User talk:144.130.106.118

Re: Dipole antenna

Don't mind Interferometrist. He's actually an okay guy, but he's some kind of an expert and understands this stuff at a higher level than I do, and doesn't want to bother with the elementary stuff. However, I think he has a point. The "hand waving" approach you and I have been taking on the Talk page, is not enough to base an article on. I'm not confident it can explain why the voltage waves on the antenna are in phase with the feed voltage, which I think is what you want to explain. The verbal description should be backed up by math, the equations of the voltage $V(x,t)$ and current $I(x,t)$ along the antenna elements as a function of time. That means solving the wave equations for the current and voltage on the antenna when driven by a sinusoidal source at its center. It would be nice to include in the article, and I've been looking around for some textbook that goes through that, but I haven't found one yet. The Telegrapher's equations give the voltage and current on a transmission line (two wires close together, or one wire close to a ground), but I'm not sure they apply to dipole elements (single wires in space) radiating radio waves. The fields react back on and change the currents, so you may have to solve for the fields as well. That may be what Interferometrist was saying.

Thanks again for replying, i have a lot to say about all this, ill respond to all the points in your reply over the next week or so if you are happy to read them, i want to make sure i get all the facts straight before i reply to you. Interferometrist i think is just doing his/her best. But i think its easy to get things out of perspective and end up with a wikipedia article that doesn't have a good structure and focuses too much on all the gory details and doesn't say how these fit into the overall picture.W0kacheeta (talk) 23:21, 18 September 2020 (UTC)

All Wikipedia content is supposed to be backed up by sources. Personally I don't want to write that the "feed voltage is in phase with the antenna voltage", until I have sources. It must be out there somewhere. Agreed, for the purposes of this discussion one of my references are 3 books by Alexander Schure, 1.Antennas 2.Resonant Circuits 3. RF Transmission Lines. If you want links just say.W0kacheeta (talk) 23:21, 18 September 2020 (UTC)

There are a number of antenna books aimed at radio amateurs with elementary explanations of dipole antennas. The ARRL Antenna Book, published for 80 years by the American Radio Relay League and now on its 24th edition, is a classic. I found a pdf of the 2000 edition online and downloaded it, I could send you a copy. It's got loads of practical info, but not much theory and no math. Other sources are https://archive.org/ and online military electronics manuals. --Chetvorno^TALK 01:19, 15 September 2020 (UTC)

I am not a fan of the ARRL, in my opinion the ARRL antenna book is pretty useless, much of the content is overly complicated and lacks any kind of perspective, and many of the explanations given are badly written, convoluted and inside out, and even incorrect. The result is that readers get confused and at first i just thought i was too stupid to understand until i worked out that it was the authors that are actually pretty stupid. Apologies for that rant.W0kacheeta (talk) 23:21, 18 September 2020 (UTC)

Hi Chetvorno, i have been doing a lot of reading on this subject. I want to find the answer to one question before i send you anything further.

For an infinitely thin ideal or theoretical half wave dipole antenna with no loss in free space :

How can the phase relationship in time between the amplitude of the voltage and current of the applied RF energy at the feed point be zero when there is a phase difference in time of 90 degrees between the amplitudes of the voltage and current of the standing waves at any point on the antenna ?

Or rephrased another way :

At resonance the voltage and resultant current at the feed point are in phase in time because there is no reactance. How can at the same time the standing waves be 90 deg out of phase in time ? In fact on the antenna elements the current leads the voltage by 90 deg making the half wave dipole appear as a big capacitor. You can't have in phase voltage and current due to resonance at the feed point and then at the same time out of phase standing waves.

Anyway, if you're interested, here's some notes I made on your last posts on the dipole talk page:

• You say "...because the elements are 1/4 wave long the traveling waves which are reflected from the ends of the antenna arrive back at the feed point in phase with the next cycles of applied RF energy." The round trip distance of a voltage or current wave traveling from the center to an end, reflecting without inversion, and returning to the center is $\lambda /2$ so the reflected traveling wave would be 180° out of phase with the incident wave, and also traveling in the opposite direction. It is only after the wave makes a full round trip, traveling from the right end of the antenna to the left, reflecting from the left end, then returning to the right end and reflecting from the right end, that the reflected wave is traveling in the same direction and reinforces the original wave. Since the length of the antenna is $\lambda /2$ the round trip distance is $\lambda$ so the reflected wave has a phase change of 360° and is in phase with the original wave. This occurs when the driving current has a frequency of $f_{0}=c/\lambda$. However the antenna is also resonant when the driving current has a frequency which is a multiple (harmonic) of this frequency: $f=Nf_{0}=Nc/\lambda$. In this case the current has a phase change of N360° and makes multiple cycles during a round trip. The radiation pattern is different at each resonant frequency. At frequencies different from these, after a round trip the reflected wave is not in phase with the original wave and does not reinforce it, so the currents and voltages in the antenna are much smaller, the antenna is said to be "nonresonant" at these frequencies.

This animation shows how a standing wave (black) is the sum of two opposite sinusoidal traveling waves (red and blue). If you take a $\lambda /2$ segment of this graph between two opposite standing wave maxima, it would show the voltage waves on the antenna. The feedpoint voltage at the center of the dipole is in phase with the red and blue traveling waves; its peak occurs at the time when the peaks of the two traveling waves simultaneously cross the centerpoint (red dot). A segment of the graph between two red dots would show the current waves on the antenna.

