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:: Return Loss and VSWR ::
 | Post date: 2019/05/26 | 

Return Loss and VSWR

Scientific-Experimental Interpretations without Mathematical Calculus

The RF energy moves along the transmission lines (coaxial cable or conductor paths on the PCB), as the sound is wrapped in an empty room. This is sensitive to reflections. When the sound waves hit the hardened tiled walls, the sound of the collision can be felt. The loss or loss of echo waves is approximately zero. Compare this with the time the sound is played in a closet and there is no echo or recurring sound. The fabric around you will absorb the sound and we will have high return loss. When radio waves (RF) are sent in a coaxial cable or PCB cracks, we expect these waves to be routed to the antenna. We do not want return loss like a return voice in the empty room.

Return Loss

These back reflections are called "return". The return loss shows the amount of reflections or small returns. We need small returns, so more losses in echo mode are better for us. return loss is less disadvantageous, which means that less energy passes through the antenna. RF engineers often measure the amount of return loss in logarithmic scale "dB", which can be much more complicated than actual. However, we must remember that better return loss is with larger return loss, and this is better for your antenna. The following table shows examples of logarithmic and loss in dB units.

Return Loss & VSWR
VSWR What does this mean? Return Loss(dB)
Unlimited 100% return, no power penetrates into the antenna and is fully returned. 0 dB
17 80% return, 20% power penetrates the antenna. 1 dB
9 63% return, 37% power penetrates the antenna. 2 dB
6 50% return, 50% power penetrates the antenna. 3 dB
3.5 63% return, 37% power penetrates the antenna. 5 dB
3 25% return, 75% power penetrates the antenna. 6 dB
2.3 16% return, 84% power penetrates the antenna. 8 dB
2 10% return, 90% power penetrates the antenna. 10 dB
1.4 3% return, 97% power penetrates the antenna. 15 dB
1.2 1% return, 99% power penetrates the antenna. 20 dB


The parameter "Voltage Standing Wave Ratio", which is expressed without the ratio (without unit of measurement), is the same return loss parameter, which is expressed only on a different scale. The VSWR is almost an old parameter and is often measured by the transmitter itself when it is transmitted to the antenna.

Return Loss Measurement

Measurement return loss during antenna design or approval is a powerful tool. Without a good return loss, an antenna can not accept RF energy, so it can not be useful for radiation. This has led to return loss as an important feature. However, return loss can not be the whole story. It is true that a poor return loss means that the antenna can not radiate, but proper return loss can not mean the effective radiation of an antenna. Unfortunately, sometimes we have test antennas that are suitable for return loss but very poorly radiate.

Radiation Gain

The problem is that internal losses or Radiation inefficiencies on an antenna can create good return loss, while the lost energy does not reflect on the transmitter. But how can one say that good return loss lead to radiation (desirable condition) or internal absorption (undesirable condition)? Most accurate methods are available to have an appropriate antenna and confirm the radiation performance of the antennas in the CMI lab. Therefore, the appropriate radiation gain is the final goal of most antennas to confirm.

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