"Oliver Gebele" wrote
i am using a Collins-Filter with 2 variable
capacitors and a switched inductor (Pi).
Obviously there are infinitely many possibilities
to match an antenna.Are there any preferred
possibilities? And how would i adjust my box
to do this? (Maybe there are already links on
the internet that i did not find.)


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Matching networks, with 2 C's and 1 Coil, can be either Pi networks or
T networks. The Pi network, with the coil in series, also behaves as
a low-pass filter and for this reason it is to be preferred. But the
T network has more convenient and therefore less expensive values of L
and C. So for economic reasons manufacturers sell more T than Pi
networks. Pi-networks have considerably larger coils and capacitors,
but in general they are equally efficient when working between the
same transmitter and antenna impedances.

As has already been pointed out, with a T network, 2 series C's and a
shunt coil, least loss nearly always occurs when the coil, in
conjunction with the C's has it's lowest inductance. But component
settings are quite uncritical.

Problems occur only when one or other or both of the capacitors are
very near to their minimum settings, ie., little more than stray
capacitance remains in cicuit. The capacitors may then arc over
(which is immediately apparent). Or the coil turns may become
overheated when attempting to run 1 Kwatt or more (which may take a
minute or so when operating CW or FM).

I mention overload circumstances only because the effects and worries
are too often exaggerated in these newsgroups. So don't lose any
sleep! Even if half the power is lost in your tuner your signal
strength falls by only 1/2 of an S-unit.

To become familiar with series T-Match Networks download program
T_TUNER from website below. Any impedance R+jX can be matched to any
other R+jX. But usually the Tx or generator impedance is a purely
resistve 50 ohms which simplifies understanding of what's going on.

Other practical programs, from the very few I've seen, only manage to
match two purely resistive impedances. Which with radio antennas
seldom exist.

Download program T_TUNER in a few seconds and run immediately. Go from
"Index" to "Download Progs from Here" page and run immediately.

There are several other programs of various sorts which include
operation and component values of T and L-match transmitting circuits.
If you are accustomed to calculating how many rolls of wallpaper are
needed to re-decorate your shack then you will feel at home.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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