[TowerTalk] SWR is what SWR meter measures

[Martin, AA6E]

I hope there's more light than heat (forward or backward) in this discussion.

Anyway,  the common SWR meter samples forward and reverse power by
adding a current sample and a voltage sample.  This makes a
directional coupler.  (See discussion of the KI6WX Tandem Match, p.
27-9ff, ARRL Antenna Book, 20th Ed.)  I guess we agree on that. The
"power" is measured by your favorite detector, after the forward and
reverse samples have been separated.  The situation is clearer when
you look at waveguide directional couplers, where the main waveguide
leaks energy through slots or holes into an adjacent one.  You detect
the power presented at each end of the adjacent waveguide.

An ancient SWR indicator was built around a twinlead directional
coupler.  You loosely couple your 300 ohm twinlead transmission line
to a short section of twinlead that feeds two pilot lamps that glow
with forward and reverse power.  (See p. 278, The Radio Amateur's VHF
Manual, ARRL, 1965. Sometimes it pays to have an OF's library!)  The
twinlead coupler effectively adds voltage & current samples, and the
bulbs do the detecting.

The forward/reflected power discussion is clearer when you talk about
fast pulse transmission instead of steady state.  You launch a
nanosecond pulse out on your transmission line.  If your mismatched
load is out there at 100 ft, you get a reflected pulse back in ~ 200
nsec.  (This is TDR - time domain reflectometry. Hopefully the pulse
bandwidth is not too large compared with your operating frequency.)
It's just one more way of thinking about the problem.

73 Martin AA6E
-- 
martin.ewing at gmail.com
http://blog.aa6e.net