# Antenna pi-network matching High or low pass?

I am a beginner in RF system design and I have some questions.

I have a PCB trace antenna, I need to tune it to 2.45GHz, this is for Bluetooth low energy communication. I want to use a pi-network for the tuning process. I chose pi-network because it is selective and allows to set the quality factor Q of the circuit and the bandwidth (BW) of the antenna as Q=F/BW.

-Can someone tell me when to choose the pi-network high pass or low pass for antenna matching? I have seen in the document "AN1275: Impedance Matching Network Architectures" by Silabs that the high-pass network allows high frequencies to pass through the antenna and the low-pass pi-network blocks the passage of high frequencies through the antenna, which also means that the matching network must allow DC current to pass through the antenna.

What does it mean to allow DC current to pass through the antenna? In general, I know that the signal transmitted by the antenna is an AC signal. Could I feed (transmit) a DC signal to an antenna?

• I am confused because I read the DC on a document. I just want to know if I could use any of the configurations of pi-network (LCL OR CLC) for my matching. We could have a high pass or low pass pi-network. • Low Pass Configuration—Series Inductor and Parallel Capacitor CLC • High Pass Configuration—Series Capacitor and Parallel Inductor LCL What is the difference between the two pi-network systems? Jul 22, 2022 at 9:29
• I measure with NanoVNA a resonant frequency of 2.28GHz, the impedance is 47.8-7.06j. Here the imaginary part is negative that's means we are in the capacitance in the Smith chart. I have used SMA connector for the measurement so my source impedance is 50ohm. Am I responding to your question? Jul 22, 2022 at 9:54
• And I want to tune the antenna at 2.45GHz, with a Q=4.9 and a bandwidth of 500MHz and match the impedance. Jul 22, 2022 at 9:56
• Ok, I understand. I want to use an online calculator home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html And with the value of my source impedance, load impedance and the final resonant frequency, and the Q factor. Find the value of C,L I have to put to match my antenna. Jul 22, 2022 at 10:25
• I don't know if I could use directly the NanoVNA to find the value of L and C I have to put for the tuning Jul 22, 2022 at 10:26

What does it mean to allow DC current to pass through the antenna?

Unless you are talking about more specialized loop antennas, DC current won't flow into an antenna (dipole, monopole, variants thereof etc.). Even if it did, there would be no benefit because an antenna cannot transmit DC; it would just be a waste of power.

Could I feed (transmit) a DC signal to an antenna?

See above.

Antenna pi-network matching High or low pass?

I would tend to prefer a low-pass pi-network because it helps reduce unwanted harmonic content from a transmitter PA.

• Okay, thanks. So the difference between low-pass and high-pass is the fact that low-pass reduces unwanted harmonics. There are no rules for using low-pass or high-pass if I understand. So I'm going to use low-pass filters with Q=4.9, at 2.45GHz as resonant frequency and 500MHz as bandwidth. Jul 22, 2022 at 9:36
• Please note that the Q factor of a pi-network is a very ambiguous and fairly meaningless quantity; a pi-network (impedance matcher) has two Q factors so, don't expect that the Q factor stated in any network calculator will be meaningful when it comes to reshaping an antenna's natural bandwidth. Jul 22, 2022 at 9:39
• Can anyone see a reason why my answer has been downvoted? Jul 22, 2022 at 9:42
• Okay, thanks for your response. I am new on stack exchange, I don't see if your comment is downvoted or not. Jul 22, 2022 at 10:05
• I've found also this software SimSmith I think I'll use it. Jul 22, 2022 at 10:56

What does it mean to allow DC current to pass through the antenna? In general, I know that the signal transmitted by the antenna is an AC signal. Could I feed (transmit) a DC signal to an antenna?

Yes, it's not useful to pass a DC current into an antenna, but sometimes you intentionally superimpose DC on the feedline for the use of some other device, such as a mast-mounted preamplifier near the antenna. In such a case the question of whether the filter passes DC or not is relevant — if it does, then you can likely add the filter without trouble. If it doesn't, then you need to come up with a way to bypass the power around the filter.

In your case this doesn't seem to be a concern, so feel free to ignore the question of DC passing and choose based on other factors.