Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!sdd.hp.com!hplabs!hpl-opus!hpnmdla!joeld
From: joeld@hpnmdla.hp.com (Joel Dunsmore)
Newsgroups: sci.electronics
Subject: Re: Impedence matching...
Message-ID: <7380019@hpnmdla.hp.com>
Date: 3 Apr 91 06:18:31 GMT
References: <2532@umriscc.isc.umr.edu>
Organization: HP Network Measurements Div, Santa Rosa, CA
Lines: 31

In sci.electronics, lrk@k5qwb.UUCP (Lyn R. Kennedy) writes:

    robf@mcs213k.cs.umr.edu (Rob Fugina) writes:

  >> I'm confused as to the importance of impedance matching...  Since I may
    .
    .
    .
  >> need, when you have a low impedence source, to have a low impedence load?
  >> Me

  >  If you don't need to get all the power from the source into the load
  >  there is no problem.

One tiny problem, (ususally not important in low power, but maybe in
dorm blasters, or ham 1kW amplifiers, and other high power devices)

That is, if the output match is a hi-Z, the voltage across the last
stage can be 2x the rated, and blow the last stage (RF voltage, not
DC).  This can be a problem, and some fixes include reverse power
protection circuits...but, as I said, is only of major concern if you're
talking high power.  (or low power 20 GHz GaAs FETs with only 9 volts VDS
breakdown :-(  ).

     The effects of mismatch can actually damage cables, if they are 
rated at right where they are being used, and the match goes bad (some
enemy fighter shoots off your antenna), the VSWR (Voltage standing
wave ratio) can cause your matched voltage to exceed the dielectric
strength of your coax (really!).

Joeld