Slightly confused on RF circuit ground plane

[Jerran]

From what Ive discovered on the net the ground plane consists of a solid piece of metal attached to the antenna ground from my transmitter/reciever modules. Do I need to attach this material to the batteries ground as well though? Aka antenna ground to a chunk of copper, chunk'o copper attached to batteries ground.

The following CAD drawing is just using the batteries ground on the antenna. Transmitter/recievers are still MKT5/MKR5, D0-D3 and ET are PIC I/O lines. The antenna's a 434 Mhz whip, purchased with transmitter chip.

 

[Jerran]

Stole this clip from one of Nigels posts I found searching this forum.


<Quote>
A whip simply connects to the output of the transmitter (and the input of the receiver of course). A whip should be mounted on a groundplane, the ground connection of the transceiver connects to that. If building it in a metal box, the metal box becomes the groundplane and the whip connects through an insulated hole to the transceiver. A whip is essentially a dipole, the other half is the reflection in the groundplane.

Presumably this unit is licence free?, if so there are strict limitations on the aerial system allowed - these are clearly given in the datasheets, so you should check on there.

BTW, for my experiments with RF modules, I simply soldered a piece of thick copper wire (2.5mm twin and earth ground wire) of the correct length to a vero pin next to the module, and obviously wired to the aerial pin. Although a groundplane is advisable (and greatly increases range), it's not essential.
<End Quote>

This has given me a better idea of whats needed but Im still a little fuzzy. Also is the connection between the metal case and the antenna insulated? Im using M4 style threaded antennas.

 

[Nigel Goodwin]

Think of it as a car radio! - the body work of the car is the ground plane, and the ground side of the radio power supply connects to that. The aerial is mounted through the ground plane, obviously insulated from it.

Your aerial should have insulators for mounting it on a metal case.

A ground plane basically provides a reflection of the aerial - so where a standard dipole aerial has two parts, one connected to the screen, and the other connected to the inner of the coax - a ground plane and whip only has one part, the whip connected to the coax inner, the screen connects to the ground plane and provides a 'reflection' of the whip, just like a lake provides a reflection of a pole sticking out of it (except a lake is an optical reflection, and a ground plane an RF reflection).

To give you an idea of how well it works, I used to have a Kenwood 2M handheld transceiver - not the tiny things you get now, but one you held in your hand and used an external microphone. The curly microphone lead was long enough to stand the transceiver on a car roof - doing so gave an increase of about 2 'S points' on the receiver meter, which made a huge difference.

Under normal use the quarterwave whip on the transceiver relied on the metal casing of the transceiver as it's ground plane, which wasn't large enough or very effective - but was easier to carry than a car roof!.

 

[Jerran]

Does the length of the coax have any bearing on effeciency? Id imagine the shorter the better correct?

 

[Nigel Goodwin]

Does the length of the coax have any bearing on effeciency? Id imagine the shorter the better correct?

Only to some extent, due to losses in the cable - an aerial system should be accurately matched to the transmitter or receiver so losses in the cable should be pretty low. If it's not matched you get SWR problems (Standing Wave Ratio), which means that instead of the RF power being broadcast it's reflecting back down the coax into the transmitter - this obviously limits your power output, and can also blow your transmitter (but it's not likely to happen with low power modules).