[TowerTalk] Reflections, Tuners and Antenna Forum Talks

Jim Reid kh7m@hsa-kauai.net
Thu, 11 May 2000 10:53:45 -1000

Steve's post today contains within:

> Now back to Jim's tuner experiment.  I ran an analysis of 
> the set up Jim described, which is the basis for the numbers 
> presented here......... I could not exactly match the tuner 
> component configuration described by Jim and get the 
> correct input impedance.  

Recall that the actual Palstar tuner component "settings" are,
and remain:

Input C at about 65% maximum mesh,  or presumably
about 0.65 of the 350 pF total available C, or 228 pF.

Inductor tap at  1.6 uHy of the total of 28 uHy inductance

Output C,  at 100% full mesh,  or 350 pF in series with the
transmission line.

>This is not really relevant since I arbitrarily set the component 
> values such that the tuner's steady state input impedance 
> was 50 + j8 ohms...........The component values I selected 
> were series C at input = 115.71 pF; parallel L = 1.8 uH; and 
> series C at output = 164.22 pF.  With these tuner components, 
> I calculated the steady state tuner input impedance to be
> about 50 + j8 ohms.

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

Perhaps from the numbers I took today about my Bird
slugs might aid in determination of the "real" forward/
reflected power values?  As I did these slug numbers,
it occured to me that the 5000 watt slug was reading
closer to the slug used to "set"  each power level
into the load;  too bad I didn't use that one also in
collecting some of the data,  hi.

Steve's post continues:

> 1)  The first point is to correctly determine the forward 
> driving voltage delivered to the transmission line.  Remember 
> that when the "system" is first energized no reflections are 
> present at the tuner output.  Therefore, the "load" impedance 
> at the tuner output is Zo.  Therefore, the initial "input 
> impedance" at the tuner input is NOT 50 + j8 ohms, it is 
> calculated using circuit theory to be 58.1 - j51.7 ohms.   

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!

Oh well,  I must read your entire post very carefully,  Steve;
you are correct:  it is long,  and different from what many of
us are accustomed to following.

Anyway,  still look forward to the ongoing discussion,  and
of course,  to Dayton and next Friday!

73,  Jim,  KH7M

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