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Testing this stub board. I understand S12 is attenuated at 1.79GHz due L=λ/4. So I calculate relative permitivity:

εr = (c / 4Lf)2 = 3.3

But what does (εr) represent in a physics perspective? Is it permitivity of the board dielectric? If so how does the signal interact with the ground plane in this stub?

Second: What is the cause of the reduced S11 reflection at 3.09GHz?

VNA

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    \$\begingroup\$ The εr you have calculated is the effective permittivity. it's not the board's (FR4) permittivity because the wave doesn't 'see' a homogeneous material - it sees air on the top. \$\endgroup\$ – Mike May 13 '17 at 17:04
  • \$\begingroup\$ But, to add to what Mike says, the "upper half" of the space it sees has lower relative permittivity, by a factor of >4, so most field energy will be below. \$\endgroup\$ – Marcus Müller May 13 '17 at 17:13
  • \$\begingroup\$ Then what does effective permittivity represent? How is the signal being slowed down? \$\endgroup\$ – bretddog May 13 '17 at 17:22
  • \$\begingroup\$ Is velocity = sqrt(L/C) ? \$\endgroup\$ – analogsystemsrf May 13 '17 at 17:44

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