[TowerTalk] Re. Impact of Reflected Power

[DOUG WALLER]

Tnx Steve for this response.......something a little more rational.  After all
this enlightment, I still plan to keep impedance mis-matches to a minimum, just
in case its worth a db or so.  Doug - Nx4d

Steve Thomas wrote:

> Thanks for this response. I got a good laugh out of the signal moving so
> fast that you don't care about phase.
>
> When energy is reflected anywhere in a transmission line system, the
> "reverse" portion that is reflected will have a varying phase relationship
> with the signal that is traveling forward from the source. This causes peaks
> and valleys in the voltage along the transmission line. Standing Wave Ratio
> is a measurement of the ratio of the voltage peaks to the voltage valleys.
>
> When the reflected power reaches the source, two things happen. If the
> source is well-matched to the transmission line, the reverse signal is
> dissipated in the source. If the source looks like a high impedance or low
> impedance (that is, poorly matched) compared to the transmission line, the
> signal will be re-reflected and again travel forward toward the antenna (or
> whatever load is at the far end of the transmission line.) Depending on the
> goodness of the match, there will generally be a combination of these two -
> with some signal being dissipated and some being re-reflected.
>
> 73, Steve N6ST
> steve@n6st.org
>
> > -----Original Message-----
> > From: owner-towertalk@contesting.com
> > [mailto:owner-towertalk@contesting.com]On Behalf Of DavidC
> > Sent: Tuesday, July 28, 1998 2:19 PM
> > To: DOUG WALLER
> > Cc: w8ji.tom@MCIONE.com; towertalk@contesting.com
> > Subject: Re: [TowerTalk] Re. Impact of Reflected Power
> >
> >
> >
> > What about the timing issue some have explained at HF -- the signal is
> > moving so fast that it obliterates the impact of phase concerns
> > -- at least
> > for the sort of narrow bandwidth signals we Hams use?
> >
> > I agree that it all sounds ugly, having our signal rattling up
> > and down the
> > feedline, but I guess it happens so fast that it is almost as though it
> > hasn't happened at all.
> >
> > 73, DavidC  AA1FA
> >
> > > Naaaahhh, cant be.  If initial sig hits antenna, and part of energy is
> > > reflected back to amp, then all of that is reflected back to
> > ant, then we
> > are
> > > already getting these two sigs out of phase.  then if part of that is
> > reflected
> > > back to amp, then reflected back, we are even more out of phase.
> > Depending on
> > > the transmission line length and its velocity coefficient, pwr
> > nulling by
> > > phaseout would seemingly take place.  Even if this is wrong, I just dont
> > like
> > > it, i tell ya!
> > >     Peace, Doug - Nx4d
> > > >
> > > > So, are we agreeing to conclude that reflected power that is resolved
> > by
> > > > the final tuned circuit of an amp, rig, or antenna tuner is
> > meaningless
> > and
> > > > to be ignored as irrelevant at HF frequencies?
> > > >
> > > > I am using "resolved" to mean that the amp or rig is protected from
> > > > troublesome voltage, current and/or impedance's.  I am also
> > using it to
> > > > mean that essentially all reflected is re-reflected back to
> > the antenna
> > and
> > > > directed to its intended destination.
> > > >
> > > > One question:  If one adds a group of toroid beads to the coax
> > immediately
> > > > exiting the amp or rig (some say this helps with impedance stability
> > and
> > > > eliminating rf in the shack), what is the actual result?  Is any
> > > > significant loss injected into the system design?
> > > >
> > > > Thanks & 73, DavidC  AA1FA
> > > >
> > > > > Reflected POWER has little to do with anything except as it affects
> > > > > voltages, currents, and impedances by presenting a different
> > impedance to
> > > > > the PA.  73 Tom
> >
> >
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> >




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