The QPack design is derived in part from a classic configuration known as the Link-Coupled Tuner (LCT).  You may have come across some other popular designs that stem from the LCT, such as the Z-Match and the BLT.  These tuners are renowned for their wide-range matching, good efficiency (in most cases), and ability to feed loads that are either balanced or unbalanced. 

Additionally, LCTs have a band-pass, rather than a low-pass or high-pass characteristic.  This is true on both transmit and receive, and the benefits of this - preselectivity on receive, and possible suppression of transmitter harmonics - are noteworthy.

All-in-all it's a nice set of features to have in any tuner, but it begs the question, "Why aren't there more LCTs on the market?"  The answer to this lies with a problem regarding variable capacitor values - it's a good story, so here goes...

This is the input stage of the conventional LCT.  L1 is the input link - it is magnetically coupled to the main tank circuit (L2-C2), and this is how the LCT gets the juice from the rig into the tuner.  The output is usually tapped off the main tank with some moveable connections - you'll often see alligator clips recommended.  What's noteworthy here is that L1 and C1 form a series-resonant circuit which is adjusted to resonate at the operating frequency Fo. 

By picking the right component values, we can not only make L1/C1 resonant at Fo, we can also make the resulting load ideal for the maximum transfer of power by making the inductive reactance (xL) of the link equal to the line impedance (rIN) of 50 ohms...

xL = rIN = 50 ohms

Since xL and xC (capacitive reactance) are equal at resonance (this is one of the primary characteristics of a resonant circuit), we can say that...

xL = xC = xIN = 50 ohms

In other words, we will want a capacitive reactance for C1 equal to the input line impedance of 50 ohms.

So simple. All we need to do is design our L1 and C1 reactance to equal the line impedance, and we've got the maximum theoretical transfer of power, and a perfect 1:1 match to our line.  And yes, it really works, and no, there are no yeah-buts in terms of the theory.  But there's a big yeah-but when it comes to practice.  The formula for capacitive reactance is...

xC = 1/(2pii FC), where F= frequency (Mhz) and C= Capacitance (mfd). 

Crunch the numbers for 14 Mhz and you get roughly 200 pf of capacitance required to produce 50 ohms reactance.  This is ok as it's well within typical component values.  But do it for 7 Mhz and you get 400pf required.  Now we're getting into some serious trouble because 400 pf variable capacitors are rare and expensive.  Do the math for 3.5 Mhz and, well...you're screwed.  

You need 800 pf of variable capacitance on 80 meters -  you may as well give up and use the phone.  And if you're good at spotting patterns you've already figured out that you need about 1600 pf for 160 meters - fuggetaboudit!

So what's a ham to do?  A tuner that needs impossible component values to get down to 7 Mhz and below is doomed forever to be great on paper, but never see the business end of a soldering iron.  Some designs have shown extra fixed or variable capacitance being switched into the circuit for these lower frequencies, but if you've ever used one of these, you'll know how user-unfriendly this can be.

Ah, but we have an answer don't we - we wouldn't be here if we didn't.  This is where we climb out of that famous box...

These problematic capacitor values stem directly from the line impedance of 50 ohms.  If we could use a different line impedance - say 200 ohms - we'd cut the required capacitor values down to 1/4 of what we saw above, covering 80 meters with only 200pf  for example.  The typical 300 pf variable, rather than being maxed-out in range well  above 7 Mhz, would now not only cover 80 meters, it'd do it with tons of additional range, giving us a good match and lots of room left to adjust for antenna reactance.  So let's do two things.  First...

Here we've just evolved the link into a tap on the main tank circuit- you'll see why in a minute. The link reactance has not changed. Now...

What we've done here is put a 1:4 low-loss, broadband transformer at the input of the tuner.  This gets our line impedance up to 200 ohms, right where we want it, and Bingo Bango - the tuner now covers 80 meters with a 200 pf variable for C1!  We've moved the tap up the tank to set the link equal to 200 ohms, which matches well with what we want for the tank anyway, so now there's just one inductance serving both the link and the tank! (That's why we showed you the tap in 2) above.)

In order to keep the output floating for balanced loads, we're now taking it from an output link L3, as the bottom of the tank will be at ground. And there, folks, is the genesis of the QPack!   Neat, huh?

And it works extremely well.  The Miracle Flatpack capacitors (another story for another day) in the QPack are about 350 pf, giving us easy access to a huge range of adjustment even at the bottom of 80 meters. ( In fact we're just about where we need to be to get down to 160, ever the bane of the LCT.  We'll probably go all the way there with our next model.)  These caps also have a wonderfully low minimum value - down around 7 pf - which helps the Qpack get right up to 6 meters without any difficulty. 

We provide both tap/tank switching to cover the whole HF range - that's the Band switch you see on the unit - and output tap switching to cover loads from very low to very high impedance - that's the Output control you see.  Then there's a switch to either ground or unground the output link, so we can feed both balanced loads, like twinlead or ladder line, and unbalanced loads like coax or random wires.

The result is a tuner that covers the HF range with tons of adjustment range, allowing it to match loads that no other tuner can get close to.  And the original high efficiencies and band-pass characteristics of the classic LCT are fully preserved, so you get a broader range and much better efficiency than almost any other tuner you'll come across. 

The mechanics and packaging of the Qpack were designed around three goals - build it to last forever, build it to operate smoothly and reliably, and make it beautiful. We think we've achieved these goals and then some.  We've gotten some very excited reviews from a few of our first users.

This first QPack is aimed at the lower-power operators who are looking for a very high quality manual tuner with much better efficiency, reliability and precision tuning than any automatic tuner - or any other manual tuner - can provide.  Remember the QPack's conservative power rating of 25 Watts is only 6 dB - that's one S-unit - down from 100 watts.  Most contacts will never hear the difference, especially when you can tune so precisely and do so with such high efficiency.  If you're dead-set on operating 100 watts, well, I guess you'll just have to wait and see what we come up with next!

                                                                         
Robert Victor VA2ERY for Miracle Antenna