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Hi all, I am a beginner in RF system design and 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?

Thanks,

If it is above all an impedance adaptation that you are looking for, it is easier to use an 'L' adapter. It will be low pass or high pass. You will choose it as impedance step-up or step-down.

You can consider that a pi-adapter consists of 2 L-adapters one behind the other. For example on a classic pi: capacitor in parallel, inductor in series, capacitor in parallel, you cut (in your mind) the inductor in two. The one on the right is step-down and the one on the left is step-up. They are both low pass.

A pi is especially of interest when the adapter must be fairly universal. This is not your case since you are going to measure your antenna with your nanaoVNA. The adaptation will be fixed (raise or lower).
--
Fran?ois F1AMM

Maybe it easier to build the antenna that it has 50 ohms input then tune a mismatch with a network
Dg9bfc sigi

Am 22.07.2022 12:08 schrieb F1AMM <18471@...>:

You would want to pass DC through to your antenna if the antenna might have a pre-amp that needs the DC for power. Also in some cases, DC grounding the antenna is good practice for safety, like when dealing with radio detonators for fireworks, explosives, or just in the presence of fuel fumes. DC grounding means no static buildup.

SherpaDoug

On 7/22/22 2:00 AM, Diane BONKOUNGOU wrote:

Hi all, I am a beginner in RF system design and 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?

Folks will chose high or low pass to meet spurious emission requirements. For instance, if your transmitter has 3rd harmonic, using a low pass matching network can help knock that down to below the (typical) -40 dBc or -60 dBcrequirement.

As to why you might want to pass DC - AC coupled ungrounded antennas can build up static charge. If you don't have some way to discharge that static, then if the voltage gets high enough, you might have a spark (which generates broadband RF power) or a component failure (e.g. a capacitor exceeds its voltage rating). Some antennas though, are inherently grounded, so in that case, you don't care.