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Grounding Besides for safety?

  1. Jun 21, 2011 #1
    In building a crystal radio one thing that comes up everywhere I look is the phrase.

    "A good ground is as important as a good antenna".

    So you have your circuit. Capacitor, Inductor, diode, resistor and headphones in series. Have a long wire just hanging from one place as a long wire antenna and another wire to the ground.

    As the currents and voltages in the circuit is going to be super tiny. Grounding isnt for safety...and I cant seem to find any explanation or maths or anything explaining what it is for.

    Any ideas?

    And is there an alternative? Such as a vitual ground? In case there is no good ground connection..say your in a building and the pipes are not metal.
     
  2. jcsd
  3. Jun 21, 2011 #2
    The ground they are referring to is a signal ground, not an electrical supply safety ground.
    Indeed the building electrical ground would be useless for this purpose.

    The explanation of signal ground works like this.
    In your electric circuit there are currents flowing and voltages changing. Both of these cause local electromagnetic disturbances around the wiring / tracks and around the other components by capacitive or inductive action.
    The purpose of the ground is to sink these disturbances sothey do not interfere with the rest of the circuit.

    In RF work it is often a good idea to provide a ground plane beneath the circuit board for this reason. You also provide a gound plane for proper functioning of the antenna.
    the dimensions of the ground plane should be similar to that of the antenna or circuit board as these are local (short range) effects.

    hope this helps
     
  4. Jun 21, 2011 #3
    Hmmm, not really.

    As people are just hooking the ground wire onto metal pipes or sticking a rod into the ground. Someone even just wired it into the ground wire of a plug.

    ...so that would mean the building electrical ground isnt useuless?...what I mean is, it seems like it is talking about actual ground ground and not signal ground?
     
  5. Jun 21, 2011 #4

    berkeman

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    I googled the phrase you quoted, and only got two hits -- this thread and a thread at the antique radio forum. Could you give us a couple links where you are seeing this topic come up? There are many, many aspects to grounding, and it would help us to answer your questions if we knew what you were reading.

    You certainly don't need an Earth Ground connection to make a radio work. Lots of portable radios just run off of batteries and have no Earth or AC Mains connections. If you want ligthning protection, then you will obviously need some way to connect to Earth Ground, hopefully through a low-impedance, high-current-capacity circuit. (Not through the building ground at a power receptacle, for example).

    And as Studiot alludes to, in RF circuits, the circuit grounding floorplan are very important to ensure good performance.

    Anyway, please post a couple of links, and we should be able to help you out.
     
  6. Jun 21, 2011 #5
    Last edited by a moderator: May 5, 2017
  7. Jun 21, 2011 #6

    berkeman

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    Ah, got it. It looks like all of those are for crystal radios, correct?

    In that case, the ground connection is needed in order to form an efficient antenna. Because you are tuning in AM stations around 1MHz, the wavelength is about 300m, so the resonant half-wave antenna would be about 150m long. To make a good efficient antenna, you need to have two [STRIKE]half-wave [/STRIKE] quarter-wave elements in a line to form a "dipole", or a single element above a "ground plane". What you are doing with the long-wire crystal radio antenna is trying to gather as much RF energy as possible, so that you can hear the AM broadcase with just a rectifier and passive resoant components. Without the Earth Ground connection, your antenna would not be very efficient, and you would have a harder time tuning out adjacent stations.

    Portable AM radios use more RF gain (with transistors) to amplify the smaller signal that you get with the ferrite rod antennas that they use. A ferrite rod antenna does not gather enough energy for you to use it with a crystal radio set with no gain.

    Hope that helps.


    EDIT -- fixed half-wave elements --> quarter wave elements. Duh, I'm brain dead at the moment :redface:
     
    Last edited by a moderator: May 5, 2017
  8. Jun 21, 2011 #7

    vk6kro

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    A typical antenna used for a crystal set is a random length piece of wire placed as high as possible. This will normally be a lot less than a quarter wavelength at the frequency of AM broadcast stations. A quarter wavelength at 1 MHz is 75 meters or 246 ft.

    Although this will pick up radio signals and convert them to voltages, the voltages are relative to real ground.

    So, you need to have the ground side of the crystal set actually grounded.

    When you do this, there will be currents flowing in the coil between the antenna connection and the ground. If this is using only part of the coil, there can be considerable step-up of voltage and hence even louder signals.

    If the mains ground is a metal water pipe in the ground, then this makes an excellent ground for a crystal set.

    A virtual ground can be made with a device called a "counterpoise". This is a long length of wire placed on insulators slightly above the ground. For crystal sets, however, this is more trouble than using the mains ground.

    For safety reasons, you should not attempt to connect to the mains ground at an electrical socket. You can connect to ground at the case of an appliance that is already grounded, but switched off.
    The metal case of a computer might be a possibility.
     
  9. Jun 21, 2011 #8

    turbo

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    Good grounding gives your receiver the best chance for discriminating signal from noise. A real no-brainer, IMO.
     
  10. Jun 21, 2011 #9
    Yep they are all for crystal radios, but I did say that in my first post :P haha
    But thanks for your help.

    So we are also using the ground to get the radio waves as it extends the length of the wire? (as I saw that the waves travel through the ground aswell).

    If so, why use a wire at all in comparison we have the whole ground and a tiny wire.

    ...But im guessing thats not the case.

    Could you expand a bit more on whats happening with the wire and ground forming the "two half-wave elements".

    And why without ground it wouldnt be very efficient.

    Also what is the ground plane element?

    vk6kro: could you elaberate on " If this is using only part of the coil, there can be considerable step-up of voltage and hence even louder signals." please.

    How about using a virtual ground? Using 2 batteries say
     
  11. Jun 21, 2011 #10

    vk6kro

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    Using a ground does nothing to increase the length of the wire. The ground can't pick up signals.

    The antenna is the piece of wire in the air. It picks up voltages relative to ground and the only way to use these is to provide a path to ground.

    If you connect to a tap on a coil like this:
    [PLAIN]http://dl.dropbox.com/u/4222062/coil%20voltage%20stepup.PNG [Broken]

    there will be a voltage step-up effect so that you can get a greater voltage out than you put into the coil at resonance.

    Could you expand a bit more on whats happening with the wire and ground forming the "two half-wave elements".

    Two quarter waves probably. At 1 MHz this is unlikely. Each side of such a dipole would be 246 ft long.
    The antennas used by crystal sets are more like large capacitors with the ground being the other side of the capacitor. The antenna has basically the same signal on all parts of it and getting this signal to be useful involves connecting the crystal set's coil between the antenna and ground.

    Virtual grounds used in opamp circuits have nothing to do with antenna grounds.

    At higher frequencies like 7 MHz where half wave dipoles are only 66 ft long, it is possible to use antennas efficiently without a ground system. The receiver is just placed between the two halves of the antenna so that one side is "pushing" while the other side is "pulling" current into the receiver.
     
    Last edited by a moderator: May 5, 2017
  12. Jun 22, 2011 #11
    Am I right in thinking.

    The radio station has their massive mast / transmitter and they are also grounded.

    And then the aerial and ground on the crystal radio completes the circuit so to speak and allows a better flow of current around the system?

    Or is our ground just for us and to give the picked up current somewhere to flow to.

    I said about the virtual ground because its basically 0V. So why wouldn't that work in giving the antenna a reference for its current?

    So your saying if I was to put the antenna connection onto a tap on the coil rather than the wire that comes out of the coil it would give an even stronger signal?
     
  13. Jun 22, 2011 #12
    Remember ............
    All current flows in a loop.

    That means it has to get back to its source and it does this thru the ground.
    If the ground is poor .... meaning not supplying enough electrons for proper
    response ..... then it will act as a resistor dropping voltage and degrade
    the signal.
    Ground needs to provide the return path for ALL the signals so
    it should be the largest conductor in the system

    Radio stations located near bodies of water will usually lay out a huge mesh extending in to the nearby water to provide a ground plane for the antenna.
     
  14. Jun 22, 2011 #13

    vk6kro

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    No, this is a particular type of antenna and it needs to have a connection to ground. If it was a dipole antenna, it would work without a ground. If it was an antenna on an aircraft, the body of the aircraft would work as a ground.

    There is no return current back to the transmitter.

    When you make the coil, you should make tapping points every 10% or 20% along its length.
    The best tapping position will be found by trial and error and it will depend on the geometry of your antenna and how efficient your grounding is.

    You would look for a point where the tuning is able to tune individual stations and still give you acceptable sound levels.

    The impedance of the headphone or earpiece will influence where you connect the diode to the coil. Again, find this by trial and error.

    There are also circuits for crystal sets that use a battery to apply slight forward bias to the diode. This makes the receiver a lot more sensitive.
     
  15. Jun 22, 2011 #14
    So I just finished making my inductor, I decided to hook it up. Diode inductor and headphones.

    I then took my aerial lead from the back of the TV which goes straight to the aerial on the roof. I touched it on the circuit in between the diode and inductor (not tapped) and was getting a pretty good loud signal. I could hear the commentary for Wimbledon really well.

    This was without a ground wire.

    So is my roof aerial a dipole antenna? (just a general tv aerial)

    Also I didnt tap my inductor, I just made it, as im building a variable capacitor to do the tuning. But as I was able to hear the radio does that mean the antenna acted as the capacitor and the antenna and inductor was resonating?

    I still dont understand by connecting the antenna to different points along the coil it can help make a louder signal, amplify it a bit. I thought just tapping the coil was to help with tuning.

    As it worked, I took it outside and stung a piece of enamelled wire across the length of my garden and hooked it up to the bit where I was previously touching my aerial connection to, I also attached a ground wire on the other side and tried different places, the ground, in mud etc but I didnt get anything...not even a buzzing.

    Oh and one last thing, When I was using my house aerial, I tried grounding the circuit by hooking it up to my radiators pipe, but it didn't change anything. But without the aerial when I touched the metal pipe with the ground cable you could hear lots of fuzzing. Does that mean the pipe isnt properly grounded? Or is the pipe trying to act as an aerial?

    ...Could I use metal window frames as an antenna?
     
  16. Jun 22, 2011 #15
    If you must have an internal aerial the neatest way is to make one from strips of cooking foil pasted on the wall to the correct shape under the wallpaper.
     
  17. Jun 22, 2011 #16

    vk6kro

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    You will need the variable capacitor and it should have a maximum capacitance of about 400 pF.
    This should be obtained from an old vacuum tube radio. These have air dielectric capacitors which are well made and you can see where the plates are.
    Portable transistor radios have a much smaller variable capacitor which is still OK if you can't get the older air-spaced ones.
    Don't try to make one.

    Crystal sets are sometimes very frustrating and results can be surprising.

    One thing you should know is that most AM transmitters are vertically polarised and this means that the vertical part of your antenna is the most important for picking up signals. So, a long horizontal antenna is mostly wasted. Height is important.

    Window frames are unlikely to be much use as an antenna at 1 MHz. They are too small.

    The coil acts as an auto-transformer and it can step up the voltage. This depends on the impedance of the antenna, though.

    You can scrape a bit of the enamel off the wire of the coil at different places along it and check how connecting the antenna at different places affects the reception and the tuning.

    Connecting a piece of wire to some mud will not be as good as connecting it to a metal water pipe which is buried in the ground. A good ground connection is difficult to achieve and commercial transmitters use a lot of copper wire and pipe to improve their ground connection for the antenna.
     
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