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I have a BLE board with a 1/4 wavelength antenna and a feed that is about 12mm long. The wavelength of 2.45ghz is 122mm. I've seen various discussions saying anything over 1/10 the wavelength you probably need to impedance control the trace? Is this true? Can I get away with not using very tight impedance control on this feed line?

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  • \$\begingroup\$ By impedance control do you mean impedance matching? If you don't match, you'll have loss. So "get away with" depends on a number of things, including how much loss you can afford. \$\endgroup\$ – Reinderien Nov 29 '18 at 0:58
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You have to use the wavelength in the medium where the wave is traveling. On PCB's, waves travel more slowly than they do in free space. For an internal trace (stripline transmission line) the speed is around 47% of free space, so the wavelength is around 47% also (waves bunch up closer together when they slow down). So rather than 122mm, the wavelength will be around 57mm. This puts you a bit over the maximum length (which would be 5.7mm according to the rule of thumb).

However, you are probably not using an internal trace. For an external trace (microstrip) the propagation speed is slightly faster (around 56% of C) but the result is roughly the same.

So I recommend that you simply design your feed trace to be around 50 Ohms. This is not difficult. You can find online tools to help you.

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Yes. For a feed this short you do not need tight control. Even if you are off by a large amount you will still probably not see a loss greater than 1dB. I would recommend using a calculator (this KiCad one is good) to find the best trace width for your board, and just use that.

You can also ask the fab house to impedance match it but for this it isn't needed.

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