Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What would happen if I were close to a powerful transmitter?

  1. Oct 13, 2013 #1
    I saw some AM broadcast radio towers, several to form an array in order to direct the energy dependant on the time of day. I guess they covered 500+ meters, horizontal.

    I am now listening to shortwave transmissions on youtube, of things like OTH radar.

    I wonder in both cases, what would happen to me if I were to approach the transmitter? If it were an AM radio tower? UVB-76? If I were to climb it? Would I die? How?

    Thank you
     
  2. jcsd
  3. Oct 13, 2013 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Hi oneamp

    in most cases the tower is the antenna and it has a huge insulator disc at its base giving it isolation from the ground. Considering than many of those AM transmitters ( of major stations) have output power exceeding 1000W and many upwards of 10kW.
    What do you think would happen to you if you stood on the ground and leant over and touched the tower ?

    The chances of becoming totally bar-b-qued are very high

    Dave
     
  4. Oct 13, 2013 #3

    Baluncore

    User Avatar
    Science Advisor

    Primarily, the effect will be deep radio frequency over-heating of your body. For microwave frequencies you can get full depth surface burns. Eye damage due to heating is also reported from technicians who have been in the beam of an active microwave dishes. They probably thought it had been turned off, or did not understand the danger.

    Technicians who tune high power LW transmitters are actually inside the matching circuit while adjusting it. Some now find they are sterile, luckily there is an insufficient sample to prove the correlation.

    If you get too close to an insulator you may trigger an electrical discharge through your body that will paralyse voluntary control of your nervous system. If you have ever experienced the cramping grip of a DC shock, or a body trembling AC power frequency shock then you are both careless and accident prone. When you get your first RF shock you will be fascinated to watch your limbs moving slowly, without you having any control over that movement. Be fascinated, there is nothing else you can do. If you are very lucky they will move away from the danger. Surface burn tracks can be expected depending on the configuration and frequency of the RF power.

    The probability of death will be determined partly by contact time. If you are alone and paralysed then death is highly likely. Safety requires that you are never alone with electricity, also that you and your mate are both trained in resuscitation and know how to quickly shut down the system.

    You are probably safe so long as you keep away from transmitters while watching to youtube .
     
  5. Oct 13, 2013 #4
    Remember the whip antennas on cars from days gone past. Some were magnetic mount and others firmly attached. The whip can be screwed off and a leaving a prong protruding from the mount. The power I think was in the watts and the frequency in the MHz. Touch that prong and your body becomes the antenna while transmitting. Fingers burn like hell even if there is no heat but that is the RF going through them. As a practical joke it is hilarious.
    Point is even with low wattage you feel it, so don`t try anything in the high wattage.
     
  6. Oct 13, 2013 #5

    Averagesupernova

    User Avatar
    Gold Member

    Plenty of those still around.

    That depends on what you are feeding it. A whimpy CB rig, yeah a couple of watts. Try it will a loaded quarter wave for 10 meter band at a hundred watts.

    I personally know a broadcast engineer who climbed an AM broadcast tower about half way up while it was on the air. The power I believe was less than 10 KW but more than 1 KW. I seem to recall it was shunt fed. In other words, the base of the tower was grounded. I am not sure the reason he climbed it but I know it wasn't on a dare or anything like that. The frequency was 1520 KHz so for a quarter wave it would have been about 150 feet tall. This was about 20 years ago. He reported no ill effects and is still working in the industry today. I don't recommend it and I thought it was a stupid thing to do at the time.
     
  7. Oct 13, 2013 #6
    Such good answers, thank you very much!! So if I touch it I die, otherwise I can be burned... even low frequencies (MW/SW) will burn? Or just microwave?

    What if I just walk up to it and stop about a meter in front of it? Will I feel like I am burning? What distance is safe?
     
  8. Oct 13, 2013 #7

    Baluncore

    User Avatar
    Science Advisor

    Answer, probably 60 wavelengths, outside the near field.

    Antennas are designed to radiate energy in a calculated direction and density. That pattern is called the far-field. The near-field is made up of all the circulating and transmitted energy close to the antenna. If you enter the near-field there are no general hard and fast guidelines to predict outcome, that is a function of circulating energy, wavelength and the RF engineering design.

    A transmission line is an electromagnetically balanced design. A momentary failure of any one insulator can unbalance the line and generate huge near fields. Don't assume that the antenna will continue operating normally. There are failure modes that can flip the energy flow situation suddenly. Your presence may trigger a change and you may become part of the circuit. In particular keep away from insulators and the ends of open wires where very high RF fields are to be expected.

    Understanding the near-field of antennas is a science that requires a decade of study and experience. Without that experience it is a dangerous field and an unpredictable black art.
     
  9. Oct 13, 2013 #8
    Don't misunderstand - I am not going to get close to one of these antennas. But I've been wondering about them.

    The hazard at close range, regardless of frequency, SW-SHF, is not due to breakdown of the dielectric between me and the antenna, but the near-field effects? Aren't these just warming of tissue (burns)?
     
  10. Oct 13, 2013 #9

    Baluncore

    User Avatar
    Science Advisor

    Breakdown of the dielectric will result in a current flow, electric shock and surface burns. That will be due to corona discharge or a plasma arc across the skin. The breakdown voltage of air is low in the presence of the corona discharge often found near the end of open wires.

