Antennas for QRP Operation
Antennas for operation at low power aren’t any different from those for use at the typical 100W level.
OK, there could be some minor differences, like the voltage ratings for capacitors, but the same electrical principles and mechanical design considerations still apply. Generally, there is no reason to do anything differently.
Now, if your vision of QRP includes taking the rig out portable, then you might want to read the Portable Antennas section, particularly Backpack Antennas for when weight is of critical importance. But those principles apply at other power levels as well.
There are times when an antenna works well enough at higher power but not at QRP. For example, an antenna with 10% efficiency still radiates 10W with 100W input, and it is still just as inefficient at low power, but that might be more apparent. On the other hand, high losses in a tuner will take longer to become apparent (by melting the coil supports, for example) at QRP than at higher power. But these are still inefficient antennas at either power level.
If you want to make sure that your antenna is as efficient as possible when operating QRP, then try running it at 100W and fix anything that arcs, melts, or even gets noticeably warm.
Well, maybe except for the capacitors, especially the “Polyvaricons” in your antenna tuner. But the best approach would be to replace them with a larger type for this test. The reason is that some of the tuner designs for QRP can actually be rather inefficient, especially when miniaturized for portable use. Yes, they might get a low SWR, but, depending on the load impedance, the losses can be relatively high. That’s why you want to test that your tuner (possibly with higher voltage capacitors) can withstand the application of higher power when matching your antenna on each band, because any losses (typically in the coil) will show up more obviously.