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PCB Design Help (315Mhz with 1/4 wave Dipole)

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Mar 19, 2008
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1/4 wave dipole

Hi, I have never done RF layout before. This is a really simple board but I would like maximum range. :)

I have a transceiver chip and antenna that are both matched at 50ohms and tuned for 315Mhz. My problem is how to get a trace from chip to antenna with as few problems as possible.

Are there things to consider like corners? Even at 315Mhz? Should the traces be really wide? Really narrow? Really straight? Parallel/Perpendicular to actual antenna? Thickness (z-direction) of copper?

Also reading some application notes they suggest a large ground plane. My antenna will be connected on a right angle near the edge of the board. I realize this will give it some directionality which will decrease range in some directions. This also means it is parallel to the board itself. Because of this I have read that the ground plane has to be a certain length (or rather multiple of some length). So do I go off of the wavelength of a 315Mhz signal an make the ground plane some multiple of that? Or does it have to match the antenna more then the raw wavelength?

Thanks to anyone that can help.

A simple quarterwave antenna will be around 1/4 meter long at 315 MHz (such as a whip of metal wire sticking up from the board). If you want to print this on the board, you will
1) need a big board
2) most likely want to stand the board on its end so that the antenna is pointing vertically (most systems use vertically polarized antennas).
3) will want the printed antenna to look very much like the whip antenna, ie with no ground plane along the length of the printed "whip"

Practically, a lot of people will prefer a smaller antenna with somewhat worse performance, just so that it will fit in their enclosure. You can then use either the same whip antenna design, but with a loading inductor (high Q please) at its base in series with the whip element. Another approach is to use a loop antenna, possibly with a tuning element in the loop. One advantage of "small" loop antennas is that the radiation pattern is in the plane of the loop, so you can have the board horizontal (sitting on a table top) and it will radiate omnidirectionally around it. Some small circuits, like wireless key fobs, have the circuitry inside of the loop, and the loop goes around the edges of the key fob in a big square circle.

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I am using a matched antenna (about 8" long encased in plastic, not sure of the style inside though but the description says "1/4 wave" that came with the modulation chip). It will attach via a RP-SMA connector.

What I am worried about is the ground plane for this antenna. I need maximum range for this, so I really want to get it right.

I read through the PDF, and it helps but I still have questions. It did give me good layour device for the other components on the board which I have changed accordingly.

Yep, that is about right for a quarterwave. As I understand it, you have a whip antenna perpendicular to the board, sticking up 8". And you want to mount it on one of the board edges.

So, something on your board is either transmitting, or receiving, the 315 MHz energy. Lets say that it is an IC with a port that says "to antenna" on it. That same chip will have ground connections. You want to run a trace on the top of the board, as straight as possible, directly from that antenna port to the center pin on the RP connector. Just as importantly, you also need to run a much wider path of metal (maybe 1" wide minimum) on the backside of the board, without any interruptions, directly between the ground pins on the IC and the outer shell of the RP connector. People love to cut this ground plane with DC power and digital signal lines, but DON'T DO IT! Once you cut the ground plane, all bets are off.

Your antenna will have some directionality. Nothing you can do about that, unless you have a 16" diameter ground plane and you put the antenna right in the middle of it. My experience with such edge mounted antennas is that most azimuths you will be within 5 dB on antenna gain ripple, and there might be one or two azimuths where the gain may dip as much as 10 dB.

Try to get as much continuously connected ground plane on the back of the board as you can. If you skimp on ground plane surface area, you will have poor radiation efficiency, AND the antenna may detune itself (not resonate at 315 MHz anymore).

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