Mel,
Your points are well taken when you are feeding an end-fed half wave or
a radiator of a multiple of a half wave.
The ideal parallel tank circuit should be fed with a coupling coil that
is isolated from the larger secondary coil - that reduces or eliminates
common mode currents in the shack.
That solution is practical for open coil configurations where the user
can tap the antenna to any point on the coil of the tuned circuit.
However, the tapping choices are limited in a compact, portable
implementation for use in portable operation which use toroids for the
inductor.
We used to use those isolated link coupled ATUs constructed of open coil
inductors where it was practical to tap the coil at any point, but
today's world of toroid wound inductors, that is not as easy.
If one has a resonant parallel tuned circuit, it will match very high
impedance, and a series tuned circuit will match very low impedance.
The link coupling will provide isolation from common mode currents.
Those type of ATU's work very well with a wide range of antennas with
varying feedpoint impedance.
However, the physical implementation of the ability to tap the antenna
to any turn of the high impedance tuned resonant inductor requires a
physically large coil. While it will work *very* well, it is not
consistent with small ATUs used for portable operation.
If you have a fixed length radiator and work only a single band, you can
devise a link coupled tuner that will do a great job, but if you are
multiband, and do not want to fiddle with coil taps, the
auto-transformer is a good compromise.
Yes, I still have my link coupled ATUs with plug-in coils for each band
and also have a Johnson Matchbox which is also link coupled. They do the
job well, but the convenience of toroid wound ATUs is an advantage in
simplicity and convenience.
73,
Don W3FPR
On 9/29/2016 8:04 PM, Mel Farrer via Elecraft wrote:
Fred said it right,
Let me go to another side of the question. No auto-transformer can match all
of the antenna reflected impedances, but using a parallel tank circuit to
ground CAN. The beauty of this arrangement is that you can tap the coil on the
input of the coil and tap the output for a VERY wide range of impedance. R and
j . What seems to have been lost in the transition from ancient and now is
that we did not have ATU's. All of the PRE now used tuned circuit match boxes.
Now you have to think about it for a few minutes.
Let me explain, the tuned parallel tank circuit can do a wide range of matching
BECAUSE, If you tun the tank to one side of resonance your get a negative
reactance, if you tune it to the other side you get positive reactance.
HUMMMMMMM.
If the tank circuit is a true resonant one, the impedance across the coil from
top to bottom is a range of the impedance available from HIZ to ZERO. This is
the beauty of the parallel tuned circuit over a auto transformer. Oh well at
least I remember it.....
Mel, K6KBE
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