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How can you make a portable earth connection?

  1. Jan 17, 2010 #1
    As it suggests, the problem I face is to how I carry about an earth connection (for an AM radio transmitter set I made) instead of burying it into the ground? Of course, if I make a radio set, I simply can't carry it about making an earth connection into the ground.

    I have provided a solution to this problem, but seems ineffective: to bury the ground wire into an earthen pot filled with soil.

    Does anyone have any other practical solution to this?

    By the way, is an earth connection really required for the radio I am making, at this http://sci-toys.com/scitoys/scitoys/radio/am_transmitter.html" [Broken]? In fact, if I remove it, will there be any harm? I suppose the range will become shorter, but the transmission distance I require is only two or three metres at the longest. So does that mean I can remove the earth connection?
    Last edited by a moderator: May 4, 2017
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  3. Jan 17, 2010 #2


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    It will probably work OK without the earth connection. Soil in a pot will not do anything.

    An efficient antenna at this frequency would be a half wave dipole, a quarter wave above the ground.
    That would be 150 meters long and 75 meters above the ground (492 ft long and 246 ft high).

    A less efficient antenna would be a quarter wave long (246 ft) mounted vertically with a very good ground connection.

    Since you are not going to do either of these, the longer you make the antenna wire, the better it will work. Do not exceed 3 ft of wire or you may cause interference to AM broadcast stations. Make it just long enough to cover the short distance you need.

    First, though, check that there is no local broadcast station on exactly 1000 KHz or whatever frequency is marked on the oscillator.

    Check the voltage required for the oscillator unit. Most of these are only for 5 volts so you could damage it with 9 volts. You could probably use 3 AA cells to get 4.5 volts and that would work OK.

    I have seen that circuit before and someone claimed the sound was distorted from it.
    Last edited: Jan 17, 2010
  4. Jan 18, 2010 #3
    You don need any earthing for such electronic circuits.

    However, The protection system to be used depends on the types of faults occur would in the circuit. In your case the fault type would only be short circuit due to power supply, so u hve to use a few fuses for diodes, transistor (if any) to protect your transmitter.
    Last edited by a moderator: May 4, 2017
  5. Jan 18, 2010 #4


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    Have you considered using a loop antenna rather than a monopole? A loop doesn't need an Earth. If you are only needing a link of a few metres, it may be more effective and more convenient. There were many such designs in the past and could be conveniently made using ribbon cable so save actually winding multiple turns. (Join red to orange, orange to yellow, yellow to green etc and then use the first and last colours as your connections - giving you one turn for each colour / strand) The (square loop) antenna can be held in shape by a cross of wood which is easily folded up for transporting.

    Take a look at old HAM radio books. Just because the people in the photos wear tank tops and have funny haircuts and their equipment uses valves doesn't mean that the antenna is also a joke. Look into it - you can forget about the need for an Earth that way and you may get very good performance for an antenna which is always going to be a tiny fraction of a wavelength..
  6. Jan 19, 2010 #5
    No, I can't use a loop antenna, taking into consideration the minimum diameter size and circumference thickness. In fact, I am making a spy bug. So, keeping in mind that a smaller size would be better, could I receive a schematic on how to make a AM transmitter using the oscillator from a clock?
  7. Jan 19, 2010 #6


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    The oscillator in a clock is usually about 32768 Hz and you could make a transmitter out of one, but the problem would be that you would then have to design and build a receiver for that frequency. They do exist but you are not likely to have one.

    A much better approach would be to use an FM transmitter on about 95 MHz.
    I would suggest something like this:

    However beware that the description of how the oscillator works is nonsense.
    The circuit looks OK though.

    There are dozens of similar circuits around. The dimensions of the oscillator coil are very important and they determine the final frequency.

    You would receive the signal on a commercial FM receiver.
  8. Jan 19, 2010 #7


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    What'cha going to use the spy bug for?

    And think about it, a spy bug using radio transmission at 1MHz... as you are finding out, the antenna issue is pretty much a non-starter. Even the FM transmission band is problematic for making something small. There are much better ways to make spy bugs, but of course we can't talk about those here on the PF. (Quiz question -- why not?)
  9. Jan 20, 2010 #8

    When you look at this image, what is referred to by "4u7"? 4 μF? And what about 4k7? Does that mean seven 4000 Ω resistors?

    Secondly, I believe sci-toys.com contains information on how to make an AM receiver. I could settle for a longer range FM transmitter, but the difficulty in obtaining the components precisely tuned to the specifications makes it difficult to obtain. Does anyone know of any place online where you can get the components?

    And finally, setting apart construction and other issues, will I get any reception of my transmitter if there is another local station on 95 MHz, provided the distance is lesser than 20 m? Take a look at the list of stations at where I live (http://www.asiawaves.net/india/delhi-radio.htm".

    If to ensure that I can use my device everywhere in India and worldwide, the frequency of the transmitter must either be below 80 MHz (there are stations in the UAE above 82 MHz, I don't know about the rest of the world) or above 106 MHz. If I'm correct, please try and change the specifications of the schematic given (atleast as to work in Delhi) . If wrong, I hope someone will come by to correct me.

