Paul Christensen wrote:
> > If the power reflects back and forth then that portion would never reach
> the
> > load would it?
>
> Correct. In the case of a 50-ohm resistive source (e.g., a solid-state
> transmitter without a pi-network, ATU, or other reactance network),
> reflected power created by a line-to-load mis-match is absorbed by the
> source (The PA in the transmitter), less a portion of the reflected power
> absorbed by line losses.
If all the reflected power was absorbed by the transmitter PA then there should
be no increased forward power reading on the bird meter whether there is
reflected power or not?
>
>
> > Doesn't a bird meter read higher forward power than there really is when
> > there is reflected power.
>
> Yes.
>
> > So subtracting the amount of reflected power from the forward power
> reading
> > would not give you the amount of power reaching the load?
>
> Someone keep me honest here with regard to the mechanics of a directional
> coupler: Notwithstanding line losses, the amount of reflected power shown
> by a Bird Thru-line Wattmeter is the amount of power reflected solely by the
> line-to-load mismatch. The mismatch creates a 180-degree reversal of the
> reflected *wave,* with respect to the forward wave at the juncture of the
> line and load.
I think that it would only be 180 degrees if it was an infinite mismatch. (short
or open) Otherwise it would be something less or greater.
>
>
> Whether the source is purely resistive, which absorbs all reflected power
> (less power lost in the line), or if the source is reactive as in the case
> of a pi-network or internal ATU, the Bird will still see the effect of the
> reflection, in both forward and reflected readings, as a result of the
> line-to-load mismatch. Except for line loss and assuring that the line
> characteristic impedances matches the charcteristic impedance of the Bird
> directional coupler, I do not believe any other portion of the system has
> relevancy with respect to the indicated reflected power.
>
I think what happens at the transmitter end is that the line is no longer the
same impedance as the transmitter when there is reflected power. So the
transmitter will never present a pure resistive load to the line when the line
is mismatched at the antenna end. (solid state transmitter)
With a pi network matching system or other type of matching system, If the
matching circuit is tuned for maximum power out then the final should be
presenting a resistive load to the line? In this case all the reflected power
would end up in the plate of the final if this were true?
73
Gary K4FMX
>
> -Paul, W9AC
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