"Earth Image" and function of ground, where to connect... - Page 1
Forum section: electronics / rf-microwave

author: 741
author_rank: Frequent Contributor
timestamp: October 08, 2020, 01:25:06 pm
content: This is the best link I have found to answer the question of why a ground connection can improve reception in a crystal set.http://www.verulam-arc.org.uk/earth-image/I still don't understand too well:QuoteWell yes I can see the distribution would be same - but opposite 'polarity'. I do not see how this concept makes a practical difference. By analogy "this push gives the same effect of that pull" - knowing that does not make a wheelbarrow being pushed by me magically gain a virtual assistant pulling to help me...If I allow that though, then are they saying that charge movement on the 'soil surface' gives the same 'aerial/antenna' effect as a vertical rod?Also I do not see where I would tap into a ground-mounted receiving aerial.The above no doubt shows my high degree of confusion!

author: TimFox
author_rank: Super Contributor
timestamp: October 08, 2020, 10:55:58 pm
content: "Image charge" and "image current" are physics textbook "fictions" that simplify calculations of real systems.The "image charge" is a fictional physical equivalent, since it gives a plane between the real charge and the "image" that is an equipotential surface. So long as you are sensing or located between the physical charge and that plane, there is no difference between having a conductive plane at that location, and having nothing between the real charge and the image charge. The mathematics for the fictional image charge is often easier than with the physical equipotential plane.

author: 741
author_rank: Frequent Contributor
timestamp: October 10, 2020, 10:40:45 am
content: Ah, right, that starts to make some sense now, thanks.

author: 741
author_rank: Frequent Contributor
timestamp: October 14, 2020, 07:57:12 pm
content: I recall (a long time ago) when I made a crystal set, there was some debate about using mains earth. The main issue being it could be unsafe of course.Technically though I do not see how this would work anyway.Consider for example a (would-be) "grounded monopole" placed on top of a table upstairs. Real earth is a fair distance away, maybe 20ft down, and the connection to real earth would be via the mains earth wire to a remote earth rod.

author: TimFox
author_rank: Super Contributor
timestamp: October 14, 2020, 08:57:06 pm
content: If by "mains earth", you mean the PE wire in a power cord to an appliance, that would not be safe because there could be a fault in the ground connection at the wall outlet, or a misguided use of a 3 to 2 prong adapter.. For example, if you have a computer with a line filter, and you lift the green/yellow PE connection feeding the outlet, then the case of the computer and thereby the PE connection will go to half the line voltage (1/2 of 220 V in your part of the world) in series with a few nF of capacitance in the filter, which should not be fatal, but will be painful if connected to earth through your body. If you are battery operated (or passive, like the crystal set), you should consider using a counterpoise for ground (for which I refer you to the antenna literature).

author: 741
author_rank: Frequent Contributor
timestamp: October 15, 2020, 08:32:23 am
content: Theoreticallythough, would a functioning PEN earth (ie. connected to substation earth and perhaps other multiple earths along the way) work/offer any benefit in the scenario described?Maybe it would allow a remote (but dangerousdo not try at home...) counterpoise ground (on a table top) - not needing any capacitive coupling to actual earth/soil?More safely, one might have a dedicated RF groundWith counterpoise ground as described onhttps://en.wikipedia.org/wiki/Counterpoise_(ground_system)]Wikipediahttps://en.wikipedia.org/wiki/Counterpoise_(ground_system), it is interesting that the connection to earth is capacitively via the counterpoise plate.Does the horizontal electron distribution now occur in the counterpoise plate - and if so, what radius should the plate be? I'd guess ideally huge but practically (if aerial/earth couipling is capacitive ) maybe half antenna height. Is there any benefit in having a radius > antenna height for example? Is a radius of say height/4 "not much worse" than a radius twice that?Ultimately it seems the grounded monopole requires a path for intercepted RF energy to return to, in contrast to say a loop antenna. That is, the counterpoise plate still requires the earth's surface to capacitively to. Thus there is no point making a satellite aerial in this style.

author: tkamiya
author_rank: Super Contributor
timestamp: October 22, 2020, 12:28:25 am
content: Using ground side of mains socket for ground indeed does work. I've done it with crystal radio set I had way back when. Risk is there, of course, but function wise, it worked reasonably well for BC band reception. I recall I had 1000pF capacitor in series right at the connection.Danger is, occasionally live and neutral is swapped somewhere between a socket and the breaker panel.Better way is to use any large metal object. It doesn't have to be connected to ground but does have to be quite large.

