MFJ-259C readings jumping all-over on skywire loop

Discussion in 'General Technical Questions and Answers' started by K9SPY, Dec 29, 2015.

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  1. K9SPY

    K9SPY Ham Member QRZ Page

    ...but when I hook back to my 10 meter wire dipole or my 2 meter ground plane, it works like it should. Loop is approx 280 ft for 80 meters. I wanted to test and maybe trim it... repeat... etc... but when I hooked up, I got readings all over. At first I thought it was the wind, as it was a windy day and the wire WAS kind of bouncing up and down between the three trees and one long swaying pole... even the tower at the feed-point (fifth and final support point) was swaying ever so slightly, so I chalked it up to movement. But its still doing it right now and the air is now still... and my other two antennas still read as they always have.

    Batteries are fine.

    Wire is 14 ga stranded THHN...
    supports are all commercially purchased plastic antenna holders...
    each is supported by a least 20 feet of dacron rope and not directly tied to metal or with metal...
    feedpoint is 4x4 Cantex box with eye-bolts,
    and inside the box one eye-bolt goes to center of an SO-239, and the other goes to the body of the SO-239... RG-8X jumper to MFJ-259C from there.

    Also makes same readings from shack over 80 ft of RG-8X... they jump all over. never settling near a measurement.

    Really baffled.



  2. KA0GKT

    KA0GKT Ham Member QRZ Page

    Are you close to an AM broadcast station? The antenna analyzer's circuit may be overloaded by a strong out-of-band signal.
    N0SYA likes this.
  3. K9SPY

    K9SPY Ham Member QRZ Page

    Thanks for the blistering fast response Steve...

    so, more info...

    I tune the analyzer to 3.75 Mhz... and this is what I get

    SWR - 3.0 > 3.4 > 13.0 > 10.8 > 3.2 > 9.4 > 6.3 > 5.1 > 11.0 > 3.9

    R - 0 > 0 > 0 > 2 > 0 > 0 > 5 > 1 > 0 > 0 > 3 > 0 > 4 > 0

    X - 55 > 52 > 66 > 53 > 59 > 52 > 62 > 57 > 55 > 65 > 58

    each ">" denotes just a split second... it just pops around all those readings...


    if I move the freq to an area that should be way out for the loop.. it of course goes to SWR >25 and stays there...
    but the R stays at zero with only occasional pops into the single digits always popping and going right back to zero
    X moves around as I think it should... but always jumps within a 10-15 point range... also never settling

    I do not get the same actions on my other antennas from my same shack bench...
    though I can imagine that said potential AM station could be marshalling via this particular loop to overload the MFJ...

    I also tried my other antennas at the same frequency (3.75 MHz) even though that is not their intended freq, just to make sure its not just the MFJ itself at those frequencies...

    Just for kicks, I just grounded the MFJ to my common ground here in the shack...
    it made the R number rise significantly... it still jumps all over and back to zero, but now it spends most of its time NOT at zero... ranges form a low of zero to a high of 16..
    at the same time X is still generally ranging over a 10-point range... but SWR is SIGNIFICANTLY stabilized - now jumping only from 2.0-7.0...

    I live in fairly urban/suburban San Antonio... many broadcast stations around...
  4. W0BTU

    W0BTU Ham Member QRZ Page

    That's likely the problem. It's a very common one.
    N0SYA likes this.
  5. WA4SIX

    WA4SIX Ham Member QRZ Page

    Try checking with an SWR meter & plot the response every 25-50 Khz. Use low power for this.
    You also may have a bad coax connection too.
    Many use eye bolts for suspension, but you should have screws & nuts for the actual electrical connection.

  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The R 0 numbers are a bit baffling. Sounds like a flaky connection that is occasionally short circuiting.

    I'd try disconnecting the coax from the loop and measuring the cable itself for intermittants using an Ohmmeter while flexing the cable near each connector.
  7. N0SYA

    N0SYA Ham Member QRZ Page

    Put a field strength meter to the loops output.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The MFJ is more sensitive than a passive FSM, although that might work.

    Even better, a 'cheap' and passive FSM would reveal any antenna intermittancies if you just transmit into the loop and use an FSM to detect the radiated power. That doesn't require much of an FSM...the cheapie diode detector plus analog meter type would work fine.
  9. KA0GKT

    KA0GKT Ham Member QRZ Page

    3750 KHz is the third harmonic of 1250 KHz. which happens to be the frequency of KZDC, the center of their directional array is at 29-29-50.0 N 098-24-53.0 W...that's just about 2 1/2-miles airline from your QTH. They are 25 KW Daytime (that's 100 KW PEP at 100% modulation peaks, Broadcasters often have a peak positive modulation of 115%) and 920 watts nighttime.

    The bouncing of the readings is probably caused by the modulation of the AM stations carrier.
    K9SPY and W0BTU like this.
  10. K8JD

    K8JD Ham Member QRZ Page

    I see the Kirby area is home to many of San Antonio's AM transmitter sites.
    Several medium power stations (5000 W) are nearby.
    KTSA is only 2.71 miles from K9SPY,
    KONO is 2.02 Mi
    KCHL, 3.58
    KLUP, 4.03
    KAHL, 4.37
    One 10 kw station and two 50000 W stations are not far,
    KSLR is 10000W at 5.42 miles, but only operates in daylight hours, KTKR is 5.1 miles and WOAI is 16.23 miles away, both full 50000W. .
    This might be a factor in your varying readings !
    K9SPY and W0BTU like this.

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