Electronics > RF, Microwave, Ham Radio

Phase diff between tx and rx antennas spaced by whole multiples of wavelength

**msat**:

I don't know if this is common knowledge or not, but I have been unable to find relevant information on this. Hopefully someone here knows. Here's the question:

Assuming two identical antennas, with one transmitting [a sine wave] and one receiving, spaced apart by some whole number multiple of the wavelength, and both probed identically and fed to an oscilloscope, would the waves be in phase, or 180 degrees out (or something else?!)?

I'm currently assuming they would be 180 degrees out of phase.

Definitive answers would be appreciated. Otherwise, please provide your supporting argument.

Thanks!

-Mark

**TimFox**:

The time delay between two antennas spaced one wavelength apart equals the (time) period of the sine wave. Therefore, the two signals (carefully probed) will be 360 degrees out of phase (receive antenna lagging behind transmit antenna), which is equivalent to zero phase angle. This assumes antenna construction such that if the two antennas coincide in space (zero wavelengths apart), the signals are in phase.

Similarly for other integer values of spacing/wavelength.

"Careful" probing requires equal transmission lines from the two antenna feedpoints to the two oscilloscope inputs to equalize the probe delays.

**msat**:

@TimFox

I understand the effects of time delay between transmission and reception based on the distance between antennas, as well as the need to consider the way the antennas are probed to maintain equal transmission line delays. I also get that if we were to plot an EM sine wave as is commonly done, we would see that they are in phase diagrammatically. What I'm interested in is not what happens diagrammatically, but what happens electrically at the antenna.

So I guess I need to clarify what I'm seeking to understand by adding some detail to my hypothetical experiment. Lets say the antennas being used are dipoles oriented vertically. In our phase diagram, the upper half of the sine wave corresponds to the upper half of the dipole having a positive charge relative to the lower half, and vice-versa for the lower half in the diagram.

In other words, for a given point in a transmitted wave, are the induced charge distributions on a receiving antenna the opposite of the antenna which created it?

**msat**:

I suppose I should give my line of reasoning which is what I'm basing my assumption off of.

We know that like charges repel and opposites attract. If some point of an EM wave was generated by a downward pointing e-field (vectors pointing from positive charges towards negative), then when that point reaches the receiving antenna, the charge carriers (electrons) would be forced to move in the direction opposite of the vector.

To my understanding, this at least holds true strictly for e-fields, but I don't know if it does for EM radiation.

**radiolistener**:

--- Quote from: msat on September 07, 2021, 04:30:50 pm ---In other words, for a given point in a transmitted wave, are the induced charge distributions on a receiving antenna the opposite of the antenna which created it?

--- End quote ---

No. If both dipoles have the same orientation (the same polarization) and placed at N*lambda distance (multiple of wavelength), then the charge density distribution will be the same for both at any given moment of time. And phase offset will be 0 degree (folded 360 degree).

Phase offset 180 degree will be at N*lambda + lambda/2 distance.

Note that there is phase delay between two antennas and resonant antenna bandwidth is limited due to high Q. So if you change sine amplitude on transmitting antenna it will affect second antenna after some period of time, which is include time delay due to antenna bandwidth limitations and time delay for wave traveling across space.

Antenna cannot change amplitude immediately and needs at least Q cycles to radiate some carrier change. For half-wavelength dipole Q is about 10-12, so it needs to wait for about 10-12 sine cycles to radiate a new sine amplitude + time delay for wave traveling at specific distance.

--- Quote from: msat on September 07, 2021, 04:46:36 pm ---We know that like charges repel and opposites attract. If some point of an EM wave was generated by a downward pointing e-field (vectors pointing from positive charges towards negative), then when that point reaches the receiving antenna, the charge carriers (electrons) would be forced to move in the direction opposite of the vector.

--- End quote ---

No. For example, let's name up side of vertically oriented half-wavelength dipole as U and down side as D.

When you apply some electric potential to the antenna A, let's say + for U and - for D. The antenna B at N*lambda distance will sense this electric potential with reduced level due to distance.

And since both antennas A and B have exactly the same orientation, antenna B will sense the same + on U and - on D side.

In other words, when you charge some thing with positive charge, you will measure positive charge around that thing, NOT negative. :)

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