Antennas are very strong throughout the fridge - that said, there is some variability. In the picture below, the red rectangles denote the areas where antenna strength is more pronounced. The blue zone is still strong, but not quite as strong as the red zones.
All radio waves are slowed when passing through materials - the denser the product, the slower the wave. Think of tuning in FM radio signals as you're driving a car through the mountains - the radio plays loud and clear at some points, but it sometimes cannot be heard. The same is true for food in the fridge.
Unlike mountains, some food products are more permeable than others. For example, salads do not slow radio waves down as much as liquids or densely packed sandwiches. This is why different tagging practices are necessary for each product. Likewise, taller and more permeable products like salads are better off being placed in the blue zone than the smaller, denser products.
Inside the store pictured above, there is a wrap sandwich packaged in cardboard in the blue zone. This product is more dense than a salad. In this case, we recommend that the sandwiches to the left (which are slightly taller and wrapped in clear saran wrap) be switched with the product in the middle to improve readings.
Antennas are mounted on the underside of each shelf (see pictured below):