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Magnetic Loop Antenna Indoors Apartment

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KT4JX, Feb 11, 2018.

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  1. K2CAJ

    K2CAJ XML Subscriber QRZ Page

    I second this. You can just buy the MFJ933 and add your own metal for the loop. I used copper tubing from the plumbing supply store, and I can clamp in different lengths that are optimal for different bands.

    The MFJ933 is $190 for the box and it will happily run 100W. I think that's the closest you need to get to "buying" a loop.

    That being said, you never want to run anywhere close to 100W indoors. At 100W the currents on a loop antenna are big and hazardous, and the fields are strong enough that you don't want people within at least ten feet of the loop. Even running it outdoors you have to be careful, because it's close enough to the ground that some kid might walk up to it.
    AK5B likes this.
  2. AK5B

    AK5B Ham Member QRZ Page

    Brilliant. Your truism could also make a great signature line, Scott!


  3. KK5JY

    KK5JY Ham Member QRZ Page


    FCC OET-65(b) contains an example (Table 17) of a "typical" STL on several different bands. The predicted necessary separation distances based on a 150W transmitter are very reasonable for indoor use, especially if you don't intend to operate at 100% duty cycle continuously over the time-average windows (6 minutes for controlled, 30 minutes for uncontrolled). The 10' rule of thumb you mentioned is good up through about 20m, even at 150W continuous transmission, and it decreases as you decrease the duty cycle. E.g., if you are running in a contest, and you transmit 50% of the time, the associated compliance distance goes down by roughly that same amount. It also decreases as you lower the peak power. So if you run lower power (e.g., 30W instead of 150W), the associated compliance distance goes down that much more.

    There's no reason not to run an STL indoors as long as you comply with the MPE limits, and none other than FCC published a guide to help you estimate the required distances.
    AK5B likes this.
  4. K2CAJ

    K2CAJ XML Subscriber QRZ Page

    And those distances listed for uncontrolled exposure (at 150W) range from 2.8m to 4.2m. Ten feet is 3.04m. This is roughly in line with calculations and simulations I've seen from other sources---10 feet is a fair distance to keep people away from a 100W STL.

    On top of that, this is an antenna that someone can easily touch because of its proximity to ground level, and its currents are much higher than other antennas in order to compensate for its sub-Ohm radiation resistance. It makes sense to stand it out of reach by at least an arm's length and a step or two.
  5. KK5JY

    KK5JY Ham Member QRZ Page

    Well, okay, but you aren't in an uncontrolled environment. You are in a controlled environment, and so are the members of your family. If you are in an apartment complex, your neighbors are in an uncontrolled environment, but in a house, you can use the first numbers, for reasons explained in the OET documents.

    You also used the 150W figure, which is 50% more than a 100W radio can produce, and those numbers only apply if you key-down that 150W transmitter continuously over the entire integration window. That might be possible for a long-winded RTTY or Olivia ragchew, but for CW, SSB, or even RTTY contesting, you won't get near 100% duty cycle.

    And even if you want to run 100% duty cycle for six minutes at a time, the way to cut those distances (roughly) in half is just to cut your power to 75W. Voila. :)

    You are absolutely correct that you need to protect people from accidentally touching the antenna. RF burns can be dangerous. But that's actually not that difficult. If nothing else, you can just wrap the large loop in a flexible PVC or foam jacket. Even a sufficiently thick layer of insulating paint might be enough to protect someone from an accidental contact burn -- I wouldn't depend on a layer of paint, but it's do-able.

    And last, the antenna pattern matters. Most vertical STLs, installed at roughly attic height, will project nearly all of their energy up or laterally in the plane of the loop. People below the antenna will have less exposure than those beside the loop, so you can achieve more effective separation, even at high duty cycle, by simply lifting the loop overhead in an attic space. Such spaces are a better place for a loop anyway, because that places them above the wall wiring.

    Remember that what you see in the OET examples are worst case calculations. They did that on purpose so that (among other reasons) you have a reliable baseline from which to extrapolate downward on the basis of reduced average power.

    There's nothing wrong with your desire to be extra-cautious. More people should be more careful in life. But the details of MPE are that a typical STL run at 100W can be safely installed and operated in an indoor environment, and that safety can be achieved and maintained by several different means. For people who want to do so, there is no reason why they should not, as long as they comply with the rather small separation distances required.
  6. K2CAJ

    K2CAJ XML Subscriber QRZ Page

    But the guy isn't in a house. He was asking about running 100W from an apartment.
  7. KK5JY

    KK5JY Ham Member QRZ Page

    Fair enough. All that changes is that he has to keep enough separation from his neighbors to comply with MPE distances for an uncontrolled environment, and from his family to comply with MPE distances from a controlled environment.

    Pretty simple. :)
  8. K2CAJ

    K2CAJ XML Subscriber QRZ Page

    I suppose I'm erring on the side of caution with respect to MPE. There are more practical reasons to consider running outdoors if possible, such as better performance and easier tuning.

    I checked the OP's location, and this isn't a high-rise apartment: it appears to be a cluster of low buildings around a common outdoor area. If I lived there, I might take the MFJ loop tuner, a tripod, and some copper tubing, and run portable right outside with a setup like I have in the first picture on my QRZ page.
    AK5B likes this.
  9. KK5JY

    KK5JY Ham Member QRZ Page

    Nothing wrong with that. :)

    Certainly. STLs are one of many antenna designs that definitely benefit from a little height, with a peak performance occurring just under λ / 4 overall height. Getting away from all the little noise-makers in the house is a nice bonus. I run mine indoors from time to time to prove that I can, but the best ones get used in the back yard.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think I have a list of all the hams who have died from RF radiation exposure. It's here:

    -End of list-
    AK5B and KK5JY like this.

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