MFJ magnetic rx only loop antenna

Discussion in 'Amplitude Modulation' started by N2DTS, Aug 8, 2019.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    I have not tried to mount it sideways...
    You could do so easy with the MFJ since its rigid tubing...
    The nulls are off the sides so one null would be pointing down to the ground and the other up in the sky.
    The nulls are useful to null out a noise or unwanted signal they say..

    If you wanted to mount it that way you could just make a pvc X with a pole in the center.
     
  2. K4KYV

    K4KYV Subscriber QRZ Page

    My homebrew one does OK, but much better when I have tested it outside, away from all the metal in the shack (transmitter cabinets, antenna feedlines, house wiring, metal shelving, etc.). It is unamplified and still delivers enough signal, but must be tuned to frequency, so if I had it outside I would have to go out and tweak it whenever I QSYed. I wonder how well those remote broadband pre-amps in the commercial units stand up to lightning? Almost anything I have that is the least bit delicate has to be isolated during thunderstorms or else it gets wiped out.

    I found that out the hard way when nearby lightning destroyed the rare and expensive 200 μA "powerhouse" meter I use as an indicator on my remote tuner control panel, just from pick-up on the DC control line running from tower to shack. Luckily, Dennis was able to provide me a replacement, which I keep disconnected when not in use.
     
  3. N2DTS

    N2DTS Ham Member QRZ Page

    Can't say Don, most have protection but how it would hold up to a lightning hit is unknown.
    MY MFJ loop currently has a preamp I got off ebay for under $20.00.

    You can buy an entire antenna, amp, power injector for $35.00...
    If you disconnected the coax in the shack when not in use, would it still fry from a close strike?

    In 45 years I have never had anything fail from lightning, low antenna's have some advantage I guess...
     
  4. N2DTS

    N2DTS Ham Member QRZ Page

    The MFJ and the w6lvp loops both seemed to work well on 40 meters and up, but not so great on 80 meters.
    So I thought I would try a larger loop.
    I got 25 feet of copper tubing ($19.00) and some pvc pipe ($5.00) and mounted the funtronics preamp on the pvc pipe.
    I made a cross with the pipe and a 4 way joint in the center, drilled holes through the top and side pieces of pipe and fed the tubing through
    so it made a circle.
    The funtronics preamp has wing nuts for the loop connections so its easy to use, I flattened the tubing, drilled a hole through it
    and attached the loop to the studs with the wing nuts.
    It worked fine but did not seem very quiet at all, it just seemed like any antenna.
    I took it down and opened up the amp and removed the thin cheap coax and added a bnc connector so I could use the quad shield RG6
    cable.
    No more noise, the loop seems to work great on 80 and 40 and on up.

    Comparing antenna's on the Flex which has many antenna inputs, the loop seems as good as the dipole and much better
    then the vertical.
    Often, the loop is best.
    Its up about 12 feet behind the house, the vertical is 30 feet tall and roof mounted, the dipoles are full size 40 and 80 meter up about 40 feet.
    The loop is pointed about NE and SW.

    If you only have a vertical, as some do because of space limitations, think about an RX loop.
    It picks up WAY less noise then a vertical.
    If you have a local nasty noise source, you may be able to null it out with a loop.

    The smaller loops from MFJ and w6lvp work well, but not so well on 160 and 80 meters, they are just too small.

    The funtronics loop is $40.00, the amp seems ok, but you can buy a loop amp on line for less.
    A waterproof electrical box, some hardware, tubing/wire, pvc pipe and some cable TV coax and you are in business.
     
  5. KK4NSF

    KK4NSF Ham Member QRZ Page

    It's funny.... but we were discussing un-tuned receiving loops in another thread earlier today.... and several folks simply could not get past the idea of a non-resonant RX only loop. While I don't have an MFJ, I have a similar "Dave-Built" antenna that is a little smaller.... but otherwise very similar. Mine works great, and I have no doubt the MFJ does, too. As I write this, I'm listening to an AM Broadcast station in Colorado.... and I'm in Mobile AL.

    The key to getting the most out of this sort of antenna is being able to rotate the null toward local sources of noise, thus blocking them. On many of them, including mine, you can almost totally block a nearby blowtorch station by simply rotating the antenna until it fades away. You can also do just the opposite, and bring in a weak station by turning the antenna so it's edgewise to the station.
     
  6. N2DTS

    N2DTS Ham Member QRZ Page

    A way to turn the antenna is my next step, and a more solid mounting.
    I think a 5 gallon bucket filled with cement with a pipe stuck in the middle is one way that will allow experimentation.
     
  7. KK4NSF

    KK4NSF Ham Member QRZ Page

    that would work. I set mine up on a photo tripod for experimentation.... and was surprised how pronounced the nulls were when I turned it.
     

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