    In reply to berkeman's last post, I think that a spy bug need not be necessarily small enough to be hidden, but can be big and even hidden in a toy duck, or so on.
    Last edited by a moderator: Apr 24, 2017
  10. Jan 20, 2010 #9


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    When drawing circuits, the value of the components is sometimes written with the modifier placed where the decimal point would be.

    Thus 4K7 refers to 4.7 K or 4700 ohms. 5u6 would be 5.6 uF. 3R9 refers to 3.9 ohms

    This is considered less likely to be misunderstood.
    This author used 3.3 p instead of 3p3 though, for 3.3 pF

    You would have to make the 15 pF capacitor variable to avoid conflict with another station.
    So, you could find a clear spot between the commercial stations and tune until you can hear it.

    This is a very low powered transmitter, but you have an obligation to avoid transmitting outside your own property. Also most countries regard eavesdropping as illegal invasion of privacy, so you would have to consider what would happen if you tried anything like that.

    Forget any ideas about transmitting worldwide with such devices or taking something like that to another country. You have to obey the laws of your country and other countries.

    If you want to learn about electronics by building such a device, that is fine, but you won't get much help if you are going to behave illegally.
  11. Jan 21, 2010 #10
    You're right, by building this I am simply trying to understand more of electronics. Moreover, as I now know from you that spying is illegal, I could use this circuit simply as a wireless microphone, and so on.

    And by the way, is it illegal to interfere with the reception of another station at the same frequency, and that too if only on my radio receiver set?
  12. Jan 21, 2010 #11


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    And by the way, is it illegal to interfere with the reception of another station at the same frequency, and that too if only on my radio receiver set?

    If you interfere with your own radio, you don't know if you are interfering with someone else's radio.
    So, if you make the capacitor across the coil a small variable trimmer capacitor, you can set the frequency to somewhere between the stations that are normally there.

    Then you won't interfere with anyone. That doesn't make it legal, but within your own property it should not be a problem.
    If you get it going, take your radio for a walk and see how far the signal is getting.

    Stay legal. It is the only way.
  13. Feb 19, 2010 #12
    You could use the - (negative) end of a battery. that what it is referred to in a schematic.
  14. Feb 19, 2010 #13


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    Neither end of a battery is, inherently, an 'Earth' connection. Sometimes the positive or negative battery terminal may be connected TO earth but that is a different matter.

    If you want a 'good' earth for the purposes of launching a good radio signal then you need to promote currents flowing literally in the ground (for a medium frequency signal, at least). Connecting to the Earth network in your home is quite effective - or to a water pipe (they are all bonded together in modern houses).

    But, if you are operating illegally, you really don't want to launch interference all over the locality. You want a short range, inefficiently launched, wave which just manages to get as far as your receiver and no further. Any thing will do, basically for your Earth- even just another length of wire, which turns your system into a 'badly fed dipole' - that's fine.
  15. Feb 20, 2010 #14
    okay then so try this,
    charge a capacitor then connect the ground to the negative end of the capacitor and the extra electrons will go in the capacitor. That's what the earth does: it accepts extra electrons. But on the downside, it only lasts for a few seconds. But if you mean portable as in carrying it in a car then you can use a big capacitor array. If you don't want to get so complicated, then you can use something as simple as a coil antenna.
  16. Feb 20, 2010 #15


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    Not many electrons will "go" anywhere. The capacity of the capacitor (I'm assuming your capacitor is of significant size) will be far higher than the capacity of the positive plate to Earth (which will be in the order of a few pF) . The charge will be shared between the capacity between what is, in effect, two capacitors connected in parallel. The charge will flow in a few nanoseconds, actually, before balance is reached.
  17. Feb 21, 2010 #16
    The positive end is not supposed to be connected to ground but it doesn't matter because a capacitor has no internal connection.

    I also meant to use the tuna-can sized capacitors: the one that is about 1 farad
    Last edited: Feb 21, 2010
  18. Feb 21, 2010 #17
    to the person who added this thread,
    read the 5th paragraph under "using the transmitter" in your link.
    i read as:

    Without any connection to an antenna or a good GROUND CONNECTION, the transmitter will only transmit to a receiver A FEW INCHES AWAY. To get better range, clip the ground wire to a good ground, such as a cold water pipe, and the antenna to a long wire, like the one we used for the crystal radio.

    That is: the transmitter would have decreased range but would still work
  19. Feb 21, 2010 #18


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    As far as RF would be concerned, the capacitor would be like a piece of wire - what would be achieved by its presence in the circuit? How would it 'encourage' currents to flow in the Earth?
  20. Feb 22, 2010 #19
    i am not talking about the earth, i am talking about the current flowing into one of the capacitance plates
  21. Feb 22, 2010 #20


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    This thread seems to have split.
    Yes, I think we agree that the best antenna for MF will use an Earth connection, really to the ground. Without it, you have an unbalanced, very short, dipole which is a poor radiator. I probably repeated much what had already been said.

    The other issue of using a capacitor is very questionable but, as the proposed 'experiment' wasn't really described fully, it is hard to see what was really meant by it. What I can say is that a capacitor will never fulfill the function of an RF 'Earth Connection' - which was what the OP was about.
    Perhaps a bit more detail about how the "1 F" capacitor would be connected . . ? (A diagram would be less easy to misinterpret)
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