Electronic – Need help with selecting an antenna for PCB

I have designed a pair of circuits which utilize the QAM-TX2-433/QAM-RX2-433 transmitter/receiver.

So far everything is hooked up on solderless breadboards, with just a length of wire for antennas.

The next stage is to design PCBs for these circuits and to find an appropriate antenna to extend the communication range. As I learned in the past few hours, antenna design is a very complex field. I am not trying to understand the subject in depth, but I would really appreciate answers to a few questions that I feel are crucial to my understanding, however vague it might be.

1. My understanding is that antennas require a closed circuit from the RF signal to ground in order to work. This connection is made wirelessly through EM radiation. So, in case of just a wire hanging in the air, the circuit is inefficiently closed to the ground terminal of my battery, which is why a wire antenna can transmit at all. Is this correct?

2. In order to increase the efficiency of an antenna, a ground plane larger than lambda/4 is required. On a PCB, this ground plane can take the shape of a large conductive area connected to the ground terminal. Correct?

3. For 433MHz, \$\lambda/4 = 17cm\$. However, this length can be shortened by providing an inductor in series with the antenna. A helix antenna combines the antenna and the inductor in one length of wire, so it seems like a very good choice for my project. I am looking at this part here: PHC-M4-433.

   Would you agree that this is an appropriate antenna to try?

4. I also found this antenna: ANT-433-HETH, but I do not understand how to use it. It is really attractive for its size – it looks like I could put this one inside my plastic case and hide it entirely. But why does it have 2 connection points? What do I connect the other end to?

5. Is there any reference material available that can explain to me exactly how to lay out the ground plane on my PCB, and where the antenna is supposed to be in relation to the ground plane? For example, should the antenna be centered above the ground plane?

6. It is also possible to make an antenna with a long trace on the PCB. Would this trace have to be 17cm long? What happens if I put it into a spiral to fit on a small PCB? Where does the ground plane go in this case?

I know that these are fairly broad questions, so I am mostly looking for practical advice on some of the options available to me for this project.