VSWR means voltage standing wave ratio which everyone knows but why it comes to calculate in Telecom?
When BTS transmit any signal (LTE, CDMA, GSM, WCDMA anyone) then in radio wave transmit through cable and reach to antenna so what?
Now I give you one example, Think you punch in wall and one man behind wall and does he get your punch effect NO why? Now think what is reflect by wall to him nothing.
So now what different in your punch and wall is impedance means wall impedance is almost 1000> to your punch so you not reflect any thing and so nothing happen with men behind wall.
This same thing in our telecom fields the impedance matching should be equal 1:1 or vey less different and the ratio of this call VSWR. now you understand why needs to tack care for BTS VSWR.
Now I write some calculation for VSWR,
In mobile communication systems, the VSWR of antenna is 1.5:1 at most. Assume that:
- ZA: input impedance of antenna
- Z0: nominal characteristic impedance
So the reflection coefficient is calculated as below:
You can also represent the matching character of port with echo loss as below:
If the VSWR is 1.5:1, the RL is 13.98 dB.
When the input impedance of antenna is not equal to characteristic impedance, the reflected and incident waves form standing wave after overlapping on the feeder.
The ratio of the maximum adjacent voltage of standing wave to the minimum adjacent voltage of standing wave is the voltage standing wave ratio (VSWR).
If the VSWR is over large, the communication distance will be shortened and the reflected power will return the transmitter.
As a result, the power amplifier may be burnt and the communication system will work abnormally.
The down tilt of antenna is an important means that you can enhance the signal level of serving cell and reduce the interference with other cells.
The common down tilt modes include mechanical down tilt, preset electrical down tilt, and adjustable electrical down tilt (RET antenna) as below:
- Mechanical down tilt: you adjust the mechanical down tilt by lowering the support.
- Electrical down tilt: you adjust the electrical down tilt by adjusting the phase of oscillators. After the preset electrical down tilt is sold out of the factory, the down tilt cannot be adjusted. The adjustable electrical down tilt is adjustable.
You can adjust the mechanical down tilt and electrical down tilt simultaneously.
My suggestion for any data network (LTE,HSD,3G,GPRS) try reduce congestion or user speed by Mechanical tilt first then try on electrical tilt because electrical tilt reduce coverage from both side (Side lobes and upper lobes).
I found it also reduce some indoor coverage by adding some resistance to antenna.now share your experience about VSWR or Tilt in comment.