Which weapons for satellites?

A mix I guess; Capital Missiles are useful.
DUC helps vs fighters and such

To protect your planets be sure to build some weapon platforms on them. They really work

~Myrath

Satellites are completely immobile. Consequently, it's a good idea to arm them with the longest range weapon available. Otherwise your enemy will destroy the satellites safely from a distance.

Capital ship missiles are a good choice. Note, however, that they cannot target fighters; you'll need other defenders or other weaponry to handle a fighter attack.

I use a mix of CSM sattilies, DUC sats, and PDC sats. Around a warp point, range shouldn't be a problem, ships will come out one at a time and your sat's will surround it.

even to defende a planet, shorter ranged wpns can be useful to stop planetary bombardment and the like. but wpn platforms and bases are good as well

Terminology help:

CSM = capital ship missiles (launches at ships, chases them)
DUC = depleted uranium cannon (shoots fighters and ships)
PDC = point defense cannon (shoots down fighters)

Depending on the situation, I've found it helpful to park a frigate on the one or two warp points that I really don't want aliens coming through. It won't stop a serious invasion but the colony ships and scouts might think twice about coming through. Note this only works if they can see your ship before entering the warp point - park your ship on *their* side. Putting weapons on it is optional as the enemy doesn't seem to identify warships from floating shells. Essentially you're making a scarecrow. You can't do this in systems that they consider theirs, they'll just send ships to destroy what they see as an invader. In neutral systems it seems to work well.

Satellites are best put on your side of a warp point so ships coming through get blasted by the ring of satellites. High damage is what you want, so they can't return fire - instant kills are the best for this technique. Don't forget to put ordnance and/or supply pods on the satellite so it has ammo to shoot with. (examine the weapon to see what you'll need) You usually don't have to worry about them having lots of supplies if you have a supply depot in their system.

If you really have to play defense, weapon platforms are the easiest way. In the unmodified game, you shouldn't have to worry about it. In the major mods, you may have nasty enough neighbors to worry, in which case it's usually better to take the fight to them than to turtle and hope they leave you alone - they usually won't.

I also like to use a mix of satellites.

Here is what I usually like to do.
1x sat with basic or other sensor, to act as early warning, plus armor and maybe a point defense weapon.
For other sats in the group, I usually go with a ratio of 2 gun armed sats for each missile armed sats, and a few sats with guns and PD. A nice well rounded defense that can handle most situations.
If enemy has mainly ships, use more missile sats. If enemy uses a lot of dronees, missiles, or fighters, go with more gun/PD sats.
Also, there is the "Annoying Cannon Fodder Satellite", few if any weapons, as much Armor as possible. Something for the enemy to waste Supply/Ordnance on and to possibly distract them from other Sats. (have yet to fully test this last concept myself, although...)

Other handy things for sats, when you get the right tech.
Weapons that need no Ordnance
Such as Energy Pulse and Energy Stream weapons.
Meson Blaster is smaller than a DU Cannon, 3 Meson Blasters or 2 Meson Blasters and a PD can fit in place of 2 DU Cannons.
Anti-Proton Beam, can have better range than DU Cannons.
No ordnance means one less thing to worry about for sats not in a system with a Resupply Depot.

Alternate Point Defense
PD Blaster, slightly weaker, but fires twice as fast.
PD Beam, slightly heavier, but has better range.

Torpedoes
They are smaller and usually a little cheaper (on average) than Capital Ship Missiles. Quantum Torpedoes have roughly equal range, Gamma Pulse Torpedoes have shorter range but hit harder. Anti-Matter Torpedoes... between the two in range, and not that powerful, but the cheapest of the three.

Weapon Mounts
Large Satellite Mount makes a weapon a litle heavier, but the +40ls range and other bonuses are a major help.
As I like to say, "Reach out and swat someone."

In later game i found null space weapons in great use for satelites at least for those in warp points.

They are fine as they skip shields and armor so if any of your let's say 50 easily hits a ship they will surely hit something that matters.

Their problem is range but that issue is cleared out on warp point assault.

My statandard tactic is to take over a new system, bring enough ships to secure the warp points on the system untill the industry is up and running. Create a Satellite Cargo vessel, and 'move up' my satellites from the inner choke point, to the new point.

I place sensor satellites on both sides of the warp point, so I can see the invaders comming, ensuring my capital ship missles are of a high enough tech to all fire at once when the enemy ships warp though.

My capital ship satellites tend to a mixure, some with just a missle and a core, and versions with extra ammo storage and point defence for protracted battles.

This means that every warp point which borders my empire is guarded with a sizable sat force. I also add weapon plantforms on my non-domed worlds using mounted DUC.

The domed worlds must rely on a small fleet assigned to each system, which often get called away for attack missions.