We see more and more reference to “QRP antennas” online these days, and it begs the question, what makes an antenna more or less suited to QRP.
To a novice, the obvious possibilities for a low power antenna system are that they are:
- highly efficient to offset the lower power; and/or
- unable to withstand higher power.
The term is often used more specifically for portable or temporary structures that due to their lightweight construction are unsuited to high power, so (2) is often the underlying reason.
Reading online articles, one comes to the view that (2) is the main factor, that most QRP antennas would simply not withstand higher power, mostly because they have low efficiency and the heat dissipated at higher power may damage components.
In some cases, the limitation is one of voltage withstand which should not of itself imply low efficiency.
In some cases, the designs appear to be deliberately inefficient as a means to taming VSWR excursions with frequency, deployment conditions etc.
Whilst it might be argued that an antenna system need not be designed to withstand more power than required, the morality of high power vs low power, and regulations etc, the simple fact remains that low efficiency antenna systems combined with low power reduce the probability of making contacts. You will still make contacts, just fewer of them.
Most articles describing QRP antennas lack worthwhile quantitative evidence of performance. It is as if QRP is a label for novelty without science.
For a QRP operator on bands where external noise is much greater than internal noise (eg the lower HF bands), antenna efficiency has little effect on their rx S/N ratio (though it will reduce the S meter reading).
Low efficiency tx antennas do not affect the noise at the remote rx site, but they deliver lower rx power at the remote site, so lower S/N ratio.
High antenna efficiency ought to be a high priority goal in a QRP station, the reward will be more contacts, better contacts… QRP should be practiced with finesse!