Everything Ron said is correct-- except that when an amplifier is generating
1000 watts and feeding an antenna through a line with an SWR that results in
100 watts reflected power, the usual SWR/power meter will show 1100 watts
forward and 100 reflected. A meter like the Alpha 45xx has the option of
computing "delivered power", which is forward-reflected.
In the situation described by Bill, the actual power generated is 900 watts,
and the antenna will radiate 900 watts less the losses incurred. The multiple
reflections of a portion of that power are what cause the additional losses due
to SWR.
Don't believe this? You can test it. Transmit a given power into a 50 ohm load
and note the reading. Say forward power is 10 watts and reflected is 0. Then
change the load impedance so there is an SWR greater than 1:1. You will note
that forward and reflected readings will both increase about the same amount.
Vic 4X6GP
> On 25 May 2017, at 2:46, Ron Youvan <ka4inm@xxxxxxxxx> wrote:
>
> Bill W6WRTTurner wrote:
>
>> There is something I don't completely understand about the effect of
>> SWR on an amplifier. Perhaps someone here can clear it up?
>
>> Let's say my SWR meter indicates 1000 watts forward and 100 watts
>> reflected.
>
>> 1. Does this mean the actual radiated power from the antenna is 900
>> watts?
>
>> 2. How much power is the amp actually putting out - 1000 watts or just
>> 900?
>
>> 2. If the answer to #2 is only 900, why does the SWR meter indicate
>> 1000? Is that a false reading caused by the actual 900 added to the
>> 100 reflected? If so, why does reflected and actual power add to
>> produce a false reading?
>
>> After 60 years in ham radio I suppose I should know this stuff, buy I
>> must have been absent that day. :-)
>
>> All comments appreciated.
>
> Do not worry about the confusion, it is all too common among HAMs even
> highly seasoned ones, to get this wrong.
> You will soon here the opposite of the following here, but they are
> wrong.
>
> The amplifier outputs 1,000 Watts.
>
> 900 Watts minus "transmission line loss" X 1.0 is radiated.
>
> 100 Watts are reflected by a line to "load mismatch" back to the
> "generator" the amplifier (the only cause of any SWR) or reflected
> power. This power when it reaches the (generator" the amplifier) finds
> a source impedance of under one Ohm, so it is reflected back up the
> transmission line experiencing a "transmission line loss" X 2.0 where
> 90% of that power is radiated.
>
> 10 Watts are reflected by a line to "load mismatch" back to the
> amplifier finding a source impedance of under one Ohm, so it is
> reflected back up the transmission line experiencing a "transmission
> line loss" X 2.0 where 90% of that power is radiated.
>
> 1.0 Watts are reflected by a line to "load mismatch" back to the
> amplifier finding a source impedance of under one Ohm, so it is
> reflected back up the transmission line experiencing a "transmission
> line loss" X 2.0 where 90% of that power is radiated.
>
> 0.1 Watts are reflected by a line to "load mismatch" back to the
> amplifier finding a source impedance of under one Ohm, so it is
> reflected back up the transmission line experiencing a "transmission
> line loss" X 2.0 where 90% of that power is radiated.
>
> 100% of the generated power is either radiated or turned into heat
> from the transmission line (and balun + antenna wire) losses. (mostly
> resistive)
>
> None of the forward power stays in the amplifier, although the
> reflected power does change the "loading" of the amplifier, changing
> it's operating conditions, from what it was with a pure reactive-less
> resistive load.
>
> --
> Ron KA4INM - Youvan's corollary:
> Every action results in unwanted side effects.
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