    Being in the electric field will result in a deeper cooking. You need to study dielectric heating, (as used to weld plastics), to understand the hazards of RF electric fields. Many dielectric welders operate between 27MHz and 45MHz, or in ISM bands.

    You should expect a patient with a fever to become irrational. The same small temperature rise happens when you are in an RF field. It does not take much RF energy to warm your blood and brain. Apart from a warm feeling, you probably would not notice it happening. There have been cases of people competing to operating dielectric welding machines on cold mornings. It seemed to be the only warm place in the factory, but really they were simply cooking themselves in the RF leakage energy from the welders.
     
  11. Oct 14, 2013 #10
    Thanks
     
  12. Oct 14, 2013 #11

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    That doesn't make any sense for "medium wave" or "long wave" AM radio, where the wavelengths are of the order of hundreds or thousands of meters.

    For example, with BBC Radio 4 long wave (198 kHz) !"60 wavelengths" would cover almost the entire width of the UK from coast to coast! (Nowhere in the UK us more than about 80 miles from the sea).
     
    Last edited: Oct 14, 2013
  13. Oct 14, 2013 #12

    Baluncore

    User Avatar
    Science Advisor

    My simple answer is overly conservative for longer wavelengths. Also, it does not consider radiated power, EIRP, or beam-width.

    If you build a resonant antenna and place it 60 wavelengths away from a powerful transmitter's antenna you can expect to build up dangerous levels of circulating energy. You will also be distorting the field of the transmitting antenna. As an example some people have milked VLF transmissions and used the power to light their houses.

    There are also situations where it is certainly not safe just outside the near-field. Consider for example a focussed microwave beam from a parabolic reflector, or anywhere near the output of an operating gyrotron.

    There can be a simple answer to a simple question about a simple subject.
    A complex question about a complex subject may also have a simple answer.
    But there can be no one simple answer to a simple question about a complex subject.

    I based my simple answer to the complex question on the concept that, if you are in the near-field then you should be considered part of the antenna.
     
  14. Oct 14, 2013 #13
    I heard of one experiment in which rats who were trained to run a maze, forgot how to run it when in a strong RF field. When the field was turned off, the rats again remembered how to run the maze.
     
  15. Oct 14, 2013 #14
    Please give me a link to this online!!!! Actually I'm pretty skeptical...
     
  16. Oct 15, 2013 #15

    Baluncore

    User Avatar
    Science Advisor

    Scepticism is good. I don't have a link because it occurred prior to the internet and was detected by the change in pattern of the transmitter. That type of activity was well known by those of us responsible for the enforcement of the radio regulations at the time, but it was obviously not advertised. If I find a link I will post it here.

    IIRC there were reports published of it being done to the 60kHz VLF transmitter at Rugby in England. Maybe it was mentioned in Wireless world. The receive antenna was multiple turns of wire wound around inside one room in a house. A parallel capacitor was then installed and adjusted to bring the LC loop to resonance. I do not remember if fluorescent tubes or filament globes were used, but the brightness varied slightly due to the modulation.

    http://forums.theregister.co.uk/forum/1/2013/08/14/boffins_hawk_powerless_radio/
     
    Last edited: Oct 15, 2013
  17. Oct 15, 2013 #16

    NascentOxygen

    User Avatar

    Staff: Mentor

    I seem to recall a report of someone being able to hear the station's program while walking near the tower. Something about a non-linearity in a dirty insulator serving to demodulate the RF. The sound was far from hifi quality!!

    I, too, recall the story of how it was discovered that an experimenter was lighting his house using power from a nearby radio transmitter. Techs were investigating a reported shadow in the broadcast's reception and found someone with aerials set up to capture the RF and use it for power. One of my uni lecturers told us that story.
     
  18. Oct 15, 2013 #17

    Baluncore

    User Avatar
    Science Advisor

    The ability to hear the AM modulation has been shown to be due to rectification by the metal amalgam fillings in teeth. There are also some people with embedded grinding grit (carborundum) that can detect AM.

    There are many semiconducting compounds such as oxides of Cu and Zn. These can generate harmonics from some transmit antennas, and cross-modulation in receive antennas. Keep those bolts tight.

    Energy theft is also done by some farmers who run an electric fence near/under three phase HT power lines. That works OK until there is a phase fault, when the phase imbalance generates a burst of energy with nowhere to go.
    Likewise, using electric detonators under power lines is safe until there is a phase fault. To survive that you need to keep the leads on the detonators short so as to reduce the coupled energy during a phase fault.
     
  19. Oct 15, 2013 #18
    If you are near enough to get significant power from a tower, then aren't you in harm's way from what we talked about before?

    Also, why the cap and resonance? What did that do for it?
     
  20. Oct 15, 2013 #19
    When I visited the AM tower array that I described above, I heard the station coming from the shack that served the towers. The friend I was with said that it may have been to some mysterious effect of the high power transmitting signal. But I've since began to believe that there was simply a radio no in the building. I wasn't able to find any effect like this on the internet, google or forums.
     
  21. Oct 15, 2013 #20
    And another question. Can I just walk up to a transmitter with a resonate LC circuit connected to a resistor and LED to make the LED light up? Or do I need a big antenna?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook