# How reflection occuring in antenna fed by transmission line affects performance?

Transmission line considered is lossless. Basically the configuration is simple.

For transmitter side : a non ideal source (can be voltage current or power) feeding a transmission line (TL1) with source impedance (Rs) not matched with transmission line and transmission line terminated with antenna with unmatched total impedance (Ra1) with respect to transmission line.

For receiver side : another antenna (Ra2) feeding transmission line (TL2) which in turns feed a load resistance (RL) here transmission line is unmatched to either antenna and load.

What I think is that as reflection of wave is internal to transmission line the power transmitted should be found using Zinput of TL1 taking Ra1 as load impedance. A part of voltage will drop at Rs and rest of it at Zinput. Although reflection can affect transients.

For receiving side the field incident on antenna should induce voltage and current again some voltage will be dropped at Ra2 and will cause reradiated power and rest of power will be dropped at Zinput of TL1 taking RL as load and ultimately dissipate at Rl.

I am not getting except of improving transient what is need of matching antenna to transmission line.

What I mean is that sure I can understand as power is reflected and reflected power cannot be fed back to source; that is if I consider a pulsed transmission. But I consider a continuous transmission the source (microwave source) will be in equilibrium with this situation that is the actual impedance seen by source will be different from the pulsed transmission case (and can be found by the Zin formulae for TL) as this impedance is calculated already after considering the reflection so the reflected power should not cause any issue with power budget of the source.

• Are you looking to find out what the reflection coefficient is? Or, are you looking to know why a reflection happens? Your question isn't very clear. Maybe this helps? Commented Jan 9, 2021 at 10:30
• @Andyaka What I mean is that sure I can understand as power is reflected and reflected power cannot be fed back to source; that is if I consider a pulsed transmission. But I consider a continuous transmission the source (microwave source) will be in equilibrium with this situation that is the actual impedance seen by source will be different from the pulsed transmission case (and can be found by the Zin formulae for TL) as this impedance is calculated already after considering the reflection so the reflected power should not cause any issue with power budget of the source. Commented Dec 10, 2023 at 3:52

The impedance of the antenna and the transmission line that feeds the antenna must be matched. If not, there will be standing waves. The reflection coefficient impacts the final power available at the antenna for transmission. If the reflection coefficient is very high, the power available at the antenna for transmission will be less than that available without the presence of reflections.

The available power for transmission at the source depends on characteristics of the transmission medium such as its reflection coefficient.

For ensuring maximum power in the forward direction, feeding plays an important role in the case of parabolic reflectors.

• Please read my approach and tell me where I missed Commented Jan 9, 2021 at 8:32

If the antenna is matched to the feeding transmission line, then there will be no reflections.

If reflections occur, that means (in the case of a transmitter) some incident power has been reflected back to the source. That is undesirable for two reasons

1. Power reflected back is then not available for transmission, your transmitted power will be lower. This means lower range, or you could have used a cheaper transmitter with a matched antenna for the same range.

2. Power reflected back could damage the transmitter.

In either case, a small amount of mismatch will not reduce your range much, and is unlikely to damage a transmitter. A severe mismatch may do. It's quite easy to get a 'good enough' match for most transmission and reception purposes. It's quite difficult to get a good enough match for accurate measurements at the 0.1dB level.