Antenna phase explanation

Question

Trying to grasp antenna phase, reading through ham.se and youtube but haven't come across a clear (enough) explanation.

I understand the basics of antenna phase (i think), where E leads I that's inductive-reactance, where E lags I that's capacitive-reactance, and where E=I that's resonance.

Easy enough, but then in phased-arrays (say verticals), how does phase relate to beam-formation?

Thanks!

Answer 1

What you are describing is not "antenna phase" but impedance. An antenna, like many two-terminal devices, is a load that can be described by an impedance. It can be purely resistive (like a resistor), meaning current and voltage are in phase. Or it can be reactive (like a capacitor or inductor) with current and voltage being 90 degrees out of phase. Or it can be some combination of both.

The feedpoint impedance does not influence the radiation pattern of the antenna per se. However, the reactive power doesn't contribute to radiation, so efficient antennas will often have a mostly resistive feedpoint impedance.

There is a notion of "antenna phase" that is different from what you describe but relevant in phased arrays. In this sense, "phase" doesn't refer to the phase relationship between current and voltage, but rather the phase relationship between different elements in the array. Assume (because it's a common case) that all the antennas have a purely resistive, 50 ohm impedance. So current and voltage and any feedpoint will have the same phase. Antenna phase is not about that. Rather, compare the phase of the signal fed to each antenna: are they all reaching peak at the same time? Or is one lagging behind the other?

Manipulating the phase of the antennas in this way influences the direction in which the array achieves maximum gain. Maximum gain is realized where the wavefronts from each antenna in the array arrive in the same phase, and thus add constructively. That phase is influenced by the distance the wave had to travel and the phase of the wave at the radiating antenna.

If you had for example 2 verticals spaced 1/4 wavelength apart, then feeding them with the same phase will create maximum gain broadside to the array. But if the signal to one of these verticals is delayed by 90 degrees, for example by adding a quarter wavelength of feedline to it, you will have created an endfire array with a cardioid pattern.

Answer 2

Well, what you describe is the phase of a complex current. That's not inherently the same thing as the phase of an electromagnetic wave! But it's the basics of all electrical theory, so it's necessary you understand it, so you've got a good basic education there.

So, sorry to drop more theory on you:

In free space propagation, there's no current, and no voltage. There's an instantaneous magnetic field $H(t)$, and an instantaneous electrical field $E(t)$.

In vacuum or air, these two are always in phase; that's a result of Maxwell's formulas, which explain how a changing electrical field can lead to an electromagnetic wave that propagates.

So, it doesn't really matter whether you look at the E-field or the H-field of an electromagnetic wave – the other is in phase.

Thus, we simply give the whole wave a property "phase", and that's a relative quality: Two RF things have a phase $\alpha$ if one thing is, at the same instant, $\alpha$ degrees behind the other.

For example, two dipole antennas that get excited by exactly the same current have 0° phase. If instead the current feeding the antenna has to travel half a wavelength more to the antenna than to the other, that antenna has a phase of 180° (relative to the other – there's not really an "absolute phase").

Now, what does that have to do with beam forming?

Because that is a complex topic, I'll remind you of the double-slit experiment:

If you have two slits on a line, in a defined distance, from which both concentrical waves emit, then there will be points on the plane where the waves interfere. The phase of the waves, and the distance from a point to the slits, defines whether that interference is constructive or destructive.

That works with water waves in a harbour, as it does with electromagnetic waves such as laser light or radio waves.

You can influence where the constructive interference happens by

• varying the distance between the emitters (your antennas!) or by
• giving one a phase relative to the other.