This is referred to as a meander antenna, which is a specific type of folded dipole.
Advantages:
- Improve omnidirectionality
- Smaller space requirements
Disadvantages:
- Tuning becomes more critical
- Losses are higher than a standard dipole
The Freescale App note, "Compact Integrated Antennas" gives a brief overview of several options, included the meander, for on-PCB antennas. It doesn't give specific design parameters.
A 1982 dissertation, "Meander Antennas" provides some guidance on the mathematical models used to understand and design meander antennas, but goes rather deeper than most EEs will want to venture for simply designing an antenna.
The reality today is that most PCB antenna design of this type is usually done with the aid of an antenna design CAD package. The antenna performance depends on not just the physical layout, but also the materials used, and the shape of those materials, for the PCB substrate, copper, and mask. The software still has some limitations, and so extensive testing is done to validate and tweak the design once it's fabricated. An example of free antenna analysis software is 4nec2 which can evaluate many types of antennas.
When designing a meander antenna, start with a trace the length of the ideal dipole, fold it into the desired shape and space, then perform numerical analysis to determine the radiation pattern and efficiency. Some CAD software has wizards that can help you choose an optimal pattern for a given space, but I have not yet seen a book or guide that gives optimal pattern information that can be applied generally to meander antennas.
No, your PCB is not "ready" if you haven't got the mechanical constraints yet!
Since PCBs can be made in almost any shape, you figure out how much board area you need, then start with the mechanical design. Having the PCB done and then looking for a box is comletely backwards. Unless this is a very high volume product, you are much better off using some off the shelf case.
For very low volumes, you buy the standard box and mill and drill the cutouts yourself or have a local machine shop do it. For higher volumes (usually a few 100), the box companies will usually customize their standard products with holes and cutouts according to your specs. They do this with a modified mold, so in volume this is cheaper than machining the custom stuff afterwards.
There are many box companies out there. We keep a few of different sized boxes around for one-off projects. We have Eagle board outline templates for boxes we use regularly, like the Unibox #128.
Best Answer
To the extent that your aluminium box doesn't have large holes (relative to wavelength) in it, it will act as a Faraday cage and significantly attenuate any electromagnetic radiation passing between the inside and the outside.
Sometimes, this is what you want. If you are trying to reduce unintentional emissions of your circuit, or reduce noise received from the outside, this is good.
Sometimes, this is not what you want. If your PCB includes an antenna, you probably want to radiate electromagnetic energy, so enclosing it in a conductive box is probably counterproductive.