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I'm designing a 345MHz transmitter using the MAX 7044 ISM transmitter with an electrically small antenna and need to test the matching network. I will connect the VNA in place of the transmitter using a short piece of coax. I also have made calibration standards out of the same coax and length.

How do I set up the circuit to measure the impedance as seen from the transmitter that includes the biasing inductor which is connected to the supply? I assume that because the DC supply and ground both appear as grounds to small signals, I should short the battery terminals (battery removed!) so that currents can flow through the biasing inductor out to the battery and onto ground. Doing this would have the added benefit of taking into account the inductance and resistance of the power supply traces.

Is this the right way to handle this?

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How do I set up the circuit to measure the impedance as seen from the transmitter that includes the biasing inductor which is connected to the supply?

That inductor is shown below inside the red box: -

enter image description here

Is often called a "radio frequency choke" or RFC and its main function is to block RF signals at the PA out whilst providing dc bias to the collector of the PA output transistor.

To that end you can ignore its presence in a circuit like this and, assume it is open circuit for AC signals.

However, if you are intent on taking it into consideration then the 100nF, 220pF and 680 pF form a pretty good low impedance to ground and, in effect, bypass any worries about regarding the battery terminals as being shorted.

If you have designed this properly (like the above) don't worry about the trace inductance unless you have an un-obvious but compelling reason to do so. In which case, don't be shy, let's here it.

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  • \$\begingroup\$ I hadn't considered the effect of the bypass caps effectively tying the inductor to ground anyway. Ultimately then, when I get this test connected, shorting the battery terminals shouldn't affect the matching. I don't fully understand how the choke can be negligible, I guess unless its sufficiently large? \$\endgroup\$ – Lance Beasley Jul 20 '16 at 13:23
  • \$\begingroup\$ Unfortunately, shorting the battery terminals ensures the device isn't powered and therefor the MAX7044 may produce all sorts of nuances that can't be explained. \$\endgroup\$ – Andy aka Jul 20 '16 at 13:25
  • \$\begingroup\$ Agreed Andy. That's why I plan to simply remove it from the circuit while tweaking the match. \$\endgroup\$ – Lance Beasley Jul 20 '16 at 13:28
  • \$\begingroup\$ The choke could be about 2uH and have a self resonant frequency pretty much mid-band where you are operating. Yes, it will have a small effect but it's likely to turn an antenna VSWR from (say) 1.5 to more than 1.55. \$\endgroup\$ – Andy aka Jul 20 '16 at 14:03

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