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'Helping Hams' - Magnetic loop conversion to pedestrian portable

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N0TES, May 17, 2016.

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  1. N0TES

    N0TES Ham Member QRZ Page


    73,
    Steve/N0TES
     
  2. VK3YE

    VK3YE Ham Member QRZ Page

    Or if you don't have one, build your own! It can be done in half a day, mostly with hardware shop items, and is a rewarding project.

    If at all possible make sure your loop tunes down to 7 MHz as it's a great band for pedestrian mobile.

    Details at: http://home.alphalink.com.au/~parkerp/projects/projmal.htm
     
    NH7RO likes this.
  3. N0TES

    N0TES Ham Member QRZ Page

    VK3YE is right! Build one! If you would like to see how, and happen to be at the SeaPac Hamfest, come on into the Forum I am giving (https://www.seapac.org/Seminars.html Saturday 1200-1250 'Magnetic Loop Antennas: How To').
    73,
    Steve/N0TES
     
  4. N0SYA

    N0SYA Ham Member QRZ Page

    I wonder what the efficiency of that thing is compared to a dipole just laid out on the ground in a stationary example. Or for a mobile wire antenna, a simple wire dragged along the ground.
     
  5. AA7EJ

    AA7EJ Ham Member QRZ Page

    Not being able to see the video, I do not want / need a video software, but I am curious.

    Magnetic loops are HIGH voltage devices when sufficient RF power is applied.
    They are also high loss devices when not constructed using correct technique, for example some construction plans recommend using silver bearing solder.
    I would not call them "pedestrian portable" - mine is pretty finicky to move around ( about 1 meter diameter loop).
    And to couple / tune the loop is a chapter by itself.
    I would suggest "ask one who owns one" before embarking on construction path.
    ( Mine is still unfinished, in the attic. )

    73 Shirley
     
    Last edited: May 17, 2016
  6. NH7RO

    NH7RO Ham Member QRZ Page

    Yes, there are high voltages present on small transmitting loops regardless of the wattage used---especially near the capacitor. Like any antenna, there are high voltages present at certain points and one uses common sense when operating close to them. It is always good practice to keep bare copper or metal insulated in some fashion for extra precaution (I paint mine and never touch the element or coupling loop when xmitting).

    All or most of these portable STLs use coax or small diameter hardline (IIRC on the AlexLoop) and the coax braid type start out with a disadvantage right there because of the skin effect playing a major role in how these radiate.

    Rather than spend several hundred buckos for one of these commercial loops one could achieve consistently better results with a properly made loop using larger diameter HVAC soft copper tubing, vacuum variable capacitor and a vernier reduction drive.

    Tune for the noise peak when receiving and after some practice one will be able to QSY and retune rather easily.

    The resulting loop might end up costing about as much (maybe a tad less) but should outperform these coax/air variable QRP substantially. While weight is always an issue with portable antennas I doubt the difference would be much.

    For those interested in learning more about STLs this is an absolute must-read:

    http://www.ahars.com.au/wp-content/...restimated-Magnetic-Loop-HF-Antenna-V1.-2.pdf
     
  7. VK3YE

    VK3YE Ham Member QRZ Page

    I've only ever used regular solder. The tuning capacitor on my 1m loop is mounted such that I can adjust it with the microphone hand while held in the same position that it is when it is being operated. There is also a vernier reduction drive on the tuning capacitor. I've found the gamma match approach works well across a wide frequency range.

    There will be performance compromises compared to a full sized antenna but I've had some great contacts on them, eg:

     
  8. AA7EJ

    AA7EJ Ham Member QRZ Page

    Good show mate. Or should it be "Mike" ?

    Not to diminish this accomplishment, would you comment on your "path over the salt water"?
    The other interesting and not unusual thing is the notable difference in US station setup.

    73 Shirley
     
  9. VK3YE

    VK3YE Ham Member QRZ Page

    Yes the above contact was likely long path to the US and over water. However water isn't mandatory for pedestrian mobile DX contacts. Although I was near the water, these six contacts into Europe were likely long path in the land direction.



    Here's some more pedestrian mobile DX with an even smaller loop - this time on 10 metres.

     
  10. W1YW

    W1YW Ham Member QRZ Page

    No, they are not "high loss devices". They are low radiation resistance devices. The ohmic losses are quite low. The problem is the radiation resistance is usually lower, so the efficiency suffers. But most are at least 50% efficient.
     
    NH7RO likes this.

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