Ground and earth

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Ground and earth

Postby lars » Fri Feb 10, 2017 10:07 am

Hi folks

I'm confused about the role played by the Earth -- that is, the ball of rock under our feet -- in RF transmission.

A dipole antenna does (I'm told) need a "ground" in the sense of an RF signal returns, since it is symmetrical. However, it seems that its closeness to the (ball of rock) earth influences its propagation pattern.

But groundplane-style antennas (e.g., verticals with radials) seem to be variously described as needing ground (to the ball of rock) or not. I've seen people physically bond the radials to the earth using spikes; I've seen people bury the radials in the ground but not actually bond them; and I've seen people supporting the radials on insulators so they aren't in electrical contact with the earth at all.

What's going on here? Why is the same basic design installed in so many different ways? Why would an _electrical_ connection to the (ball of rock) earth have any bearing at all on RF performance?

Best wishes
Lars
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Re: Ground and earth

Postby m0lsx » Fri Feb 10, 2017 11:32 am

This whole earth thing & what is an earth is & what is called an earth is a big can of worms. Mainly because some of what we call an earth is different to some other things that we call an earth & then add in some confusion & then add in a little stupidity & things start to get really interesting.
Counterpoises are not an earth & which antenna benefits from being earthed & which does not is not straight forward. I have a multi band vertical & any form of connection to earth, even for lightning protection sends the VSWR higher than it needs to be. Other antennas will benefit from an earth.
Counterpoises well clear of the earth are often the best available & allow for the best radiation pattern. However if they are already close to the ground, then giving the end a good earth stake is sometimes beneficial.
The earth below our feet varies from area to area, so that changes things too.
The thing to remember is that not everything is what it seems. A lot of small verticals use the coax as part of the antenna system & with those getting a really good earth & counterpoise is beneficial, as the antenna is not a complete antenna, that is balanced.
An antenna should be two halves. One radiating & the other a balanced counterpoise. But with a vertical antenna, you only get one half of that. Unless it's shunt fed, but I'll not confuse things.
Also don't get confused by antennas with radials, I can think of several designs that have so called radials, none of which are a quarter wave in length. A multiband antenna can be made to be more efficient by using multiple cut for band counterpoises at each desired band & the number of radials & their pattern will effect the antennas radiation pattern.
Sorry to not be more specific, but I could be here all day, if I cover even one area of what earths are or are not.
And to add to the confusion, it is possible to make or buy am Artificial Earth. These a great on short end fed antennas where RF can get onto the coax as a result of the compromise. But these artificial earths need a decent length of wire attached & those wires need caution as they carry very high voltages at their ends & can thus set fire to things like carpets.
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Re: Ground and earth

Postby lars » Fri Feb 10, 2017 11:56 am

Yeah -- can of worms is exactly my impression.

Are you able to explain why any kind of antenna would benefit from being connected to ground (the ground under our feet?) What current would flow through such a connection, and where would it go?

Best wishes
Lars
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Re: Ground and earth

Postby m0lsx » Fri Feb 10, 2017 12:24 pm

lars wrote:Yeah -- can of worms is exactly my impression.

Are you able to explain why any kind of antenna would benefit from being connected to ground (the ground under our feet?) What current would flow through such a connection, and where would it go?

Best wishes
Lars


Me in some instances yes, in most others no. You are starting to talk about academic paper level discussion & very specialist fields.
This is all dependant upon very specifics. For example could you tell someone who understands it properly, what the soil measurements for things like moisture content are? Because if not, then they could not answer.
Soil has both a Conductivity measurement & a Dialectic mesurment. But the measurements are not stable across all frequencies & can change due to things like manure which will change the organic content & thus the moisture content within the soil.
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Re: Ground and earth

Postby lars » Fri Feb 10, 2017 1:15 pm

Sure. But I wonder how much of this academic work really applies to practical installations?

When I was a student, back in the (gulp) 70s, I remember being shown a diagram of radio transmission, that showed an electrical current coming out of the transmitting antenna, through the air, into the receiver, and then back through the earth to the transmitting station, completing the circuit. We had to build "crystal" (unpowered) receivers, where one end of the tuned circuit went to the antenna (typically a long wire), and the other end went to a water pipe or some such. It was clearly implied that an electric current would flow from the antenna, through the tuning circuit, and to "ground", and thus back to the transmitter.

Now, I'm 99% certain that electrons don't fly out of the antenna to form a current in the air. Most practical antennas are covered with insulating material anyway, to keep the rain out. I'm 99% certain that the "ground" in my early radio experiments was nothing more than one half of a horribly untuned dipole. Any other large object, reasonably conductive, would do just as well.

My feeling is that when we "ground" some part of an antenna, all we're really doing is adding some volume of partially conductive mass to it; so a "ground plane" is just a mass. But I keep hearing and reading about "ground returns" and all the rest of it, as if there were more to it than this. But is there?

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Re: Ground and earth

Postby G4RMT » Fri Feb 10, 2017 5:41 pm

I always use an audio loudspeaker as an easier to understand comparison. If you look at a loudspeaker, it's job is to turn electrical energy in compression waves that travel through the air to our ears, where these changes in pressure get converted back to electricity that our brain 'decodes' as sound. Dangling a loudspeakers on it's two wires naked works pretty badly. A number of things are happening - the physical back and forth needs to couple to the air. Imagine the speaker is a big one, like on the parcel shelf of teenagers cars. as it kicks forwards, the air resists, and the actual speaker on it's dangly wires actually moves the other way, then the cone reverses, and the speaker gets pulled back the other way - net result very little sound from even quite big movements of the cone with plenty of power. This of course is low frequency operation. If that same speaker is fed with higher frequencies, then the physics of moving the entire heavy loudspeaker works for you (momentum and inertia, in this case), and more of the speaker movement makes it into the air, being carried to the ears. however, as a quality loudspeaker it's still pretty hopeless - it's inefficient because some of the energy going forwards gets counteracted by the rear of the cone - which is working in reverse. So this could be a cheap speaker or an expensive one - it has nothing to work against - so as a speaker is pretty poor. However, if you stick it in a box, the contains the reverse side of the cone, it dramatically gets better. The design of the box could have a total seal, or a speaker port - the difference is in how well that loudspeaker, in it's box handles the low frequencies through to the highs. The internal volume of the box impacts on the frequency response, and the efficiency. Suddenly that useless loudspeaker driver works much, much better.

This broadly aligns with how aerials work, but we do have to make sure the basics are understood. Dipole, for instance. The basic definition is something that has the capability to hold two opposing electric charges. So the usual device we thing of has the end of the coax feeder taken in two separate directions, extending from that point. That is a dipole. A half-wave dipole is just a description of a popular type that has a length determined by the frequency we wish to use. The half-wave measured from tip to tip - the longest it can be. A quarter wave dipole is the bit from the middle to one of the tips. If instead of another piece of wire, we plant the quarter wave element rising upwards from the roof of say a car, the car roof becomes the magically spoken about ground plane. A surface the quarter wave element can use to work against, instead of the missing other quarter wave element. In a way it becomes that element - but with it's surface area stretching to infinity. In the real world we can't get that big, but when it's biggish - it works much like that missing other element. It's related to frequency. So a UHF quarter wave rising from a car roof is pretty efficient - length against surface area. Move to CB frequencies, and that surface area (the ground plane) is really a bit small - hence why CB aerials are less efficient on a car. Stick one on a boat on the sea, and that surface area is much, much larger, and the aerial works an awful lot better.

When you use a simple half wave dipole - it's balanced. Each side of the thing is the same. The quarter wave on the car roof is unbalanced the two parts of the aerial being very different. For what its worth, ordinary coax cable is not balanced. There's a thin conductor in the middle, surrounded by a 'tube' of another conductor - not at all balanced, and it makes a good match with an unbalanced aerial system. If you want to connect a balanced aerial to an unbalanced feeder - you need the balun - simply a device to convert balanced to unbalance and vice-versa. A good one is almost transparent to the RF, while a bad one loses some of what you send through it.

We can use loudspeakers again to show what happens with aerials with gain. The typical small hi-fi loudspeaker gives listeners in the room pretty good sound no matter where they sit. They're a bit louder in front than behind, but the change is gentle. Compare these to those old fashioned how type speakers you still see at outside events. The speaker sits at the entrance to a trumpet like horn. This focusses the sound so that it gets directed in one direction. Imagine a loudspeaker as something rather like a globe, floating in space. The entire surface of the globe producing sound. Now imagine you have wrapped amazing soundproof material all around it except for the UK. None of that sound can escape anywhere apart from our little island. ALL the sound has to come out this way, in a beam. This is what those horn speakers are doing. Their design is also limited in frequency - hence why these horn speakers are great for speech and rubbish for music. One of these things might only handle a small amount of power from the PA system, but throws it a very long way.

PA systems for outside festivals have worked in this way for a long time now to appease the noise police. You want it loud where the people are, and quiet everywhere else. They are beams - working a bit like torches. Walk outside their design angles and the sound drops very quickly.

Sound and RF energy can reflect of things. They can also scatter it to places you don't want.

Most of this analogy works pretty well. There are differences of course that it's a bit too simple for, but in general terms, the similarities are quite surprising.

It also explains how many other radio systems work. The old pirate stations in the north sea - a huge tower that is actually the aerial - perched on top of a mass of salty water stretching for miles. It is the same as the quarter wave on the car roof. Salty water works even better than fresh. So remember that lattice towers may not be simply supporting aerials but could be the aerial itself.

All the talk of earths and grounds confuses people. Electrically, we kind of thing of earthing as safety based and something hurried in the ground - which of course it is. Radio wise - earthing can work for you, or against you depending on what you want to do. It's not good or bad, its more to do with what your aerial system needs to be efficient. If you had a huge tree in your garden, you could arrange for your feeder cable to emerge in the middle of the garden, and then be pulled up vertically towards the supporting tree branches. At the bottom, you bang in a nice metal rod, and connect to the cable screen. Your garden - wet soil mainly - becomes the 'sea' in my pirate example above. Not as good as salty water of course. but in the winter - pretty close. In the summer, with hardly any moisture in the soil, then not so good. Depending on your tree height you could have a nice quarter wave vertical for loads of bands, and quarter waves are pretty efficient aerials. You could even have 3/4 or 5/8 vertical elements and get a bit of extra performance from the gain these provide.

On the subject of gain - and again with some simplification we're doing two things. We have a longer active part of an aerial, on the principle that the more RF we get into the air, the better it is, but the actual aerial (remember the idea of it have electric charge) can work differently. If a bit of it is pushing while the other part is pulling (like the speaker) some cancellation can take place. It can also work for you and the two properties add up, so pushing, then a bit more pushing. The aerial design can also change how the RF leaves the aerial, direction wise. Remember the globe example again). Ig half of your signal goes upwards, and nobody is in that direction apart from the space station, if you can take this wasted energy and stuff it in the wanted direction you have created gain. It's not magic, this gain idea. You cannot increase the energy 'in' the aerial - all you can do is arrange for it to escape in useful direction and make better use of it.

There you go - my as non-technical as I can do attempt at explaining how these things we take for granted work.

Just keep in mind the basic principles - when you get to study it, you'll find many examples of much stranger things, some contradicting the above, but the basic ideas and physics are sound.

Maybe this will help get the basics sorted for beginners, as it does explain what appears to happen.
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Re: Ground and earth

Postby lars » Sat Feb 11, 2017 2:34 pm

Thank you for this very thorough response. I confess that I have not yet understood it all, but I will persevere.
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Re: Ground and earth

Postby G4RMT » Sat Feb 11, 2017 3:31 pm

Feel free to ask the questions. Like most things really, it's little chunks that kind of link up and make sense.

I think one of the really critical things about aerials is that what applies going out, applies to receive as well. One of my early discoveries was that with a tuner, you really can make almost anything have a VSWR of virtually 1:1 which means it's a perfect match to a transmitter or receiver. Electrically, you did it - a brilliant aerial. Performance wise, this perfect aerial probably works as well as the proverbial but of damp string! Almost anything can grab a bit of the RF energy in the ether. Exactly how much is what all the fiddling is about.

