I have read this about 4 times and I still can't figure out what you are
saying. "there is no such thing as reflected power but, oh wait, we can
measure reflected power just like real power but the reflected doesn't exist
it is just fooling the wattmeter".???
Maybe I am just not understanding what you are saying?
What is this "portion of the waveform that gets reflected"?
It is true that most all of the power either gets radiated or is dissipated
in heat in the feed line and is not dissipated in the transmitter. However
the reason is because with a conjugate match to the feed line at the
transmitter all of the power that has been reflected from the mismatched
antenna is re-reflected back toward the antenna to be ultimately radiated.
Less of course any that didn't make it to either end because of feed line
loss. This is why the wattmeter will read higher forward power than is
actually coming out of the transmitter. The re-reflected power gets added to
the forward power.
Where this doesn't hold true is if the transmitter had an isolator on it.
Any reflected power that returns down the line is now dumped into the load
on the isolator and dissipated as heat in that load. There is no
re-reflected power in this case.
You can prove that by putting a wattmeter between the transmitter and
isolator and another wattmeter between the isolator and the antenna. If the
transmitter puts out 100 watts the first wattmeter will show that. If say
there is 20 watts of reflected power coming back down the feed line, the
second wattmeter that is between the isolator and antenna will show 100
watts forward and 20 watts reflected.
If the isolator is removed and a conjugate match is provided in place of the
isolator the second wattmeter will show 120 watts forward and 20 watts
reflected while the first wattmeter shows 100 watts forward and zero
reflected.
Another situation where all the reflected power does not get re-reflected
back toward the antenna is with a fixed tuned output on the transmitter such
as a solid state transmitter. In this case there is no conjugate match and
some of the reflected power will be dissipated in the transmitter circuits
and finals. All because there is not a perfect conjugate match to the line
at the transmitter.
A conjugate match is an equal resistive part and opposite sign but equal
amount of reactance. The conjugate match cancels out any reactance presented
by the line to the transmitter due to the mismatch at the antenna end.
But reflected power is real! It is just as real as the forward power.
> If the generation source is "matched" to the feed line but the feed line
is
> not "matched" to the load then VSWR or "reflected power" is going to
> occur. There will be no additional losses at the transmitting end but
> there will definitely be additional losses in the feed line.
Being matched at the transmit end is called a conjugate match and it will
re-reflect any energy that comes down the feed line back up towards the
antenna. All the reflected power will reach the antenna to be radiated less
the feed line loss suffered in both directions of the reflected and
re-reflected power.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Glen Zook
> Sent: Thursday, April 23, 2009 4:37 PM
> To: Amps; Jim Brown
> Subject: Re: [Amps] High SWR,
>
>
> Power itself is not reflected. What is reflected is a portion of the wave
> form and, as such, that can be measured in the same manner as power. But,
> in the true sense of the word power is either dissipated as heat in the
> feedline or is radiated.
>
> If the power was, in fact, actually reflected then the transmitter would
> definitely have to dissipate this power. But, in the real world, what is
> dissipated in the final amplifier of the transmitter (as heat) is the
> power that cannot be transformed into r.f. energy. If the r.f. coming
> from the final amplifier is not properly transformed into the feedline
> then additional losses (again as heat) will happen and have to be
> dissipated in the final amplifier. The match between the feedline and the
> load (generally the antenna) is another cause of heat generation (this
> along with the IR losses in the feedline).
>
> The "reflected power" can be measured. But, since the reflected power is
> added to the actual power of the transmitter by the instrumentation (this
> is read a "forward power") and the reflected power reading has to be
> subtracted from the forward power to indicate the actual power being put
> from the transmitter into the feedline, therefore, the reflected power can
> be considered "phantom". Yes, it can be measured. But, there is no real
> physical effect of "reflected power". Now an improper "match" between the
> generating device (in this case the transmitter) can result in additional
> heat being generated. But, this is different from "reflected power".
>
> If the generation source is "matched" to the feedline but the feedline is
> not "matched" to the load then VSWR or "reflected power" is going to
> occur. There will be no additional losses at the transmitting end but
> there will definitely be additional losses in the feedline.
>
> There used to be a "white paper" on the Telewave website which gave a
> detailed explanation of this. However, I could not find it this morning
> when I was looking.
>
> Glen, K9STH
>
> Website: http://k9sth.com
>
>
> --- On Thu, 4/23/09, Jim Brown <jim@audiosystemsgroup.com> wrote:
>
> Basically, "reflected power" is not real.
>
> The author of this false statement would be far better off learning how
> transmission lines work by studying the ARRL Handbook and ARRL Antenna
> Book than the manual for someone's SWR bridge.
>
> The concept of reflection energy on transmission lines is QUITE solid. To
> deny that is to deny Maxwell's Equations! By the principle of
> superposition, the current, voltage, and power at any point on a
> transmission line are the sum of the forward and reflected current,
> voltage, and power at that point.
>
>
>
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