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I am designing a board having nrf52832 and a chip antenna (2450AT18B100 Johanson Technology) for 2.45 GHz (BLE basically). But I have few questions regarding standard 50 ohm characteristic impedance of Antenna Feed Line.

In my PCB, antenna feed line will be coplanar waveguide with bottom ground plane. As far as I know transmission line will have capacitance, inductance and resistance. But the standard suggests to keep impedance of antenna feed line to be 50 ohm (which is resistive).

Does that mean the transmission line will also resonate at RF frequency (2.45 GHz in my PCB) and it will be resistive at that frequency only?

The chip antenna (2450AT18B100 Johanson Technology) states that it has impedance of 50 ohm. Does that mean if my antenna feed line is of 50 ohm, then I won't need any matching components?

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  • \$\begingroup\$ They mean characteristic impedance. So that is the ratio of width to distance from ground plane. There are many calculators online. You most likely do not need any matching network. \$\endgroup\$
    – MadHatter
    Oct 30, 2017 at 12:08
  • \$\begingroup\$ @MadHatter: Do You mean the characteristic impedance of CPWG will be 50 ohm resistive at 2.45 GHz? \$\endgroup\$
    – abhiarora
    Oct 30, 2017 at 12:32
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    \$\begingroup\$ 50 Ohm at any frequency. Characteristic impedance does not shift much with frequency. Think of a coax cable... \$\endgroup\$
    – MadHatter
    Oct 30, 2017 at 13:55
  • \$\begingroup\$ But Then how could it be resistive only? Don't you think it should be having capacitance and inductance? \$\endgroup\$
    – abhiarora
    Oct 30, 2017 at 14:29

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If the chip antenna has an input impedance of 50\$\Omega\$, and the line has the same impedance, then the line loaded by the antenna will also look like 50\$\Omega\$ impedance looking in. If that's what your amplifier is expecting to drive (usually is), then you won't need any matching.

If you place a line with the correct geometry, that is correct width on a substrate of specific thickness and dielectric constant, with correct gaps to the topside ground plane, then it will have a broadband 50\$\Omega\$ impedance.

There are many online calculators you can use to give you suitable dimensions. Coplanar with ground has too many variables to keep dimensions in my head, but note that with ordinary microstrip (ie coplanar with ground with very big gaps!) on FR4, you get roughly \$50\Omega\$ with a line that's twice as wide as the substrate thickness. A coplanar line will be narrower than this, as there is some field coupled to to the top metal.

Although the line has capacitance to ground, and you can define an inductance per unit length, they are both uniform. The effect is that when a voltage signal passes along the line, a current signal runs along with it, and they are in the ratio 50 volts to 1 amp, or \$50\Omega\$. This impedance is a nominally constant 50 ohms from DC up to frequencies where the imperfections of the line geometry or dielectric uniformity start becoming significant, often many GHz even for plain FR4, and 10s of GHz for 4350.

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  • \$\begingroup\$ According to you, 50 ohm standard impedance is the magnitude of complex impedance of Transmission line (in my case CPWG)? \$\endgroup\$
    – abhiarora
    Nov 6, 2017 at 9:50
  • \$\begingroup\$ @abhiarora I'm not sure I understand your comment/question. Ideally the real part of the line impedance will be 50ohm, and the imaginary part (the attenuation) will be as close to zero as possible. Though as the attenuation is usually small, the magnitude of the complex impedance is pretty much the same as the real part. When I say 'the line', I mean 'the 50ohm line we're talking about', not 'all transmission lines'. Obviously a line a different geometry will have a different impedance, and so a different volts/amps ratio for signals passing along it. \$\endgroup\$
    – Neil_UK
    Nov 6, 2017 at 10:12

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