The actual situation in the antenna is a little more complicated:

• There are separate voltage and current standing waves on the antenna, each consisting of a superposition of a left-traveling and right-traveling wave.
• At the ends of the antenna: the voltage wave reflects without a phase change, so the incoming and outgoing voltages add, creating a voltage standing wave maxima twice the amplitude of the traveling waves. The current wave has a 180° phase change on reflection from the ends (because the incoming and outgoing current must sum to zero), so the currents cancel, creating a node at the ends. Since the zero of the current standing wave coincides with the maximum of the voltage standing wave, the two standing waves have a 90° phase difference.
• At the center feedpoint: the feedpoint is $\lambda /4$ from the ends, so a traveling wave that travels to the end, reflects and returns to the center travels a round trip distance of $\lambda /2$ and so has a 180° phase difference. The voltage wave reflects without a phase change, so the phase difference between the two traveling voltage waves is 180° and they cancel, resulting in a node in the voltage standing wave there. The current wave has an additional 180° phase change when it reflects, so at the center the phase difference between the two traveling current waves is 180° + 180° = 360° so the waves are in phase and add, creating a current standing wave maxima twice the amplitude of the traveling waves. Again the voltage and current standing waves have a 90° phase difference.
• Since the feed terminals are in series with the two dipole elements, the feed voltage wave is superimposed on (added to) the antenna voltage wave. The current through the transmitter is just the same as the antenna current.
• As you said, the feed voltage is in phase with the traveling voltage waves, not the standing wave, at the feedpoint. So it adds to the voltage, replacing energy that is radiated by the antenna. However, the feed voltage is only a small fraction of the amplitude of the traveling voltage waves. You can see that in the 2nd antenna animation, because the traveling waves are half the voltage of the standing waves at the ends, much larger than the small feed voltage step at center. Thus the feed adds only a small increment to the energy each cycle, most of the energy in the antenna is stored from previous cycles.
• This phase relationship is only true for a dipole connected to a transmitter, as in these animations. Note from the moving arrows that during each half cycle, the current enters the feedline on the low voltage side of the voltage step, and leaves the feedline on the high side of the voltage step. This means that the charges are gaining energy flowing through the transmitter, which is radiated by the antenna. If the antenna is connected to a receiver, the phase of the feed voltage step at center will be reversed, it will be opposite to the traveling voltage wave. The current will enter the feedline on the high voltage side and leave at the low voltage side, so the charges are losing energy in the receiver, the receiver absorbs energy from the antenna.
• The electric and magnetic fields in the near field region within about $\lambda$ of the antenna are 90° out of phase, like the current and voltage, because they are temporarily storing energy that is returned to the antenna each cycle. In the radiation fields of the radio waves, that dominate in the far field region beyond a wavelength, the electric and magnetic fields are in phase, because they carry energy away from the antenna permanently.

--Chetvorno^TALK 01:19, 15 September 2020 (UTC)

I just realized that the previous editor that I threw under the bus in this remark by suggesting he is just interested in showing off his math skills was Interferometrist; he wrote a lot of the existing article. Thankfully he doesn't seem to be too offended by my snarky comment. --Chetvorno^TALK 21:23, 15 September 2020 (UTC)

Chetvorno, hi again, i'm not sure if you are still reading this, but if you are, i wanted to let you know that i think i now finally understand most of the aspects of half wave dipole operation, and your new animation and the associated text has helped to find the last missing piece. You are the only person that has come close to helping with this so thank you for your interest and patience. Please feel free to comment if anything i write below is incorrect.

The main points which are never explained well anywhere (maybe they are and i am too stupid to understand) are :

Assuming an ideal center series fed half wave dipole in free space when used for transmitting. And keeping in mind that voltage causes current flow and not the other way around ie: impedance = cause (voltage) / effect (current).

• A half wave dipole by definition is always exactly resonant. A half wave dipole always has zero reactance.

• The length of one half a wave of RF energy in free space isn't the same as the length in wire or aluminum, and so a half wave dipole does not have an impedance of 73 +j73 ohms as erroneously stated in many descriptions of half wave dipole operation.

• The standing wave represents the actual conditions on the antenna which can be measured. You can't see the original incident and reflected waves as these are obscured by the addition of each to the other.

• The voltage of the standing wave lags its current by 90 deg.

• The standing wave is a common mode current where the same current flows through both halves of the antenna in the same direction. The standing wave only appears on the antenna halves and doesn't travel back down the transmission line because a transmission lines cancels out common mode current.

• Resonance is determined only by the fact that the antenna halves have an electrical length of 90 deg.

• The antenna has zero reactance at resonance only because of the fact that at resonance the current of the standing wave is in phase with the applied RF voltage.

• A half wave dipole is not the same thing as a series resonant circuit with lumped constants, it just behaves in a similar way at resonance.

• The capacitance and inductance of the antenna as per the well know formula for resonant frequency of a series LC circuit with lumped constants fo = 1 / 2π √(LC) doesn't determine the resonant frequency of a half wave dipole. At resonance the reactance is zero because the length of the antenna elements are such that the current of the standing wave is in phase with the applied voltage. Resonance doesn't occur "because the capacitive and inductive reactances cancel out" as erroneously stated by many explanations of dipole operation because at resonance there is no reactance present because the current on the antenna is in phase with the applied voltage.

Andrew 23/11/20 9:44 pm.

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