Sounds like someone turned a radio on with the same channel. Could be someone at home close to the field working on a plane, not knowing he well crash a plane at the field. Turned on the radio to check something turned it off then back on. 1/2 hr lator the someone at home did the same. I'd talk to the club memebers with the same channel as you and see if they live close to the airfield to cause this. Not that it's his falt but would be something to bring up at the next club meeting.