[rabbit73]

is loosening the screw that Bell Canada is using on the service ground to add a second ground wire considered removing the house from its ground?

I can't see it from here, but if loosening the screw that Bell Canada is using also loosens the ground connection for the house electrical system ground, don't do it. Use a split bolt to connect your new ground wire to the Bell Canada wire or the house electrical system ground wire.

could i not simply run my coax and ground wire from the antenna and mount to the grounding block and screw the ground wire from the mount to the same screw as the ground wire leading to the service panel. In this case, on the grounding block there would be the mount ground wire and the ground wire leading to the service ground under one screw and on the actual service ground there would be the the existing Bell Canada ground wire and i would add the grounding block ground wire under that same screw.

That is called a piggy-back arrangement and is not according to the US code, but it is often done. Our code says to run separate 10 gauge wires from the mast and the grounding block to the house electrical system ground.

http://www.dbsinstall.com/diy/Grounding-2.asp

A common grounding practice is to connect a 17 Gauge copper coated steel ground wire (messenger on your coax) from the dish mast to a coax grounding block. Then ground the ground block with 10 gauge copper to an NEC approved grounding point. By grounding the mast to the ground block and sharing a single 10 gauge ground wire for both mast and coax, the NEC grounding requirement of the mast is not meet. This sharing, or piggy-backing of grounds is a common practice within the satellite dish industry and is fully supported by both Dish Network and DIRECTV.

Grounding thread on Canadian forum:
Grounding Info & Standards For OTA
http://www.digitalhome.ca/forum/81-o...dards-ota.html