What is radio spectrum, and why is it auctioned?
Spectrum is the range of radio frequencies usable for wireless communication — and it is auctioned because it is a genuinely finite shared resource where two users of the same frequency in the same place simply destroy each other's signal.
Why it is scarce. The usable range is bounded by physics. Below roughly 1GHz signals travel far and penetrate buildings well but carry less data. Above a few GHz there is far more room for capacity but range collapses and walls become opaque. Every band is a trade-off between coverage and capacity, and the desirable middle is crowded.
Why it must be allocated. Radio does not queue. Two transmitters on the same frequency in the same area produce interference that degrades both, so someone has to decide who transmits where — nationally by a regulator, and internationally by treaty, since radio does not respect borders and satellites cover everyone.
Why auctions specifically. Historically licences were awarded by comparative hearings — beauty contests — which were slow, opaque and vulnerable to lobbying, or by lottery, which produced windfall resale. Auctions were adopted because they reveal what operators actually believe a band is worth, allocate it to whoever can use it most productively, and return the value to the public rather than to licensees.
How they are designed. Modern spectrum auctions are carefully engineered — typically multi-round and ascending, with rules on how much one bidder may win to preserve competition, coverage obligations attached to licences, and sometimes reserved blocks for new entrants. Auction design is a serious applied-economics discipline precisely because badly designed ones have produced both collusion and ruinous overbidding.
The criticisms are real. Very high prices are ultimately paid by consumers and can delay network investment; the 3G auctions of the early 2000s are the standard cautionary example.
Unlicensed spectrum is the counterexample. Wi-Fi, Bluetooth and many other technologies operate in bands nobody owns, under power limits and politeness rules — and the resulting innovation is an argument that not all spectrum should be sold.