rigidbody velocity

am doing a pong game. i have a script in the ball, where it has to bounce back when it hits a racket. it works good but the speed of the ball always gets low even if i give high values for the velocity. i have used a method for ball’s movement in the start function so the speed of the ball wont be adding every frame, but only on the first frame the speed is as what i desired. need help for increasing the velocity of the ball and please help me where am making mistake.

using UnityEngine;
using System.Collections;

public class ball : MonoBehaviour {
	
	public float ballSpeed = 5f;

	// Use this for initialization
	void Start () {
		rigidbody.velocity = Vector3.right;
	}
	
	// Update is called once per frame
	void Update () 
	{
		rigidbody.velocity =rigidbody.velocity.normalized;
	}
	
	void OnCollisionEnter()
	{
		rigidbody.velocity = rigidbody.velocity * -1.0f;
	}
}

rigidbody.velocity =rigidbody.velocity.normalized;

You normalize your velocity. This means the vector is always magnitude 1.