On Mon, 17 Nov 1997, John Mitchell wrote:
>
> In that case, what happens to the 300 Watts that is measured as reflected?
>
> According to at least one book I've read (Maxwell's Reflections), that power
> is re-radiated down the feedline, where the same proportion of RF bounces
> back, and is again re-radiated, until it finally either all goes out the
> antenna, or gets eaten up by feedline loss.
>
> So, the power isn't lost, but is it measured? Does the Bird directional
> meter account for this re-radiation due to lack of a conjugate match?
>
> 73, John K4IQ
Hi John - This is a good question about what's going on "inside" the
transmission line.
Assume you're transmitting a continous carrier and think of the
transmitter's output as a sequence of wavefronts in one continuous stream.
Each wavefront traverses the line, being partially reflected at the load.
As it goes past the meter outbound, the meter registers some forward
power. When it comes back (now smaller), the meter registers some reverse
power. When the inbound wavefront encounters the transmitter output
circuit, some of it is reflected again, making another outbound wavefront,
which repeats the cycle until completely radiated or absorbed in the coax.
At any given time, there would be a Very Large Number of wavefronts all
bouncing back and forth.
If the meter was really fast...such as an oscilloscope with a
high-bandwidth detector...and you transmitted a REALLY short dit (even
faster than K7SS in the Sprints) then you would see it banging back and
forth in the line. Time-Domain Reflectometry (TDR) does just this and
cable repairmen use it to find breaks and taps.
In fact, for fast digital signals on a PCB, you can see the wavefronts
piling up at each end of the trace, making a stairstep-like leading and
trailing edge to a pulse. This is usually a big surprise to new digital
designers who have never dealt with RF issues before :-)
The total meter deflection is the sum of all the wavefronts passing in one
direction during the period of time over which the meter electronics
average the reading. The meter is *far* too slow to show the effect of
each individual wavefront.
All the meter can tell you is that, "Right now, the total power of all the
wavefronts passing me in the forward direction is 1000W, while the sum in
the other direction is 300W." Note that the meter is completely unable to
tell you anything about the relative phase of the wavefronts and, so,
cannot tell you anything about the total instantaneous power in the line
at any given moment.
This somewhat oversimplifies the exact explanation and does not address
the issue of why a meter might indicate more instantaneous power than the
transmitter is supplying. Hope this helps.
73, Ward N0AX
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|