If your boat has a keel stepped mast the mast cables – antennas, navigation lights – can exit the mast below decks. If, however, you have a deck stepped mast you’ll need to get the mast cables through the deck another way.
You’ll want to look at the Cableport, a wonderful Swedish design that’s standard equipment on many top quality boats.
Its low profile doesn’t snag lines and doesn’t obstruct action around the mast – you can step on it without damage. Check it out at: Cableport
Other choices would be deck plugs, deck glands or a swan neck.
Deck plugs are notoriously corrosion prone and need meticulous maintenance. Make sure to locate them where they can’t be accidentally stepped on.
There’s a wide range of deck glands available; you’ll probably want the type through which you can pass a 19mm diameter PL259 connector, especially if you drop the mast each season. Plastic ones can break if you step on them so aluminium is a better choice.
Swan necks are usually seen on larger boats where there is more room around the base of the mast; their biggest drawback is their tendency to snag halyards and other lines.
By the way, if you need to take your antenna cable through an internal bulkhead one choice is the PL363 connector with two PL259 plugs.
The one in the picture is a 1.75” connector fitted with two large stainless nuts. These connectors come as standard with rather small nuts, quite useless, particularly if you want to use sealant at the joint. So, add the hefty large diameter nuts.