[Elecraft] The use of a doublet

[Ron D'Eau Claire]

Can anyone point to why the single coil Z match wasn't commercialised rather than the T match?  (Perhaps it's a historical thing and manufacturers just copy each other, notwithstanding the Johnson Matchbox of course, which is no longer produced.)

I'm in process of building one to cover 160 to 10 from VK2BR (?).  I'm hoping that the link coupling will help with "balanced" feeders and antennas, though one side is bound to have more capacitance to earth than the other, so "balanced" is fairly relative.

David
G3UNA

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I believe it comes back to addressing the largest possible market with the simplest product, David. 

Relatively inexpensive Shunt-L T-network commercial tuners (e.g. MFJ) appeared in great quantity as "no tune" solid state transceivers became popular. These transceivers are really not "no tune" but, instead, are "pre tuned" for specific load impedance. The user is required to provide a "pre tuned" load that shows the proper impedance, indicated by a low SWR at the transmitter output. 

Unfortunately, many antennas in common use as the no-tune rigs came on the market - trap dipoles, multiple paralleled dipoles, trap verticals or, frequently, an end-fed "random wires" - provided the required match across the bands.  

The external "tuner" solved the problem, providing the low SWR the modern rigs required when feeding these antennas and able to handle an impedance range similar to the popular Pi-network used in older transmitter outputs that previously fed these antennas. 

In short, "no tune" rigs just moved the output tuning from the transmitter or transceiver itself to an external box: the "antenna tuner". 

Although the Shunt-L, T-network can be very lossy, it can be very efficient too when used properly. The key is to adjust it for lowest Q where the circulating currents in the coil are lowest. Just like feed lines, With the Shunt-L T-network the lowest Q and lowest loss occur with the *least* inductance in the circuit that can provide a match to the rig. 

Again, keeping things simple to address the basic needs of the most customers, manufacturers like MFJ use an inexpensive switch-tapped inductor in their original tuners. The taps, inductance and capacitor ranges were established to provide a match from a few tens of ohms to 600 ohms or so. They'd match much wider ranges of impedances, but it was not unusual to discover that the tap settings weren't optimal. In some cases that led to situations where high circulating currents and losses occurred because the taps were too far apart.

And of course, some tuners are used by Hams who have never carefully studied the manual that came with it, so they ended up using the wrong settings with the mistaken belief that all was well if the SWR on the link to the rig was low. 

Some years ago, Kevin Schmidt, W9CF, wrote a neat little Shunt-L T-Match tuner simulator in Java that's available to play with on line at many sites. One is here:

http://fermi.la.asu.edu/w9cf/tuner/tuner.html

You can download the program or run it on the WEB page, twirling the knobs with your mouse. It gives you the input/output impedances with the inductance and capacitance needed and the loss as a percentage of power and in dB for a given setting. I'm not sure what all the assumptions about conductor losses, etc., were for this simulator, but playing with it provides an intuitive "feel" for how the T-match works.

I ignored one type of antenna that was quite popular about the time the "no tune" rigs appeared. That was the "folded dipole". It was cheap and easy to make from 300 ohm twin-lead or home-made open wire line, avoiding the expense of coax for a very efficient and relatively broad band antenna, covering much if not all of the 80 meter band with a decently low SWR. But it needed a balanced feed. So MFJ and others provided a 4:1 toroidal balun in their tuners specifically to provide for these types of antennas (including the variant of the Windom fed with twin lead that was becoming popular too). Those baluns work well when feeding an impedance somewhere around 300 to 600 ohms, transforming it to about 50 or 100 ohms, right in the middle target range for the T-network to match most efficiently. 

Of course, many Hams immediately attached those baluns to their open-wire fed multiband doublets. That works fine where the antenna and feed line combine to provide a low impedance at the rig. Where the impedance is very high it doesn't. Again, the manuals with most Shunt-L T-network tuners I've seen say to avoid situations where the feed point impedance is high. MFJ, for example, goes so far as to give owners combinations of feed line lengths and antenna lengths to avoid with a doublet and cautions about using them with "Windom" antennas under certain situations to avoid these impedance extremes. 

That's why I have a wide-range link-coupled tuner for use with a doublet. That sort of tuner can handle the impedances efficiently, but it's bigger and more complicated to set up and use than most other types of tuners. 

More recent Shunt-L T-Network tuners use roller inductors - often low-loss silver plated roller inductors - instead of the cheaper tapped coil. They allow one to find exactly the optimum inductance for a given situation to avoid unnecessary losses. An alternative that I've used with a T-match is to adjust the positions of the taps on the coil for the optimum inductance for each band for *my* particular antenna.

Ron AC7AC