Coax?

[davy28]

Is it possible/probable to have compatibility issues when connecting a high end coax HD100 to bog standard RG6?...

[Satdumbum]

There should be no compatability issues if both cables have a rated impedence of 75ohm, assuming they are for satellite or terrestrial aerial connection. Joints are always going to degrade signal gain (strength) because they increase current resistance so it is important to make as few as possible. Another consideration is to maintain the screen to avoid interference, so it is probably best achieved with 2 f plugs and a barrel connector.

[Satdumbum]

Another quick thought if the joint is outside, exposed to the elements it needs to be protected from water ingress. This can be achieved by wrapping the joint in self amalgamating tape, or covering it with silicon sealant - not as pretty but works in emergency.