Saturday, May 18, 2013

QRP Antenna Tuners

In my junk box I have a few antenna tuners, small and large. There is also a selection of components that can be cobbled together to make more should I be so inclined. One of those commercial products was put to work in my current station to coerce my eaves trough into an impedance that my KX3 will (mostly) tolerate.

The one I chose is a small L-network tuner by MFJ that is suitable to end-fed wires. Other network topologies would also work, but I chose this one because it is small enough to comfortably (and unobtrusively) sit on the narrow window sill just under the wire coming in from its attachment point to the eaves trough.

The thing is, aesthetics aside, was this the proper choice for my QRP station? Consider the two (very dusty) tuners in the picture below, both of which I dug up from my basement horde of ham paraphernalia. If you had to choose, which do you think is the best antenna tuner for QRP? (For the moment please ignore the fact that these are very different tuner circuits, one being an L-network and the other a general-purpose transmatch.)

Have you picked one? The thing is both are correct, under the right conditions. It all has to do with why one uses QRP and one's personal circumstances.

Apart from the obvious size difference there are others.
  • Price - Price tends to rise in proportion to size and features. For example, a little coil is much cheaper than a big coil, a built-in SWR bridge costs extra, and increased size and weight mean higher costs of transport and inventory for the retail supply chain. Buying used equipment can change the equation, as will the price sensitivity of the buyer.
  • Power - When properly designed bigger components means a higher maximum power and mismatch tolerance. While this is not relevant to QRP, the operator may have aspirations for QRO in the future and so may wish to plan ahead.
  • Performance - As a general rule, the greater the mismatch the greater the losses within the tuner. Even if you find that perfect 50+j0 match to satisfy your finicky transmitter it may be that 9 or more of your precious 10 watts are spent heating the shack (and the tuner). This is not an exaggeration. It often occurs with small tuners driving non-resonant antennas.
For the QRP operator who does it under the principle that "small is beautiful" the choice is clear: pick the smaller, cheaper alternative, even if it entails reduced performance. If that is not your style you will need to review your options.

My choice was not based on price or principle. I simplify opened boxes until I found some tuners and proceeded to pick one that met my immediate needs. That was one suited to an end-fed antenna (the eaves trough) and would be easy to get close to the feed point. There was just the one that fit on the window sill. That it is the "best" tuner configuration for the antenna was a bonus, not mandatory.

The price I pay is performance. In practice it is a high price on several bands, particularly on 160, 80, 10 and especially on 15 and 6 meters. This is unsurprising since tuners, and most especially the small ones, get terribly inefficient at their range extremes, in regards to both frequency and mismatch. If you happen to read QST you should pay attention to these measurements in their tuner reviews so that you'll see how true this is.

I did not bother to open either unit to take pictures of their insides. However I'm sure you can guess the difference in coil wire gauge and capacitor plate area. Unlike with a kilowatt, even if every one of my 10 watts is dissipated by the tuner there will be no meltdown. Nevertheless the losses are there just the same. If the tuner has a 3 db loss (500 watts out of a kilowatt) it will also be 3 db with QRP (5 out of 10 watts).

The losses are due to resistances in the coils and capacitors, and core heating of any toroidal transformers. Ohm's Law tell us what we need to know:

P = I²R

For variable coils the resistance increases as the length of wire in the active section of the coil increases and as the wire gauge decreases. For variable capacitors the resistance increases as the plate overlap decreases or the plates are smaller to begin with (less surface area). We typically use more of the coil as the frequency decreases (A moves to the left in the diagram) and less of the capacitor plate area as frequency increases. That helps explain where much of the losses occur on the lowest and highest bands, respectively.

Current compounds the losses. Inside a matching network -- whether in the transmitter, tuner or antenna -- the currents circulating through the components are often higher (sometimes much higher) than the current in a perfectly matched antenna system:

I² = P / 50

This is why the best (and most expensive) components have large surface areas, and those surfaces are silver-plated. For a fixed R the loss increases in a component by 6 db for a doubling of the current. The square of the current quickly rises as the mismatch between the transmitter and antenna system grows.

I would be better served by the larger antenna tuner in my present situation even though it can be tricky getting it to match an end-fed antenna. There is no QRP "aesthetic" standing in my way. It's just that it too big to place near the antenna and it takes longer to change bands since the roller inductor takes more time to adjust than a tapped inductor. Speed and annoyance matter.

For an outrageous example of how inefficient the little tuner can be I tried to use it on 6 meters. To be fair this is well outside its design range. I just had to make the attempt when I noticed a sporadic-E opening one evening. I heard many stations but not one of them heard my 3 watts -- the most power I could coax out of the KX3 into a poor match. When I bypassed the tuner the band noise and received signal strength rose at least 10 db. The SWR was a high though usable 2:1. This time I did log a few stations.

What the actual loss of the tuner was I don't know since I can't easily measure it. I expect that the results on the other problematic bands (mentioned above) are of lesser degree though still significant. It's a handicap to add to my other handicaps of QRP and poor antenna, though a necessary one.

I do enjoy a challenge but this is ridiculous. Should I require a tuner when I get some real antennas erected it will be a big one.

No comments:

Post a Comment

All comments are moderated, and should appear within one day of submission.