I understand relay antennas are used to relay and direct antennas aren't, but direct antennas can call home and are more light-weight than relays. I was thinking of the following system:
The idea is that I'd like to put this satellite on different planets, and use the small relays to connect with other near-by vessels. My hope is that the other vessels will be able to access the main CommNet through this satellite's direct antenna, even though it's not a relay. Will it work?
Best Answer
I wondered the same myself, but came to the conclusion that it won't work. I believe this is (more or less clearly) documented in the in-game KSPedia, and of course it's also reasonably easy to test.
I finally had some time to actually run such a test. The test craft I used consisted of two probes:
Probe 1 had a single Communotron 88-88 antenna (antenna power 100G, direct only), and two HG-5 relay antennas (antenna power 5M each, relay capable).
Probe 2 only had two Communotron 16 antennas (antenna power 500k each).
These probes were launched together into a solar orbit and, once sufficiently far from Kerbin, separated. After undocking the two probes, and setting them to float right next to each other, the results were as expected:
Probe 1: 100% signal strength (direct connection to Kerbin)
Probe 2: 23% signal strength (relayed through probe 1, using only the HG-5 antennas)
Just to check, I closed the 88-88 antenna on probe 1, and observed its signal strength also drop to 23%:
Repeating the same test further out, beyond the range of the twin HG-5 antennas, probe 2 could not get any signal at all, while probe 1 (with the 88-88 antenna open) still had 100% signal strength.
Indeed, the wiki also confirms this behavior, stating that: