# Forum

## Differention of Multiple Waves when sent over the air

• Hi,

If I have two waves that are on the same frequency, but are 180 degrees out of phase - is there any way I can recieve the two original waves? I thought that they would cancel eachother out?

Thanks
Nik

• How far apart are the two sources from each other?

Has diversity helped?

• If the two TX antennas are 1/2 wavelenght apart and of equal amplitude and polarized in the same direction, then yes, they will cancel. (The RX antenna will feel two equal and opposite forces.) Otherwise, depending on the amplitudes and polarizations, you will get an elliptical, circular or more complex wave, which you can recover the two signals by orienting TWO RX antennas exactly the same as the two TX antennas. And if the TX antennas are far apart, then consider them as independent systems.

• [quote]If the two TX antennas are 1/2 wavelenght apart and of equal amplitude and polarized in the same direction, then yes, they will cancel. [/quote]

That really depends on the placement of the receiving antenna's, and reflecting surfaces, multipath, etc.

• Thanks guys - I am reading the CWNA and using the EMANIM-CWNA application; so I am a real beginner - please bear with me!

Just to make sure I have properly understood: If the amplitude and polarization are identical at transmission so effectively they cancel eachother out - could I still receive these waves independantly if I arrange the recieving antennas so that they are half a wavelength apart?

Thanks very much for all the assitance!

• Nik

When we transmit a radio signal, the amplitude and polarization are characteristics of the radio wave itself. It's like saying someone is 6 foot tall and has brown hair.. The height and hair color are inherent characteristics of the person, but are independent of each other.

When we transmit a radio signal from one antenna, we have a wave which spreads out in a pattern dependent on the type of antenna and environment. Depending on the environment, "parts" of that wave may be reflected, refracted etc. Multiple copies of that wave may arrive at the receiving antenna or antennae. If two signals of the exact same amplitude and exact same frequency and exact same polarization arrive at one antenna at exactly the same time, complete cancellation would occur. Real life is a bit different. Signals may actually reinforce each other for periods, then partially cancel each other etc., etc. Real RF environments are usually dynamic, with people moving around, doors opening and closing etc. Even changes in atmospheric temperature, pressure, humidity etc can cause changes in radio wave propagation.

Often we will have what is known as "diversity operation" whereby the radio receiver "listens" at each antenna and makes intelligent decisions about which signal is best at any time.

Later in your studies, when you look at 802.11n, you will find that sophisticated electronics actually allows us to take advantage of all those signals which have been reflected etc.

One very important thing to remember is that in a real-life Wi-Fi system, we do not have one or two nice sine waves being sent out ( as per the instructional tool ), but rather a highly complex modulated signal containing a vast number of frequencies within the operational bandwidth of the signal.

Dave

• Dave is correct...theorectically, two sinusoidal waves of the same amplitude, polarization, direction and of integer 1/2 wavelength(s) apart will completely cancel...but practically, each may encounter different forms of interference after leaving the TX antenna leaving two distinct waves at the RX antenna.

• Thanks all, and I have yet to get to 802.11n - I very much appreciate the help. I am trying to get to grips with the basics first, learning to walk first etc. so I made a load of assumptions! cheers all.

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