default "all" would be nice, since as we get nearer to a release, there will be more and more "instant" servers (people just hosting without config, or serious config...)
and although many people would change it to enemy inhibiting, but personally, a field to block a jump drive would be omni-directional, hence, it would affect all ships in range...
like the suggestion though, as stated, defenses in this game are lacking, "technically" you could power drain and shield drain a enemy vessel... but that's not the same as stopping them entirely.
the jump drive does need a serious defense, but it should not be an assault ender, i'd recommend having a hidden "charge" meter, so the first ship would be halted at max distance, but until it fully charged, it got worse and worse, meaning the more ships jumping in, the further the next one would get. this would easily halt the AI fleets, but would still allow larger fleets (factions) to get surprisingly close, especially if they use jump drones to wear the field down.
in case your wondering why i'd suggest such a obvious flaw, it's because i fell everything should have a weakness, this would probably stop ships at the edge of the sector, at a size of say 5000, that's a good minute to the centre of the sector. (and i use a higher max speed then 75, i think it was 300~ish)
if you always had a minute before even being attacked, that would allow even battles, but if they can overwhelm the system, the enemy could easily score a few shots before you made it to a ship, or even a ship core, but still have time to get astronauts to cover before the fleet actually arrives, maintaining the ability to do considerible damage before response, without allowing easy player farming...
that's the important part, if we were the attacker, we'd feel cheated if the defenders could never be ambushed, or surprised. but being a defensive tech, it needs to provide some advantage, so a diminishing effectiveness is the easy remedy.
it provides the advantage, and a method of defeat. pirate fleets would see some of the benefit, being large enough to jump probably about half way. that said, each ship should wear it down less and less, we don't want ships within about 30 seconds of a station... under no circumstance, so the lower that charge, the more resistant it should become to being lowered further.
(sorry for the long post, got carried away.)