The US is suffering a heatwave, and so, it turns out, are the early millimeter wave (mmWave), high-frequency phones being used to test initial 5G networks in cities across the country.

The expensive ($1,300) and short-range (up to 2,000 feet) Samsung S10 5G phones that are being used for early consumer tests of 5G promise speeds of over 1 Gbit/s on 5G networks.

The catch? They can overheat as the temperature gets hotter.

"When I ran tests, the phone's 5G often switched off due to overheating, leaving me with a 4G connection," reports the Wall Street Journal's Joanna Stern in a multi-city test of the 5G devices recently.

During 80 and 90-degree days, In Atlanta, New York and Chicago, Stern used ice packs, air conditioning in cars or simply the sun going down to cool the phones.

Samsung says the S10 5G phone can switch to a 4G connection when the phone reaches a certain temperature to preserve battery life and optimize operation. "As 5G technology and the ecosystem evolve, it's only going to get better," a Samsung spokeswoman told the Journal.

AT&T, T-Mobile and Verizon presently operate limited mmWave 5G networks in cities in the US. As we've reported already, the higher-frequency 28GHz and 39GHz bands used in millimeter wave can have short signal coverage ranges, don't work indoors and could even suffer from fingers directly blocking the high-frequency antennas.

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— Dan Jones, Mobile Editor, Light Reading