All QRP Labs kits use low loss multi-layer ceramic NP0 capacitors for all capacitance values below 5nF. This includes all the LPF and BPF kit capacitors for HF bands. These capacitors are also sometimes known as C0G, CC4 or Class-I dielectric types. The voltage rating of these capacitors is adequate for up to 10W.
The very first QRP Labs kit, introduced at Dayton FDIM 2010, was a simple ATtiny13-controlled FSKCW transmitter. If you are interested you can see details of it here: . Hundreds of people happily built these and operated them. However a small handful of customers possessing VNA or Spectrum analyser/tracking generator equipment spent some time examining every detail of the kit, as they enjoyed doing. These LPFs were lossy and it was because I had used the cheapest common type of ceramic capacitor. One OM called them "molded mud"! I have learned from my mistake, and all the kits since then have used the more expensive NP0 type. Well, when you buy 1000 each time, they anyway become not so much more expensive? ;-)
QRP Labs uses toroids from several suppliers, sometimes micrometals, sometimes equivalents, practically-speaking according to availability. Batches from every supplier have been tested by me, and verified to have equivalent characteristics to the specified micrometals ones (within tolerance). I'm fact sometimes the alternative manufacturer toroids outperform micrometals on their Q. This has also been verified by other constructors.?
People should also note that there is a range of inductance of a toroid with n turns, depending how tightly the turns are wound and how spread they are around the core. This range is quite wide! The theoretical calculation of inductance for a given number of turns (which is also found in many online toroid calculator tools), assumes an even distribution of turns around the core. In practice everyone always leaves a gap - in fact they SHOULD do so, because otherwise there may be capacitance between the two ends of the inductance, which will alter the filter performance also. But I have found that many people tend to leave a larger gap than is necessary or desirable. Squeezing the turns of a toroid increases the inductance. Therefore leaving a larger gap increases the inductance. Actually ANY kind of non-even winding will cause the inductance to be higher than expected!
This quote is from the Micrometals app note entitled "Iron Powder Cores for High Q Inductors" states:
"Another characteristic, which affects the apparent inductance, is leakage inductance. Leakage inductance acts in series with the coil's self-inductance.? This is a result of un-coupled flux and
becomes most apparent in high frequency, low inductance coils particularly when the turns are not evenly distributed around the core. Here is an example where a T50-17 is wound with 10 turns #20 (μ = 4). In cases like this where it is possible to drastically change the positioning of the turns, and the permeability of the core material is low, very large differences are seen. In higher permeability materials this affect is much less. In a number of applications, toroidal coils are tuned by this means."
Where the number of turns of wire is smaller, the range of adjustment is therefore larger. In any event, everything always seems to get worse when you go up to higher frequencies. I believe this is why we more often hear of "low power" problems on 20m, than say, 40m.?
Some people feel that measuring the inductance value with an LC-meter prior to installation is helpful. In general I would agree with this. But even then, caution is required because in many cases the accuracy of these LC meters is unknown or suspect; and some people say that the measurement of inductance at much lower frequencies than will be used in the application, also gives a differing result (I am not sure how true this is).?
Heating of components can occur when the Low Pass Filter has attenuation at the operating frequency. It is a simplification to say that if the PA produces 5W of power, and if only 3W come out the other end of the LPF, then 2W has been lost in the filter - and this 2W must go into heating the components! Nevertheless, the simplification does have some validity. And since the capacitors are small, if they dissipate power their temperature can rise quickly (or in other words, not much power dissipation is required to raise the temperature significantly).?
The output power of the QCX is dependent on supply voltage. This simple fact is often missed by constructors. Getting 2.5 or 3W at 12V supply is normal. To get 5W output does require 15 or 16V. This is what I have found on all my QCX here too. The power output of the three BS170s is a little less than the maximum that would be possible; this is because a little voltage is dropped across the 1N5819 Schottky diode which is used for reverse polarity protection of the radio at its power supply connector; and a little more voltage is lost in the key-shaping transistor. These two features (reverse polarity protection and CW envelope shaping) are very important, so worth the little reduction in power.?
It is also the case that many constructors do not appreciate the difference that 1W of power makes (for example, whether they use 4W or 5W) is rather smaller than it looks, if you consider it in dBm terms, which is what is actually important. Or equivalently, a quite small fraction of an S-point.?
Finally, the measurement of power itself is difficult to make accurately. To get an accurate power measurement requires transmitting into a known good 50-ohm dummy load. Many power meters have accuracy limitations. Usually a small difference in peak or RMS voltage measurement, leads to a bigger difference in calculated power (since the voltage is squared in the formula). So any inaccuracies are multiplied.?
There are people who build their QCX and plug it in and measure 5W at 12V supply and are very happy about that. Personally I find that much power, at 12V supply, rather unlikely. Then there are other people who measure less.?
The simplest way to deal with it, for people who think that they have less power than they expect, but lack the means to make detailed measurements, is to just remove a turn or two from the inductors L1, L2, L3.?
All of the above is a theoretical explanation, and a description of the quality of components that are used in the QRP Labs kits. However, none of this removes the possibility that a particular component might be drastically out of tolerance; or defective, or the possibility of supplier error, etc. In these cases I would need to undertake a further investigation if there was a widespread problem. But I do not think there is any widespread problem, from all I have heard to date.?
73 Hans G0UPL
