visualizing e and h fields - Page 1
Forum section: electronics / rf-microwave

author: Beamin
author_rank: Super Contributor
timestamp: July 31, 2019, 09:37:53 pm
content: https://youtu.be/-F7KYLO4BkgMakes creating plots easy to understand

author: hagster
author_rank: Frequent Contributor
timestamp: August 01, 2019, 06:05:27 am
content: Its a good video. Probably the best and most accurate description on YouTube.This more recent Learn Engineering video is pretty good too, but there is a bit of artistic licence in it.https://youtu.be/ZaXm6wau-jc

author: radiolistener
author_rank: Super Contributor
timestamp: August 01, 2019, 09:21:41 pm
content: nice videos, but they are explaining how works basic oscillator. But actual antenna works a bit complicated, because there are a lot of electrons and they all produce EM waves and they all affected by EM waves. Some EM energy is radiated by one electron and consumed by other. This is why half wavelength dipole antenna has Q factor about 12, it shows how many cycles energy is circulating within antenna before loss for radiation and heat.In short, antenna radiates E and H fields with different combinations, but only these E and H which are perpendicular to each other will fly away. Such combinations happens at some specific directions which forms the radiation pattern. All other E and H fields will return back into antenna, they will create electrons movement which will leads to produce a new E and H fields and this will continue in a loop. So, antenna actually works like oscillating LC circuit which has energy loss for EM radiation.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 02, 2019, 12:33:28 pm
content: I don't get some parts:- 1. What does it means in the second video by "however, this fluctuation magnetic field is of no use in transmitting signals" (at second 45,https://youtu.be/ZaXm6wau-jc?t=35). In my experience a closed loop antenna can send/receive, too, so why would that be of "no use in transmitting signals"?- 2. In the first video, when E and H field lines are drawn on the straight wire, their maximum seem to be shifted by 90 degree, when one is min/max the other is 0, but when the H and E are represented as vectors in the propagation diagram their maximum is in phasehttps://youtu.be/-F7KYLO4Bkg?t=647- 3. Also in the first video,https://youtu.be/-F7KYLO4Bkg?t=461, why E changes its polarity when a reflector is hit, but H doesn't?

author: hagster
author_rank: Frequent Contributor
timestamp: August 02, 2019, 02:24:51 pm
content: Quote from: RoGeorge on August 02, 2019, 12:33:28 pm1. He means a fluctuation magnetic field invite own is not sufficient for radiation. It's important that the sum of the fields do not cancel each other out.3. Not sure entirely. The E field is zero at the surface. My best explanation based on my understanding is that assuming an ideal conductor, there is no energy lost from the field, so we imagine the wave being cancelled out by a new reversed polarity wave. There is no such mechanism to 'short' the h field.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 02, 2019, 03:37:32 pm
content: Also about no 3, I know the sign of the reflected wave can be either the same phase or anti-phase, depending of the ratio between the impedance of the two media. As an example, if I send a rising edge on a transmission line ended in a short circuit (|Z2| < |Z1|) then the reflected wave sign is opposing the incoming wave, while if I left the transmission line open (|Z2| > |Z1|) then the reflected voltage will add up with the incoming one, so why in the animated example the E field is reversed still puzzles me. Maybe they are exemplifying the most encountered type of reflectors in real life for the radio waves (where |Z2| of the reflector < |Z1| of the air), i don't know.I still don't have a good grasp of the electromagnetic field, AKA intuitive understanding of Maxwell's equations.As an example, even if I already knew about the memristor for dacades now, I only very recently understood that the memristor is not just a hyped fancy concept, but a 4'th fundamental type of a passive component (and there are only 4 possible in total, according to Maxwell's equations), together with the other 3 types we already know and use: R, L and C.https://regmedia.co.uk/2011/12/22/hp_memristor_history.pdfAt this point I don't know if what I am missing is a better understanding of Calculus or Physics, or maybe both, but I'd really like to have a better grasp for the electromagnetic fields. Any hints of how to achieve that, please?

author: radiolistener
author_rank: Super Contributor
timestamp: August 02, 2019, 09:22:43 pm
content: Quote from: hagster on August 02, 2019, 02:24:51 pmvoltage and current are shifted by 90 degree. But E and H fields of EM wave are in phase. There are some parts of E and H fields around antenna which are in phase due to antenna geometry and these parts are flying away, they are radiated as EM wave. And some parts of E and H fields are not in phase, they are return back into antenna and make oscillations at antenna resonant frequency.Antenna with higher Q produces more oscillations, this is why it has narrow bandwidth.