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I have been using a TAOGLAS 915MHz PCB antenna for my remote control. I'm about to redesign the PCB and wanted to try including my own PCB antenna on the board. For the 915MHz band, a quarter wavelength is ~80mm, however I measured the trace on the TAOGLAS board using a ruler, and got something closer to ~117mm. The difference between these is too big to be attributed to inaccuracies due to the use of a ruler, so I was wondering why the trace length they use is different than the theoretical optimal value.

Additionally, I was thinking of making my trace ~80mm, and then putting a solder bridge connecting it to another trace for a total of 117mm. I know this wouldn't be as great as a single trace of length 117mm, but is there a reason this wouldn't work for testing purposes.

EDIT: (for more information)

I've found online that the optimal length for an antenna is 1/4 of the wavelength (although now typing this I realize this probably does not apply to PCB antennas, as that research was done for a monopole wire antenna). I have tried looking for resources on how to design a PCB antenna, but could only find equations for a patch antenna. I used an online calculator, and used the bounds of 3.8-4.8(through quick google search) for the dielectric constant of FR-4 just to get an idea of what it would look like, but it is wayyyy too big ($>7,000 {mm}^2$). If anyone has any resources describing how to design a monopole antenna PCB send them my way.

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Velocity factor, probably. A transmission line with FR4 PCB as dielectric has a velocity factor of around 50% (meaning its electrical length is half its physical length). An antenna is a radiator, its field isn't confined to the PCB material (we hope!), but a good chunk of it is there, so it makes sense to expect a Vf somewhere between 50% and 100%. 70%'s as good a guess as any, given no real information.

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I must admit I'm a bit confused by the question – the datasheet behind the link shows you the efficiency of the antenna for different frequencies (i.e., different wavelengths). That is what the antenna was designed for – your notion of how long it should be doesn't matter to the antenna designer. So it's not "intentionally long" or such; it's exactly as long as the designer needed to make it to get the desired frequency behaviour.

If your theory disagrees with the reality of the antenna measurement, that is indicative of your theory not accounting for all effects well enough.

I'm especially confused that you have a theory on the length of the antenna at all – for that, you'd need to know at the very least material constants of the PCB substrate, and these are not specified in the datasheet at all!

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