solid
state amplifiers. The output impedance is merely part of
the amplifier design. What matters is the rated load resistance,
the power delivered into that rated load, and the way the
efficiency varies with other loads.

This is a rather different way of looking at the question,
hence the new thread. Considered comments welcome.

Guy Fletcher VK2BBF

The power will all get dissipated in the load if there is no resistive component in the source or transmission line. This is true regardless of frequency. In real systems the source and T line both have a real R component which will dissipate the reflected energy as heat, high SWR means less power to the load. In systems where the T line has very little restive component then huge power can be transmitted. For example: a small optical fibers are used to carry 100KW of power without issue.
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
FA linear power supply with toroid power xformer jeff Equipment 0 March 19th 06 08:07 AM
Collins R390 power cord and power line filter Vollumscope Boatanchors 0 December 26th 04 06:26 AM
Collins R390 power cord and power line filter Vollumscope Boatanchors 0 December 26th 04 06:26 AM
Using a power mosfet to select from various power sources for HFrig James W Homebrew 0 March 8th 04 07:33 AM
Astron RS-20A Power Supply Great Condition - used to power a VHF radio Les Smith Swap 5 October 16th 03 09:47 AM


All times are GMT +1. The time now is 06:09 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