[TowerTalk] Reflections, Tuners and Antenna Forum Talks

[Steven Best]

Hi Jim:

<SNIP>

Jim Said: Steve,  I just don't understand why using the acutal Palstar
component settings would not be relevant.  All your assumed
settings,  except for the inductor,  appear to be quite a bit
different from where the "real" capacitors are sitting;  I assume
the capacitors do have the 350 pF full spec. rating which the
mfgr. of the Palstar tuner specifies.  Maybe I will go back
and redo the input Z measure;  I could have messed it
up!!

Jim:  No reflection on your measurements or setup.  Maybe the MFJ meter was
not giving correct Z measurements?  Also, I don't know the Q's of the
components.  I was really just trying to come up with a lossy network where
the effective power delivered to the line would be somewhat consistent with
your power measurements.


Jim said: Hmmmm,  well,  Steve,  I know how fervently you hold this
to be true.  I am still having a difficult time with the idea
of a "transient"  changes in impedance because a reflection
has not yet occured.  You claim the tuner output sees only
the transmission line Zo,  and so therefore the input impedance
of the tuner is greatly different from what it will be some a
microsecond or so later (whatever the round trip propagation time
may be in the feedline up to the antenna input terminal and
back).  Still seems to me it is better to use true wave behavior
than  this "introduced"  transient change of nature,  hi!

Jim: This is a tough one.  The physical impedance at any point in the system
never changes.  Only the forward and reflected voltages do.  At any point,
the "effective impedance" is simply the ratio of total voltage to total
current.  As the system transitions to the steady state (granted too fast to
worry about - but the reflection concepts can't be overlooked) the effective
or line impedance does change with each reflection. However, the driving or
source voltage delivered to the transmission line does not change.  The
total forward voltage changes as the sum of the driving and re-reflected
voltage and the total voltage changes as the sum of the driving,
re-reflected and reflected voltage.  These are concepts that can be easily
illustrated with a SPICE analysis. I have done this to illustrate these
point for an upcoming QEX article.  I can send it along if you like.

73,
Steve, VE9SRB


--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm