Arduino UNO + Ethernet Shield + Ultrasonic Sensor = Fail

With my Arduino Uno I measure the distance using HC-SR04 ultrasonic sensor with no problems at all using the wiring below. When I attach ethernet shield, my ultrasonic sensor does not measure distance any more, it constantly says 0cm no matter what. I have tried different digital pin pairs such as 5-7, 6-8, 5-9, 3-5, 2-8 but no luck.

I suspect that HC-SR04 is not compatible with my Ethernet shield but I haven't seen such warning anywhere on the net.

There are no components attached to arduino besides ethernet shield and the ultrasonic sensor itself.
There is no SD Card in SD Card slot.
My ethernet shield works fine while running a web server or web client script.
Digital pins of ethernet shield works fine with all other components such as temperature sensor, motion sensor etc.

Here is the ethernet shield I have; http://www.ezshopfun.com/product_info.php?products_id=169

Here is my actual circuit;

Here is my code;

#define trigPin 6
#define echoPin 7

void setup() {
  Serial.begin(9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}

void loop() {
  int duration, distance;
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(1000);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH) / 2;
  distance = duration / 29.1;
  Serial.print(distance);
  Serial.println(" cm");
  delay(500);
}

Circuit

Solution

Today I bought a multimeter and tested my circuit. Here are the results;

When my circuit directly attached to Arduino itself;

4.80V & 5.7mA

When my circuit attached to ethernet shield;

3.06V & 3.8mA

I think the problem is that 3.06V is not enough for my HC-SR04 to operate.