I've been making some aerials half wave dipoles in a tube, and fed at the end. A bit of internet research, plus a few ideas of my own, and the strange thing is that when I stick them on my aerial analyser, I see the VSWR curve, and it tells me it is resonant at say 120MHz - excellent for the air band. Another, that shows it is resonant at 145MHz, for the ham band actually works better on air band, if maximum signal strength is the decider. I haven't yet worked out exactly why. At air band, the VSWR is 3:1, so not good for transmitting - but it seems to be better for listening to my local airport at thirty miles.

In your first question you mentioned the radials. The idea of radials is to simulate a continuous area that in most cases is swappable with a continuous surface.

Remember the people with fibreglass sports cars who always have grief with aerials? if the aerial needs a ground plane as part of it's design, it needs something that acts like the real thing. Let's imagine water, earth and sand. With water, especially salty water, you could probably not even bother with the metal radials. If your earth is boggy peat marsh - full of water, and squidgy, then while in the winter, you could dump the radials, in the summer, it could be hard and not remotely a good conductor. You could bond the radials at the centre, maybe in a ring with the vertical rising from the centre, then if the ground was electrically rubbish, the radials would do the work. On dry sand, it makes a very poor ground plane - so the radials would do pretty much all the work. Even banging a copper stake into the earth doesn't guarantee good electrical performance.

Electricity supply to homes and businesses all nowadays have a ground connection, mainly for safety reasons. Many of these are described as Protective Multiple Earth systems - where the Neutral cable is bonded to the earth near your consumer unit, near the meter. Even with lots of earth connections going down the street, it's not uncommon for the actual earth potential to vary by a few volts up and down the street, as the ground type changes. In bigger buildings, putting a volt meter between the earths in two distant 13A sockets shows maybe 4 or 5 volts between them? They should be the same, but grounding/earthing is not an exact science. With radio systems - the earth is often a really important part of the aerial. Well worth spending time making it work.
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Re: Ground and earth

Postby m0lsx » Sat Feb 11, 2017 5:10 pm

G4RMT wrote: One of my early discoveries was that with a tuner, you really can make almost anything have a VSWR of virtually 1:1 which means it's a perfect match to a transmitter or receiver. Electrically, you did it - a brilliant aerial. Performance wise, this perfect aerial probably works as well as the proverbial but of damp string! Almost anything can grab a bit of the RF energy in the ether. Exactly how much is what all the fiddling is about.


A manual tuner will give more than one SWR point. One being better than the other.
At an SES several of us were setting up & my SEM would not tune the antenna very well on 80 meters. With the SWR being just under 2:1 which was high for that. When we checked the antenna had not been conected, I was tuning a length of Coax& got !:1 on 20 & 40 with a few meters of Coax :huh: :biggrin: :biggrin: But then again so does a light bulb & that does not radiate very well either.
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Re: Ground and earth

Postby lars » Sat Feb 11, 2017 8:34 pm

I can understand why an antenna that is designed to have a groundplane needs a groundplane. What I'm struggling with is whether it makes any difference whether the groundplane corresponds to the ball of rock we stand on, or whether the groundplane needs simply to be some substantial conductor. Of course the ball of rock is, to so extent, a conductor. So I can see why it is convenient to use it. But does the fact that the (ball of rock) ground is a reference potential for electrical equipment, as a well as a safety conduit, have any bearing on its RF properties? Suppose I dug up a patch of earth the size of a football pitch and (with the help of aliens, or something) hoisted it a mile into the air. Would that still amount to a groundplane? Or is there some necessary property of the "true" ground that I'm overlooking?

I've seen vertical antennas advertised with paltry verticals -- perhaps 20cm long. In what way can these puny conductors substitute for the huge volume of the earth? Or do they not have to? Or is it that the significance of a groundplane becomes rapidly less, the further from the antenna vertical it is?
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