author: 741
author_rank: Frequent Contributor
timestamp: October 22, 2020, 08:03:18 am
content: That is very interesting.Does the "large metal object" need to couple capacitively to "Planet Earth ground" or not - does that matter at all in principle? Suppose the receiver aerial was on board an airship, and a large metal plate was available. Then, would the charge distribution in that plate mimic that on the Earth's surface near an aerial mounted on the 'real' ground?Say there are two large area, co-planar metal sheets out in free space. On one is a transmitter, on the other is a receiver. Both use grounded monopoles where 'Tx ground' is entirely unrelated to 'Rx ground'. Can Tx and Rx use the earth image effect?Basically, do we need this common DC conductor called "Planet Earth" when using a grounded monopole? I'd have thought not, as this is finally RF transmission. Thus there is no need that the 'large metal object' couples capacitively to 'conductor Planet Earth', thence ultimately back to the transmitter.However, descriptions of the counterpoise setup do indeed mention capacitative coupling to earth, so why is that important?

author: 741
author_rank: Frequent Contributor
timestamp: October 22, 2020, 08:53:56 am
content: Further to my questions above I found this linkhttps://ham.stackexchange.com/questions/7089/is-a-ground-plane-necessary-for-transmitting-and-receiving-with-a-vertical-monoAs I read it, this suggests that no, actual planet earth is not essential. What matters is to form the equivalent of the "lower half of a dipole". Then, the "conductive currents" are forced to stay in the antenna, which means the "radiated energy" is higher.This then leads me to wonder why/how connecting mains earth to a crystal set earth helps: The geometry would seem to be wrong. It's basically a long wire to the substation earth, then conductive via earth's surface.It seems to be like starting with a standard grounded monopole, then inserting a long flying lead between ground and the aerial's base, which can now be (say) your kitchen table.So now there is coupling to a complicated ground, part of which is your house wiring, part of which is earth's surface.

author: tkamiya
author_rank: Super Contributor
timestamp: October 22, 2020, 10:49:41 am
content: An honest answer to your very reasonable questions is, I don't know. I am not a physicist. I am just speaking from experience.But let's theorize... and talk about practicality.Let's say I'm in Japan and you are in US. Let's say our antennas are using ground as "virtual the other side". Because of huge distance and the fact earth is not a perfect conductor, we can't reasonably assume they are connected in the same sense they are copper wire. Yet, it works. Plus, a radio wave is a EMF emission. Electrical and magnetic field in 90 degrees difference. Ground takes no part in this type of propagation. Using the common "virtual the other side" theory.... they are more like a vertical dipole with no ground. I've done this many times. Using railing of a high rise building, many times using cars, outside shed, sheet pan and so on. They all can be tuned to have no reflection. Worked well, too. Aircraft antennas on small planesare aircrafts are vertical antennas often with metal sheet on inside of fiberglass body.I don't think of antennas as capacitively coupled. If that's the case, magnetic field will not be involved. But low band antennas are often wires on ferrite rod. I think they magnetically couple. So I don't know.The reason why connecting to neutral side of outlet works is that long wire in the circuit works as ground plane. A reference for how vertical antenna works against and kind of forms "the other side". Also, wavelength of AM broadcast band are long enough that length of this wire is less critical. I assume you are aware neutral side is connected to earth ground at your breaker panel and at the pole? Not an ideal situation but practical albeit risky.A very ODD thing I used to do. Remember rotary telephone? Remember the finger stop? THAT works as an excellent antenna. This never made any sense to me as these are not connected to anywhere. But worked. Explain that!I think of antennas as piece of wire with something to push against. I don't necessary subscirbe to virtual antennas in the ground theory. It explains many antennas but not all. It may be easier for you if you think about how dipole works and ground plane works. Even in absense of ground side, that side is coupled to ground somehow - very lightly and inefficiently.

author: TimFox
author_rank: Super Contributor
timestamp: October 22, 2020, 02:03:36 pm
content: In electrodynamics, thebasicorelementarydipole antenna is two charges, of opposite polarity, oscillating back and forth along the line separating them to form an oscillating dipole moment. Everything else, including lengthening the distance to make a resonant dipole, is derived from that model. "Ground" may be part of the structure of the derived apparatus, but it is not necessary to launch an electromagnetic wave into the luminiferous aether (sarcasm). At a great distance, you receive the oscillating E and B fields from the wave propagating from the transmitter.