The L-network, when used as a matching network over wide frequency ranges
and/or impedances does have its limitations. When the impedance to be
matched approaches 50 ohms, the values of the components get so small that
strays prevent a good match, particularly at the higher frequencies. To get
around this problem Ten Tec switches in what they call a "compensating"
capacitor in switch Hi Z switch position 1 and a "compensating" inductor in
Lo Z switch position 1--but now the tuner is no longer an L-network. In Hi
Z switch position 1 it is configured as a CLC pi network, in Lo Z switch
position 1 as an LCL T-network. I think that the Ten Tec solution is
elegant. There are still only two controls (if you don't count the switch
as a control) to set--but in these two switch positions an L-network it
ain't..
While the L-network has the lowest Q possible for a given match, it doesn't
necessarily have the lowest loss. In February 2003 QST reviewed five
high-powered antenna tuners. On 10 meters the Ameritron ATR-30 had, with
the exception of one test point, lower loss, and greater 1.5 SWR bandwidth,
than the Ten Tec 238A. The one exception was at an SWR of 1:1-they both had
he same numbers for loss and SWR bandwidth.
I use both a Ten Tec 229 and a home-built tuner that can be configured as, L
step up, L step down, or T, feeding multi-band wire antennas. I like them
both. I have no trouble with "the T-network is harder to tune." A simple
relative output meter on the feedline lets me tune the T-network properly,
then log the settings vs frequency. Can reset any time with no thinking
required.
73 Paul W5DM
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