# Matching network for a resistive matched source, line and load?

Suppose we have a well designed 50ohm antenna and source (as there are in many RFIC, and antennas), and we design/use a noticeable long 50 ohm trace/cable in between. If I recall right, this design would lack bandwidth. I'm wondering:

1- What are benefits here for an impedance matching network?

2- And if we need one, how this matching network should be designed, as all source, line and load are 50ohm?

3- How much bandwidth we have without matching network and by only a constant width matched trace/cable?

Edit 1:

Explaining the lack of bandwidth above, in most real life applications we would have signal with a bandwidth, and this can go up to a some GHz in some recent years applications, or atleast circuit needs to be designed for. I've read somewhere before that stack of matching network would help less Q and so more bandwidth in a design. and of-course a simple constant width trace by a set of "Structure/Width/Height/Gap/..." would be designed for an specific frequency and its impedance (as well as VSWR) would change when frequency goes up/down.

I just barely remember someone said to me that this would have less bandwidth in contrast to matching network, but I don't have anything to compare a constant trace to matching network for bandwidth, noise, distortion and etc., so I asked this question here.

• It wouldn't lack bandwidth so, maybe you can explain that assertion. Jan 29, 2023 at 19:32
• You seem to be asserting the transceiver and antenna are both matched for 50 ohms already. In that case, VSWR --> 1 and no length of 50-ohm transmission line, of any type, will affect the result. Note that this is only true at the frequency(ies) where these assumptions are the case. Jan 29, 2023 at 23:24
• Thank you for your comments, I've edited my question, hoping to clarify the problem.
– 2i3r
Jan 31, 2023 at 16:31

we have a well designed 50ohm antenna and source ..., and we design/use a noticeable long 50 ohm trace/cable in between. If I recall right, this design would lack bandwidth.

That doesn't follow. Certainly no circuit or antenna has unlimited bandwidth, but being well-matched to your system doesn't mean they have inadequate bandwidth for your system.

And real transmission lines often have attenuation at high frequencies that limits bandwidth. But if they do we characterize that with an imaginary component in the characteristic impedance. If the characteristic impedance is taken as purely real, then we are assuming the line is lossless. If we also assume the line is single-mode at the frequency of interest, and the characteristic impedance we've been give is correct at our frequency of interest, then the line is not limiting the bandwidth at this frequency.

What are benefits here for an impedance matching network?

An impedance matching network is used when your source or load don't match the line impedance. In your case, you say the source and load are well-matched. Therefore no matching network is needed, and there is no benefit to adding one.

How much bandwidth we have without matching network and by only a constant width matched trace/cable?

Even in cases where a matching network is used, this question can't be answered with the information given. Rather, you should determine how much bandwidth is required, and then design your matching network to meet that requirement. A wider bandwidth might require a more complex matching network.