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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
