I'd like to use a 315MHz RF module and a 1/4wave antenna (I think) should be about 226.5 mm long. The PCB will be shorter than this so is it ok to bend, fold or spiral a PCB antenna?
Are you sure?Yes. It's in the right spot for the RFC; i.e. C2 is to ground, R5 is to VCC (C5 to ground also).
I'll have a look for the recommended RFC inductance value for the 434MHz Tx... just a sec..
EDIT: unfortunately I can't find the design note I was thinking of, but here's a snippet from an elector article from 05/98 on the transmitter bit.
what this "E" emits? E-waves? i find weird all these antenna shapes out there... sometimes i'm wondering if they are really operational or just decoration_al
Designing antennas is a very creative process. There are some antennas that are designed using what is called a Genetic process (the antennas may be called Evolved or Evolutionary antennas) and the appearance of these antennas is often very sculptural and mysterious. This process uses a computer simulator. In the process, you start with some structure, simulate its performance, then allow one part of it to grow or shrink in a random direction. Then you check its performance again. If it got better than before, you keep that change, if it did not, you back up and try another change. This is a pretty dumb way to design an antenna, but with a computer doing all the dumb guesses and calculations it is fairly easy to do. It is really just automated trial and error (of course I am oversimplifying the details). It is surprising how interesting the results can be sometimes.
Evolved antenna - Wikipedia, the free encyclopedia