Thread: How do you make a PCB antenna?

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?

Disclaimer: I don't have much experiance with RF.

Bending it would make it inductive.

You could go for a loop antenna and put the correct capacitance in parallel with it so you have a resonant circuit.

The trouble is if it's short, then it'll be slightly capacitive, you could put an inductor in series to see if it helps. At this frequency you can probably make good enough small inductors and capacitors by drawing them on the PCB.

The short answer is yes it is ok, but of course the devil is in the details. Assuming you are not transmitting a lot of power (like more than 1 watt), then we don't need to worry too much about overheating the transmitter due to a bad impedance match, you can bend things around a bit and get away with it. Bear in mind that even if your circuit board was long enough for the full quarter wavelength, it still would need a large ground plane to meet all the performance expectations of a typical quarter wave. This just means that a full quarter wave needs more than 226mm to work well.

Anyways, it is fairly harmless to bend the top 1/3 of the antenna at a right angle and still expect decent radiation from it. Another thing that I have done with good success in the past is what we call a "meander line" where you zigzag the pcb trace in a rectangular pattern. This starts to fall apart when you shorten by about 60% so don't push it shorter than 40% of that 226 mm. A reasonable meander dimension might be horizontal lengths of about 20mm and vertical part about the same. I'll run that through a simulator and tell you how it looks. But first, tell me what ground plane you have available.

edit: after some quick analysis, I should adjust the meander size recommendation to more like 5 to 10 mm on edge because it looks like a lot more inductance is needed than 20 x 20 mm would provide.

BRE,

Are you sure about the length? I have an FM tx made by Velleman for the usual VHF band, it is on the pcb and isnt that long...

Seems Rr posted before I did...mebbe that's What Velleman did...

Well it's a new layout on a 100mm x 50mm doUble sided PCB. I've not started layout yet but it's for a thermostat and the RF is a premade 315MHz Sparkfun $4 module.

antenna options

Cant Sparkfun assist?

Surely they have data on aerial design..or am I being too naive again?

A loop antenna might be a reasonable alternative. Nordic Semi has some app notes on this:
NORDIC SEMICONDUCTOR - nRF905 Multiband Transceiver
(click Application Notes on the right hand side)

You could also consider just buying a pre-made pcb antenna but that wouldn't be any fun, would it?

I would be able to simulate something in a pcb antenna for you to get you close, but would need to have some idea of how you will partition your pcb for electronics vs antenna space. Perhaps PM to collaborate.

The other squiggly trace is the RFC

Are you sure?

Chokes are normally quite high impedance at the intended frequency, that squiggly trace will have an inductance of about 10nH at the most which is an impedance of just 27R.

[QUOTE=Hero999;758507]Are you sure?[QUOTE]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.

Is C3 needed?

For UHF it can be omitted, the internal CE capacitance of the transistor is normally large enough.

Here's a PCB antenna for 2.4GHz I have plenty of space for that style.

This is the Microchip MRF24J40MA 2.4GHz module with antenna. Range is 400 feet. The antenna is about 1/2 by 3/4 inch.

Hmm a big E, neat.