Posted by
Don Wilhelm-4 on
Nov 17, 2007; 5:22am
URL: http://elecraft.85.s1.nabble.com/K2-s-n-4276-Output-Power-Instability-tp366380p366416.html
Yes, I fear that many hams do not realize how much loss is being
introduced by their tuners. It has been stated that MFJ makes good
tuners, but my experience with their MFJ-963D and the MFJ-971 tuners say
otherwise - the inductor is mounted too close to the enclosure which
reduces the inductor Q and thereby reduces the efficiency. BTW, there
is no real need to have the tuner inside an RF proof enclosure, a tuner
will not introduce harmonics or other spurious responses that have been
adequately suppressed by the transceiver - a tuner built on a plain
wooden board will work just great.
The fact that there are tuners and then there are tuners is true
indeed. The easy to manufacture T network tuners can have multiple
setting combinations that produce a low SWR at the input, but many of
those combinations will have a high loss factor due to RF currents
circulating through the inductor. This is a recognized problem with the
easy to implement T section tuners. The PI section and L network tuners
do not have that ambiguity, but the only PI section tuner that I know of
is the old Collins tuner - it is almost as efficient as the classic link
coupled tuners, but due to the range of practical variable capacitors,
its matching range is limited - the L network tuners similar to the
Elecraft design are much more flexible and have a wide matching range.
Every tuner design has its limitations. The T section, L network, and
PI network tuner designs are nothing more than a transmission line
section implemented with lumped components - the limits are defined by
the range of adjustment for each of the components, and bandswitching
with these designs is not difficult although the Pi network tuner does
have limitations due to practical capacitor values. Iin bandswitching
tuners, the L network is the most efficient - the Elecraft tuners all
use the L network configuration.
The link coupled tuner is usually the most efficient, but does not lend
itself easily to bandswitching (the Johnson Matchbox is one link coupled
bandswitching design that works well, but even it has a limited matching
range). When the going gets tough, I fall back onto the classic link
coupled tuners - they are the most efficient by far, but the use of
plug-in coils for bandswitching is a drawback to many hams. With
renewed interest in low loss tuners, it may be time to accept the
reality that efficiency must be sacrificed for the convenience of
bandswitching designs. Unfortunately, there is no one correct answer,
each situation must accept its own consequences. I myself use the
KPA100 in the shack for fine tuning of my coax fed resonant antennas at
power levels up to 100 watts and an MFJ-962 tuner for times that I add
the linear. It is a compromise, but it does work well for me. Since I
cannot have open wire line coming into the hamshack, any link coupled
tuners must be fixed tuned and located remotely (relay switched). That
is not fully implemented yet, but is in the planning stages.
73,
Don W3FPR
Darrell Bellerive wrote:
> The big problem as I see it is that those who buy the tuners don't realize the
> tradeoffs they have gotten. After all it tunes to a 1:1 SWR and contacts can
> be made, so it must be working well. The reality could be quite different.
>
>
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