I think that it's relatively common for people to put Active Sensor PDCs on the moon, as they are the most likely to be attacked and the least sensitive to range.

I generally don't use PDCs that much at all though. I prefer Orbital Bases, not so much for the beam weapons (although that's not true anymore in my "no missiles" game) but for their mobility. It's nice to be able to tow them around as the front lines change, and they aren't all that expensive to refit given their lack of engines, so once you build an Orbital Base they can remain useful in combat for a long time.

My thought if you have enough tech to mount beam defenses on luna (reasonably high up there!) earth is unlikely to be threatened xD

Well, not all inhabited planets have handy moons, especially if they are moons themselves.  Other than that, there seem to be basically three downsides.  One, the distance (~400k for earth-luna) makes beam defence impractical and gives your countermissiles increased travel time. You may also have lockon issues depending on enemy ECM. Two, in the case of a ground invasion there is no PDC to resist enemy attack.  Three, most times Earth has been bombarded its because the enemy ships instantly appeared in earth orbit because I mashed way too long of a time interval.  Lunar PDCs are problematically slow in that case.

They are underground installations... that's why they get a few free layers of armor, and why they can't be towed to a new location.

I used to use 2500ton PDCs, but I find it annoying how they tie up commanders and clutter my list.  So lately i've been building 10000 ton PDCs. Also, I found that transporting PDC components is not nearly as big of a deal as i thought it would be. 

I also prefer to use 0.25xsize 100x reload missile launchers. These have a few advantages.  Civilians do not look at reload times for PPV, so the more tubes you have the more impressive the base is.  Second, you can generate much larger salvos.  Third, missile silos just seem so in character... xD

Fourth, a salvo-based PDC is a lot harder to turn against you if it gets captured, though I've never had an NPR land ground troops on me.

The problem of course is that minaturized launchers require research.

Another consideration is the use of a powerful active sensor. Tonnage isn't really a problem for PDCs, and it is good to have early warning of enemies approaching your planets.  These large active sensors also work better in either a larger PDC, or a dedicated sensor PDC.   For similar reasons, I use a anti-ship design with a 100-res sensor and an anti-missile design with a 1-res sensor. Once you develop these sensors you can also use them for scouts, jumpships or flagships.
Zulu class Planetary Missile Base    20,950 tons     697 Crew     1439 BP      TCS 419  TH 0  EM 0
Armour 5-67     Sensors 1/256     Damage Control Rating 0     PPV 128
Troop Capacity: 5 Battalions    Magazine 512   

Cellular Launch System (128)    Missile Size 4    Rate of Fire 4000
Goliath-48 (4)     Range 115.2m km    Resolution 100
Shrike-25 (128)  Speed: 30,000 km/s   End: 31.2m    Range: 56.2m km   WH: 4    Size: 4    TH: 100 / 60 / 30

SPY-256 (1)     GPS 25600     Range 204.8m km    Resolution 100
If you take out the PDC barracks this design goes down to 8300 tons and 1300 BP. The downside of this design is that bad luck will result in a premature knockout - it only has basic PDC armor. Small PDCs have redundancy advantages.  That is another reason it is designed to salvo all of its missiles at once. 

Thanks.   This made me go back and rethink a design I'd just made for a basic PDC.    I was using a design limit of 2500 tons to keep this as a one-section item, that's also hopefully cheap to build, that I can take and dump on a brand new colony or a base somewhere. 

DC 2x4 -1 Sec class Planetary Defence Centre    2,500 tons     178 Crew     342.08 BP      TCS 50  TH 0  EM 0
Armour 8-16     Sensors 1/16     Damage Control Rating 0     PPV 12
Magazine 286    

Size 1 Missile Launcher (4)    Missile Size 1    Rate of Fire 15
Size 4 Missile Launcher (2)    Missile Size 4    Rate of Fire 60
Missile Fire Control FC2-R1 (1)     Range 2.9m km    Resolution 1
Missile Fire Control FC28-R100 (1)     Range 28.8m km    Resolution 100

Active Search Sensor MR0-R1 (1)     GPS 16     Range 960k km    Resolution 1


This design is classed as a Planetary Defence Centre and can be pre-fabricated in 1 sections
I'd had an earlier design where I was trying to put protected spots under armor for stuff like troops, fuel, supplies, etc.    But, then I realized that since anything shooting up at aliens is going to be the first thing the aliens shoot back at, I decided that was a bad idea that was just making my design too big and too expensive.    Still, I might design a no-weapons PDC to be that sort of armored storage bunker some day.   

This also has no sensors.    Well, not really beyond the short range missile detector array.    I'll either put a tracking station on the base.   Or again, maybe I'll design a couple of other PDCs as a companion to this one.    I'm thinking the big passive sensors I like are best placed in another no-weapons PDC so it doesn't draw fire.    And I wonder if its worthwhile to put the Active Sensors over in a separate bunker as well.    If it's going to draw fire, maybe I want those some place away from my missile sites.    IIRC, that's the most SAM systems are designed since they know missiles will come down on top of anything broadcasting like an active sensor. 

PS . .  this also reminds me that I need to go research special PDC versions of my missile launchers since they get a reload time bonus.

Definitely. And probably a sensor that can track missiles, if you intend for it to operate without support of ships.

I'd also use missiles, and take advantage of the planet's deep space tracking capacity.  The large sensor radius lets you fire some longer range missiles more easily.

You still need to aquire the target with active sensors before shooting at it so deep space tracking is no substitute for active sensors although they do allow you to know about enemy fleets early