[TowerTalk] Tuner Loss - Do Not Panic
Tue, 16 Nov 1999 15:04:20 -0800 (PST)
> > I have a friend who peeled the paint on one of the Dentron tuners a few
> > years ago, and he only had a FL2100 at the time for an amp.
> > Gary
> You folks are all making me nervous.
Hold the phone, guys...
Almost any tuner can be put in a situation where it is trying to match
extreme loads. In these cases, efficiency will be less than if it's
trying to perform an "easy" match. Very low, very high, and very reactive
loads all stress a matching network.
If you look at a Smith chart, you will see these stressful loads near the
outer circle. The tuning network "moves" the impedance along the circles
of constant impedance or constant admittance until the resulting impedance
is close to the center of the chart - 50 resistive ohms (in a 50-ohm
system). Long trips are associated with high voltages or high currents in
the network because the trip on the Smith chart goes through or approaches
a high impedance or low impedance point, respectively.
As a general rule of thumb, the farther the trip to the center, the more
losses are incurred. A tuner can be inefficient because it's components
are undersized or poorly made OR it can be inefficient because its circuit
topology makes for a long trip when presented with certain loads.
Certain types of tuner circuits do better than other for certain types
of loads - a discussion rather beyond the current scope. For example, an
L-network matches half the Smith chart rather well. For the other half,
it won't even get close - you have to "turn it around" or swap the L and
C. Because of the simple nature of the circuit, there are fewer parasitic
elements to compromise performance and it's easier to justify the use of
high-quality components that handle those long trips better.
Pi- and T-circuits, and the various hybrids, cover more of the
Smith chart, but don't typically go as far into the extreme matches as the
simpler L-network. It's a design trade between flexibility of match and
This is why it's a good idea to have some "extender" cables around to
change the impedance presented to the tuner - whether the line is open or
coax. Note that it does NOT change VSWR - it just moves the impedance
around on the Smith chart to a point that may be a little easier for the
particular tuner to match. Learn the trouble signs (heating, arcing,
extremly sharp match tuning) and do something to make life easier for the
Here's some trouble spots to look for in a tuner:
- thin wire, particularly in coils
- close spacing in capacitors
- small toroid cores
- small switch wafers with closely-spaced terminals
- components mounted close together or close to metal
All of these can cause trouble on a long trip to the center of the Smitch
chart. You have to be the judge - do you want cheap? or small? or light?
If so, you will have to trade efficiency or matching range away.
Smaller, more efficient, cheaper - pick two.
73, Ward N0AX
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com