I have been looking at 868MHz through-hole spring antennas for an application, but I can't seem to find any sort of information in datasheets regarding layout recommendations.

Here's the most detailed datasheet I can find for the type of antenna I'm talking about.

I was hoping to have this mounted in a corner cutout of a PCB, which means it will have no ground plane directly under it, but it will have ground plan on two sides (feedline side and parallel with the coil direction). Similar to the orientation below, but with the feedline coming in from the right side.

Has anyone used an antenna like this in a similar orientation/setup? Does anyone know of any doucumentation/guidelines for helical antenna orientation and ground plane keepouts?

enter image description here


Yes, but the photo is of an "Inverted F" PCB antenna. VERY different beast. Missing from the "F" photo are via stitching of the ground plane around the perimeter of the antenna element. Without via stitching, a conductor greater in length than 1/10th the wavelength will start to behave as an antenna itself, and induce voltage onto it. Also notice that the feedline necks down to a very small and thin point right after C8 - this could be a drawing error and may cause problems.

Anyways, for a copper spring antenna, just like the F antenna, every aspect of the physical dimensions are critical to correct operation. This includes not only the length of the spring, but it's spacing and orientation to the ground plane and any nearby metal objects. Also critical are the impedance-matching components - L and C values in series and to ground to control the impedance from source to antenna. The width of the trace leading to the antenna is critical, as width has a corresponding impedance. Every aspect of this antenna - matching component values, distances, orientations... all influence how the antenna works and how efficient it is. Get any one of them wrong, and it may not work at all.

To calculate all of this by hand would require a college degree in RF engineering. Usually much easier, is to find a reference design using this antenna (at 868MHz; everything must be the same) and modify it to physically fit into your design. Ground planes can usually be extended without much influence to operation, so stick it in the corner of your PCB.

Even so, the matching component values may benefit from slight adjustment in your finished product. Usually this is determined by measuring antenna performance using a VNA to obtain the s-parameters. This indicates the efficiency and what component(s) need changed and by how much.

  • \$\begingroup\$ Thanks for the reply, very informative. The lack of via stitching is a bit of an oversight from whoever laid out the example I provided, but I showed it only to give an indication how how I hoped to set up the helical antenna. I've got a VNA so my intention is to tune the antenna once I have some samples made, but there I have had extreme difficulty finding any data sheet that gives mounting/ground plane suggestions, which is what I'd hoped someone might have insight into here. \$\endgroup\$
    – Sensors
    Sep 3 '18 at 19:20
  • \$\begingroup\$ I did search a bit, but like you, came up with nothing for this antenna. There has to be a reference design somewhere. \$\endgroup\$
    – rdtsc
    Sep 3 '18 at 23:44
  • \$\begingroup\$ I found a datasheet for a 433MHz ISM antenna. Perhaps this can get you looking in the right direction. \$\endgroup\$
    – rdtsc
    Sep 22 '18 at 3:19

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