Posted by
Ron D'Eau Claire-2 on
Aug 09, 2005; 5:28am
URL: http://elecraft.85.s1.nabble.com/Re-Elecrafts-T-1-tp381070p381081.html
Paul, KD3JF wrote:
My only comment is, first of all I am not an engineer and I am not savvy
always on the ins and outs of antennas, but my gut feeling is that a "L"
network tuner is the better for all circumstances than a "T" network tuner.
----------------------------------------
Good 'gut' feelings, Paul.
I try to minimize the number of reactive elements (coils & caps) in the
circuit on the theory that all such elements have SOME loss.
Measurements show that there is little difference between L and T networks
at nominal impedance levels. However, when transforming to very low
impedances, the losses in a T-network tuner go up very quickly compared to
other types. Also, T-networks will typically show a match at more than one
combination of settings, some of which will show much higher losses in the
tuner (usually in the inductor) than others.
In their defense, T-networks do a great job of fixing nominal impedance
mismatches. They became very popular some years ago when the popular tunable
pi-network outputs in our rigs were no longer adequate. Of course,
pi-network outputs were very versatile, allowing us to not only handle just
about any SWR we might encounter on a coaxial feedline, but even to load up
quite an array of end-fed wires and other "casual" antennas.
But tunable pi-network output were doomed by two problems. One problem was
that, here in the USA at least, the FCC tightened up the specifications for
spurious radiations beyond what pi-nets could provide. The other problem was
the solid-state RF power amplifier. The circuit values required for a
pi-network were beyond that available with reasonable parts. More complex
networks were needed. So rig manufacturers started building rigs with
fixed-tuned output networks.
>From a marketing viewpoint they were heralded as "no-tune" rigs. No longer
did we need to dip the plate current and then set the loading for every QSY.
That was sort of true. They were no-tune as long as our antenna provided a
very good 50-ohm match to the rig on every frequency we wanted to use. Open
wire feeders and end-fed wires virtually disappeared from most Hamshacks in
favor of a variety of trap verticals, trap dipoles and multi-band beams. But
that didn't fix the problem for everyone. Many Hams found it very difficult
to erect antennas that provided the low-SWR required by their 'no-tune' rigs
on all bands.
Companies like MFJ and other tuner manufacturers stepped in to fill that
gap. They chose the T-network because it was simple and cheap to implement
to cover a wide frequency range and, when used to correct nominal mismatches
in a 50-ohm line, they are very efficient. Typically a 4:1 balun was
included for those wanting to feed antennas with balanced lines like the
popular folded dipoles that required a 300 ohm feed. The 4:1 impedance
matching ration provided a good match for such an antenna to the most
efficient working range of the T-network.
But, Hams being Hams, some of us decided to use them to load up all manner
of strange random wires and antennas. And Marketing folks being Marketing
folks, the manufacturers didn't exactly discourage that, although if one
reads the MFJ manuals there are plenty of warnings about the limitations of
the T-network and its tendency to become very lossy under certain
conditions.
Things really got crazy when the WARC bands were made available. The Ham
bands no longer had a simple harmonic relationship to each other, so the
design of a single antenna that would provide a low SWR on ALL the HF bands
became very complex. Doublets and random end-fed wires and a whole bunch of
"untuned" multiband antennas started appearing on the scene again, and the
tuners designed to correct minor mismatches in coax lines were simply not
adequate for the job. Really efficient wide-range tuners were needed.
To handle these new requirements, Elecraft has opted for the L-network with
its wide matching range and good efficiency in all of its tuners, even
though it requires a more complex switching system to handle all of the
possible combinations of inductance and capacitance that might be needed.
But we don't really notice the more complex tuning since it's now done
automatically for us by controller systems that were only available on Star
Trek back in 1960.
So things have come full-circle. Instead of dipping the plate current and
adjusting the loading for the output power, we now adjust the tuner for a
low SWR on the link to the rig to make sure the transmitter's output network
is working efficiently.
And, with fully-automatic ATU's like Elecraft gives us, we still don't have
to twiddle with any knobs when we QSY. We just listen to the latching relays
go "crrrrrrick!" <G>.
Ron AC7AC
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