Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

Yes, Emory Roberts did pick up a radio device. Read his statement:

12:30 pm. First of three shots fired, at which time I saw the President lean toward Mrs. Kennedy. Why didn't you do something, Roberts? I do not know if it was the next shot or third shot that hit the President in the head, but I saw what appeared to be a small explosion on the right side of the President's head, saw blood, at which time the President fell further to his left. Mrs. Kennedy was leaning toward the President, however, she immediately raised up in the seat and appeared to be getting up on back of same. About this time I saw SA Clinton Hill trying to get on left rear step of the President's car. He got aboard and climbed up over the back of the car and placed himself over the President and Mrs. Kennedy. After SA Hill got on rear step of the President's car it appeared that SA John Ready was about to follow and go for the right rear step, however, I told him not to jump, as we had picked up speed, and I was afraid he could not make it.
It is estimated that we were traveling approximately 15-20 miles per hour at the time of the shooting and it is believed that the follow-up car was approximately 20 25 feet behind the President's car. No, Roberts. The limo had slowed to 5 miles per hour or less. Many said it came to a complete stop. It was going nowhere near 15-20 miles per hour.

The crowd was very sparse, in fact only a few people were along the motorcade route at the time of the shooting.
Just after the third shot was fired, I picked up the car radio. You God-damn right you did, Roberts. You probably had it in your hand when Altgens took his picture. I said "Halfback (code name for SS. Follow-up car) to Lawson, the President has been hit, escort us to the nearest hospital, fast but at a safe speed." I repeated the message, requesting to be cautious, meaning the speed. I had in mind Vice President Johnson's safety, as well as the President's, if he was not already dead.
The Vice President's car was approximately one-half block behind the Secret Service car, at the time of the shooting, and some of us waved for it to close in closer to the Secret Service car. The Vice President's car quickly closed the gap. 
When I turned around to wave the Vice President's car to come closer, at same time, trying to determine where shots had come from (what for, Roberts? What could you do about it even if you knew where the shots had come from?) I said, pointing to SA McIntyre, ''They got him, they got him," continuing I said "You (meaning McIntyre) and Bennett take over Johnson as soon as we stop" (meaning the hospital).
I turned around a couple times, just after the shooting and saw that some of the Special Agents had their guns drawn, I know I drew mine, and saw SA Hickey in rear seat with the AR-15, and asked him to be careful with it. (Yeah, you wouldn't want anyone to get hurt now, would you, Roberts?)