Enclosures, aerials and ground planes

What's the ideal enclosure material for an RFM12b in order to maximise RF performance?  Assume we're using an external antenna (i.e. the aerial output of the RFM12b is connected to 50-ohm coax which runs a few cm to a panel-mounted SMA connector so a 1/4-wave whip antenna can be mounted on the outside of the box).  There are three main options I've been considering:

  1. Plastic enclosures are cheap, easy to cut and there's no risk of the case causing a short.
  2. But metal enclosures would appear to have two advantages: shielding the RF circuitry from unwanted electrical noise, and acting as a ground plane for the antenna.
  3. A third alternative (and my current favorite) would be to use a plastic enclosure with an added ground plane (either by gluing alu foil to the underside of the lid or by using four 1/4-wavelength wires radiating horizontally from the SMA connector).  This option would be cheap, easy to cut but wouldn't shield the RFM12b from noise.

Thoughts?  I'm especially interested to hear opinions on how important it might be to shield the RFM12b from electrical noise?

hhovde's picture

Re: Enclosures, aerials and ground planes

A metal box can give you some shielding, but most noise would probably originate from within the box itself or come over the wire. For best RF-shielding you would need an RF-shield cage encapsulating the RF-unit, on the PCB.

That said, this is generally something you don't need to worry about.

When it comes to the box as a ground plane, just remember that the 1/4 wavelength still applies, so you need a fairly large box at the given frequencies. This also applies to a plastic box with added ground plane. (In practice, a ground plane less than 1/4 will work, but with a significant performance hit)


In my opinion you should pick the box that best suit your needs (or wallet), and use one of the many alternative antenna configurations out there that doesn't need a ground plane. (fx: 1/2 wave or dipole) A good antenna will do more for your performance than any box ever will :)

jack_kelly's picture

Re: Enclosures, aerials and ground planes

that's great, thanks loads for the reply :)

rthorntn's picture

Re: Enclosures, aerials and ground planes


Rather than start a new thread I thought I would ask in this one :)

I have an emontx and would like to extend the range by adding a better antenna, the problem is I can't figure out what I would solder the ground wire to.

The build guide shows the supplied piece of wire soldered to the ANT pin but I will either be using the following design with each element being a 1/4 wavelength:


So the shield wire in the link above needs to be soldered to something and I can't figure out what?

The other option is something like this:


Ready made, it says it's a dipole and it is ground independent but they don't show a schematic of it so I can't see how it could be made up, anyway with this one I would want to attach an SMA connector to the emontx and that would require the centre to be wired to ANT and the outer part to be wired to ground, so the same issue?

Can somebody point me in the right direction on where to solder ground to on the emontx?

Also would anyone have any comments on the two antennas above, I am tempted to pay for the Radio Industries one because it already seems weather proof (it will be attached to the outside of my metal meter box)?

Thanks in advance.


MartinR's picture

Re: Enclosures, aerials and ground planes

You would normally connect the coax shield to the ground of the RF12B, which is one pin away from the antenna pin (after Vcc).

The first link you suggest is a dipole, which is a balanced aerial and isn't really suitable as the RF12B has an LC network to make it compatible with a simple whip aerial. (A dipole should also be connected by a balanced feeder not a coax as shown). I'm not sure about the second link, it certainly doesn't look like a dipole!

For increased range you need an aerial with gain, which basically means a directional one since the gain is achieved by concentrating the power in one direction.

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