[AMPS] Lets chew on this

Tom Rauch (W8JI) w8jitom@worldnet.att.net
Wed, 05 Mar 1997 19:22:53 -0500

"Dick Green". wrote:
> Assuming you are describing a situation where the transmitter and antenna
> system are not matched, I wouldn't say that the reflected power is
> "retransmitted" on the next cycle, but rather that the 1000 watts of
> reflected power on each cycle is dissipated by the finals.

As a general statement we couldn't say what would happen. That's because
the tube is not the source, it is a time varying resistance. The tube's
dissipation is only (neglecting minor dielectric and resistive losses)
affected by conduction angle, and voltage times current integrated over
every point during the conduction period.

An example of a PA with 100% (or near 100%) reflection is a PA dipped to
minimum current with no external load at all. Dissipation can be less
than the quiescent (idle) dissipation of the tube under that condition.  

> Yes, but it is only correct to subtract reflected from forward power if you
> take the measurement at the transmitter output terminal. If you take the
> measurement after the matching network (which could include part of the
> feedline in some systems), subtracting reflected from forward power would
> be incorrect, at least from the FCC's point of view.
> In other words, if you use a tuner or matching network to obtain a match
> between the amp and the feedline, the amp will not see the reflected power
> (and hence the finals will not dissipate it). But a meter placed at the
> antenna end of the feedline will show both forward and reflected power.
> Both are absorbed by the antenna (and either radiated or dissipated as
> heat.) However, in this case it would be incorrect to subtract reflected
> from forward to get the output power.

The only error would be in the power loss in the tuner, After the tuner,
even in this case, the real power fed to the antenna would still be
forward minus reflected power.

This rule always applies. Nothing you do in front of or after the meter
can change that.

The whole concept of power bouncing around really confuses everyone. It
is a description useful for analysis by wave theory, nothing more. The
whole problem can also be viewed as impedance changes, with equally good

73 Tom

FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Sponsored by:             Akorn Access, Inc. & N4VJ / K4AAA