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I powered up my Kenwood TS 520 without connecting the antenna and after 1 hour realized it was not connected, after connecting the antenna I noticed the S-meter was not giving me a reading and I have no idea why
Unless you tried to transmit without an antenna, it should be fine.
If the radio is in receive mode without an antenna, it won't hurt it. But if you transmit without an antenna, especially at high power, the high SWR could damage the final amplifier in the radio.