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The method/steps to use a VNA to determine a matching network for a 50 Ohm (or whatever impedance) antenna seem mostly clear to me now. My current knowledge assumes though the transceiver (source) is basically the same impedance as the desired impedance (well, complex-conjugate) of the feed line + antenna. That's the whole point after all, to match them to the transceiver.

What I don't really understand, is how to tune the system when my transceiver is known to have a complex impedance other than the antenna (in my specific case the manufacturer specifies it to be 35+10j Ohms).

Say my source is Zs and my load is Zl. Is my goal then, when measuring the impedance using a VNA, to see Zl when my probe is placed at the antenna in, and to measure Zs when measured at the point of the transceiver? Should the transceiver be connected during these measurements at all? Should it be powered (but not sending)? How does this comply with the general advice of connecting the probe at the point of the matching network?

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  • \$\begingroup\$ Assumptions regarding transceiver Zs=35+j10 are suspicious. The manufacturer likely specifies something like "50 ohm load nominal". So don't worry about the radio-end. Work to make your antenna appear as 50+j0 Zl. \$\endgroup\$
    – glen_geek
    Commented Jul 23, 2019 at 12:20

2 Answers 2

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This is not at all clear. To stay healthy radio transmitters need a certain load. If that load is also used as your receiving antenna, the receiver in the same box probably is designed to work well with just the same antenna. Do not expect you can measure the optimal antenna impedance by connecting a network analyzer to the antenna connector of your tranceiver box. The result will not tell anything for which load impedance the system has been designed. You must find it from the system specifications. Some radio tranceivers are their own network analyzers as well, they have antenna tuner and a special tuning mode. I guess this is not your case.

If it happens that the right load to be connected to tranceiver's antenna connector is (30+10j) Ohms you should connect the antenna+cable+possible tuning elements to your analyzer and tweak the connected system until your analyzer shows impedance = (30+10j) Ohms at the operating frequency. You use the analyzer in one port mode.

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  • \$\begingroup\$ It is not about measuring the optimal antenna impedance, just matching two known impedances. The antenna itself (without the feed line) is known to be 50Ohm, and the transceiver is specified as above by its manufacturer. My idea was to match the transceiver to the 50Ohm of the antenna. Do you mean it would be easier/better for performance to just tune the "antenna side" of the matching network to be also 35+j10? Given that the antenna is already 50Ohm, isn't that the same as my original intention? Sorry, these things are new to me, you might have confused me here a little. \$\endgroup\$
    – ultimA
    Commented Jul 23, 2019 at 11:16
  • \$\begingroup\$ @ultimA If your antenna + its cable already happens to be a perfect 50 Ohm resistive load, it should not be especially difficult to design a two reactance matching network which converts that 50 Ohm to (30+10j) Ohms when it's connected between the tranceiver and the antenna cable - at least if you operate at so low frequency that inductors and capacitors are useful. \$\endgroup\$
    – user136077
    Commented Jul 23, 2019 at 11:37
  • \$\begingroup\$ To rephrase, I have a transceiver with known impedance ZS, and an antenna with known ZL. There are parasitics and a (electrically long) transmission line between the two. The way to design the matching network, the way I understand now, is to tune the matching components, until I measure ZS* when I replace the transceiver with the VNA, and measure ZL* when I replace the antenna with the VNA. Correct? \$\endgroup\$
    – ultimA
    Commented Jul 23, 2019 at 11:41
  • \$\begingroup\$ Not at all correct. The only goal you should have is to make the transmitter see its specified load impedance and that should be achieved with a reactive tuning circuit to minimize power losses. That should be ok also for receiving if your tranceiver is designed properly. Measuring with an analyzer the tranceiver doesn't give info of the internal impedance of the transmitter during it transmits. \$\endgroup\$
    – user136077
    Commented Jul 23, 2019 at 12:15
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You want a conjugate match, so replacing the transceiver with the VNA, you should be seeing ZS* and replacing the aerial with the VNA you should be seeing ZL*, with in both cases the other end of the network connected appropriately (BUT NEVER TRANSMITTING).

Personally for this sort of thing I reach for the Smith chart, then tweak it on the VNA if required.

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  • \$\begingroup\$ Because VNAs are 50 Ohm terminated, am I correct a 2-port VNA would be pretty useless in my case? Since it would need to be 35+10j terminated at the transceiver side when doing measurement from the antenna. So a 1-port is VNA should be "enough" for me. \$\endgroup\$
    – ultimA
    Commented Jul 23, 2019 at 10:11

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