We know that "Setekhs' crystalline detection arrays provide their fleets with doubled radar range within the nebular region." So I would say that it's a pretty safe assumption to say that the GTVA uses radio waves for something.
I wouldn't say it was safe. They might be, they might not.
If the captain of a GTVA destroyer says to fire all guns at the enemy are we to assume that he means chemically propelled slug throwers? Or is it just that the term guns is so engrained in the vernacular that it's now used when it's technically incorrect.
Same goes for radar. I can easily see the term being used long after the system no longer works on radio waves.
But anyway, even if the GTVA does use radio waves for radar it's unlikely that they have the power to be picked up 4.4 light years away.
Security risk? What sensitive information do the marines have that could be of any interest to shivans? Not that hte shivans really do care a lot about talking with us in the first place or that they understand our language at all 
And the shivans were the only design consideration when FTL comms were designed? The engineers figured that humanity were so peaceful that they would never fight amongst themeselves or with the vasudans ever again and thus didn't build basic security options into their devices.
I'm not saying that security against eavesdroppers was a consideration in this particular mission. I'm saying that only a fool would design a radio system that could automatically connect to and piggyback off any friendly radio system in the vicinity. You're just asking for your ships comms to be hacked and have false orders sent if you do that.
It's one thing to say that the Qeb would have acted as an FTL relay point using a carefully designed system that would allow the marines to send their signals back to it for relay. It's a completely different matter to say that they can simply connect to any other ships present and use their comms to talk to Command
without any interaction from the pilot of said ship.
Why would you be using subpsace communicators on a planet when radiowaves do the job just fine already?
Or are you seriously telling me that a tech that manipulates time-space will somehow be cheaper and more compact that something we already posses now?
Dear Mr Marconi.
I read your proposal for a system of communication using radio waves but do not believe that it is workable. There have been various attempts to do something similar using radio waves in the past and they have all been financial failures. It is simply much cheaper to use the electric telegraph than your so called wireless device. I see no reason why anyone should invest in this system when the electric telegraph works just fine.
Do you believe that your system will be cheaper than the telegraph? Smaller? Do you believe that a system involving sending and receiving aetheric waves will really be able to take over from the current one using simple electricity?
The future of communication is obviously the electric telegraph and I have no doubt that you will find it still in use 300 years from now. Even if the subspace phone/TV is more expensive (and to be honest that's a fairly big if) you are forgetting the cost of the infrastructure that keeps radio working. In order to make a cellphone call to America I have to connect to a cell phone tower. It then has to pass the signal on until it reaches a transmitter that can beam the information to a satellite which then beams the signal back to a receiver then to a another tower and then finally to the cellphone of whoever I'm talking to.
A subspace phone does it directly. Furthermore it doesn't necessarily require a provider. All calls might be free once you buy the equipment. That's an enormous incentive to buy one.
Radio requires a strictly regulated set of frequencies. Not everyone can have one, meaning that the government has a reason to step in and control them. Without that need TV and radio stations could do whatever they wanted to. Not only that but they could reach anyone on the planet or even everyone in the Solar system from one small transmitter. No need to faff about with keeping affiliates happy in order for them to keep transmitting your programs. So I can lots of reasons why TV stations would change to a subspace based system once the phones had put the infrastructure in.
And I say again - nowhere in-game have we seen a ship jump even remotely close to a planet. (aka- the planets in game were al ldistant, approximately as distant as the moon) I'm not talking jumping in in high/low orbit.
In every example the reason ships have been jumping in has had little to do with the planet itself. The only event even involving planets in FS2 is the first mission where the refugees are being escorted away from one. And in that case the ships don't even have jump drives. FS1 quite clearly states that the Shivans showed no interest in planets and were only interested in holding and controlling jump nodes. This behaviour changed later on in the game but in FS2 it appears that the Shivans have simply gone back to their original game plan.
Secondly even if I assume you are correct about gravity affecting subspace jumps that still doesn't mean subspace comms are affected. Jumps involve a transfer of mass. Comms involve a transfer of information. Even if you are correct it could be that the gravitational field has an effect on the mass of the object being transferred. Since information has no mass it wouldn't be affected in the same way.
