Euro: the jet with the tcas will be constantly interrogating. When your transponder picks it up, it will reply. I think the jet’s transponder will be much weaker than atc’s, so that may be why your transponder doesn’t flash at long distances.
My experience is with an Allied Signal tcas 67, and this response is based on that unit in our aircraft only.
As people have said, whether you are shown on the tcas display depends on whether the jet can pick up your reply, and whether the altitude difference meets the preset requirements to be displayed.
If you are not squawking altitude, then you will show up on the jet’s tcas display anytime you are within range, with a strong enough signal for the jet’s transponder antennas to detect your signal. The jet will also get a proximity alert (filled diamond), followed by a traffic advisory (yellow circle, and "traffic traffic" audio). Since there is no altitude, no climb or descent commands can be issued.
Operationally, when we get this, we see that there is no altitude reported, so understand that the tcas does not know your altitude. Normally, these are low vfr targets, which we can sometimes see.
AND NOW, the big question, Why do high flying tcas equipped aircraft not see a myriad of low level vfr type aircraft that are not squawking mode C?
I believe the answer may be two fold.
1. The low vfr traffic may not receive the high flyers interogation, since antenna on the low vfr aircraft are oriented for a ground atc interrogation. EURO, DO YOU REPLY CONTINUOUSLY to these jets, until they are well past? OR does your reply cease before?
2. TCAS antennas are not particularly suited to receiving responses from aircraft WELL below them. In fact, in some cases we’ve had a "no bearing" traffic advisory a few times, where the tcas is unable to compute the bearing. This is even with the gear up, so upper and lower directional antennas should be in operation.
Of course, I could be wrong.
Hawk