[TowerTalk] Open Wire

[Jim Reid]

Phil wrote:

> The proof of the pudding in store-bought tuners is usually 
>160 meters.  The two capacitors in the "T" configuration 
>are usually 200-250pf each;  not near enough for the 
>proper L/C ratio on Top Band. This causes you to
> switch in more coil (or roll in) to get a reasonable match. 
>Most of the coils are on the puny side to start with, and a 
>melt-down process begins.

> I would never push the so called "legal limit" tuners past the 
>500 watt level on 160 meters.

Another very nice design parameter of the Palstar AT4K 
antenna tuner:  350 pF variables and the option to switch 
in another 7.5kV 350 pF in parallel with the 350 pF output
variable C.  28 uhy roller is an edge wound copper strap
(3/8" wide conductor, .062" thick) silver plated unit rated
at 5kV, 30 amps.

And,  yes,  160 meters is the band of concern when
a tuner is used.  The higher bands are usually only
a problem when the real part of the antenna impedance
gets below 25 ohms,  and/or the reactive part,  either
inductive or capacitive gets well over 100 ohms.
The higher the real part of the antenna impedance,
even up to 1600 ohms,  the higher the reactive part
can also be,  and still have reasonably low tuner
loss,  well under 20 % when the real part is high,
way above 50 ohms.

You really need an antenna analyzer telling you what
the complex impedance at the input of your feed line
is just where you will attach to it in the shack.  Then
you will know whether you should worry about tuner
efficiency losses as a result of a way out load impedance.

AT4K user manual comes complete with a page of operating
loss data for each band vs.  load impedance showing loss
vs.  XL or Xc vs. R of the load.  Loss is color coded:  bright
red if loss is well over 20% in the tuner,  which can occur on
160 when the antenna impedance is between reactances
of up to +112.5 ohm inductive,  and down to -100 ohm capacitive
with the real part,  R,  being as low as 6.25 to 12.5 ohms.
Loss of 20 % or more,  but not super high loss as in the
above extreme Z mismatch cases,  can also occur on 160
when the load reactances are above 400 ohms inductive
with R as low as 25 ohms;  and for capacitive reactances
as high as 200 ohms with the same 25 ohm real part of
the load impedance.

So,  this is a complex impedance problem which changes
dramatically with load impedance (which depends upon
the operating frequency and the antenna design,  of course).

Not sure if this helps any in understanding about "store
bought"  tuners.  Don't have the '99 ARRL Handbook,
but do have a Palstar with which I am pleased.  And
with the AT4K manual,  and my MFJ-259B antenna
analyzer,  I pretty much know my operating losses;
they are very low.

73,  Jim,  KH7M



--
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