MFJ-1778 80-10m - SWR is very confusing

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD2AMY, Nov 1, 2020.

ad: L-HROutlet
ad: l-rl
ad: abrind-2
ad: Left-2
ad: Subscribe
ad: QSOToday-1
ad: L-MFJ
ad: Left-3
  1. KD2AMY

    KD2AMY Ham Member QRZ Page

    Hi all,

    I recently purchased and put up an mjf-1778. I know it is not efficient in a lot of ways, however, the SWR results I am getting are very confusing and contrary to what I am understanding they are supposed to be.

    I originally had half the antenna in my attic and half outside at an 90 degree angle (an L shape).

    The SWR dips (without an antenna tuner) I was seeing where at
    3.368 =1.45
    7.623 =1.08
    13.300= 1.9
    19.200 =1.15
    27.102 =3.3


    I have since strung the antenna in a relatively straight line across my yard, with the ladderline dropping about 18ish feet ( not enough room to go any higher) and I am getting the following results.


    30.7 -1.6
    23.800 - 2.5
    19.050 - 1.06
    13.330 - 2.7
    7.390 - 1.7
    3.320 - 1.6


    20m band which from what I understand this antenna is supposed to match to is nowhere near these results.
    I am using an air baulin, as recommended by the manual. R8 coax in 10 turns that are 6 inches wide.

    Overall, with the drastic changes made in the antenna positioning, the SWR and frequencies dont seem to have changed all that much. I'm concerned I may have missed something, or there is something else that is wrong.

    73,
    Josh
     

    Attached Files:

  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    That antenna requires a tuner. It is only resonate at one frequency.

    The ladder line needs to be away from everything.


    "Covers all bands, 160-10 Meters with antenna tuner. 102 feet long, shorter than 80 Meter dipole. Use as inverted vee or sloper to be more compact. Use on 160 Meters as Marconi with tuner and ground. Handles full legal limit power. Add coax feedline -- get on the air!"
     
    AK5B likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The MFJ-1778 is intended to be a conventional G5RV doublet.

    The window line really needs to be perpendicular to the axis of the wire for its entire length, which should be around 31-32' long. If it's not, that violates the design and all bets are off.

    I've installed lots of "G5RV" antennas over the decades and always found the window line really needs to drop vertically when the antenna is horizontal, and end above ground; this means the antenna needs to be 32' above ground at the feedpoint as a minimum.

    All linear antennas that have "ends" have their highest voltage (and highest impedance) points right at the element ends, or tips, so those must be kept away from everything other than air, with means good end insulators (the MFJ probably comes with those) and then non-conductive rope for some distance, to keep the wire ends away from everything.
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    AK5B likes this.
  5. KH6AQ

    KH6AQ Ham Member QRZ Page

    I had an MFJ-1778 and found the weak point to be the UHF connector. At 500W, 50% duty cycle it was close to melting down on 17 meters. I removed the connector and connected the ferrite balun directly to the ladder line. The balun was one dozen turns of RG-8X on two stacked FT240-43 ferrite cores. The MFJ recommended air-core balun cannot be effective on 80-10 meters.

    The odd SWR curve is to be expected. One can adjust the length to possibly improve the SWR on any one band at the expense of higher SWR on some other bands. A tuner takes care of it nicely.
     
  6. W4HWD

    W4HWD Platinum Subscriber Platinum Subscriber QRZ Page

    I, too, had an MFJ-1778.

    My 1778 measured 106' on the flat=top and 35' on the 450 ohm ladderline. In case you didn't know, those dimensions are not correct.

    I junked it and used the insulators to build my own, more acceptable G5RV.

    The one I built is 101' flat top with 30.5' of 300 ohm DXE ladderline. At the bottom of the ladderline I hung a BD 1:1 dual-core balun. RG-213 handles mismatch better than RG8X so that's what I use for coax. This system works great on 80, 40, 20 and 17.

    Shoulda skipped the 1778 all together and built my own to start with.

    In any configuration, the G5RV is NOT an all band antenna. Mine presents < 3:1 for the tuner on 80, 40, 20 and 17. On other bands it's ridiculous so...I don't use it on any other bands.
     
    Last edited: Nov 1, 2020
    AK5B and AI3V like this.
  7. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Am I correct in understanding this means the feedpoint is only 18' above ground? That is far too low for any horizontal antenna intended for use on the HF bands. Having the antenna that low, and some part of the balanced line on the ground, is probably only going to lead to frustration.
     
    AK5B likes this.
  8. AI3V

    AI3V Ham Member QRZ Page

    The G5RV was obsolete the day it first apoeared :)

    The whole idea that there is one "magic" combination of dipole length, dipole height, parallel wire length, parallel wire impedance, balun ratio, coax impedance and coax length that will provide a "low vswr" on every hf hamband is a problem that has eluded about 2 million hams around the world for just about 100 years.

    For some odd reason, hams are drawn to ramdomly mixing and matching antenna system components like moths to a flame :)

    Rege

    Here is G5RV's original description of his antenna.

    Please refer to his vswr charts :)

    https://forums.qrz.com/index.php?attachments/g5rv-pdf.269429/
     
    KC3PBI, AK5B and W4HWD like this.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Maybe, Because it works, IF used properly. o_O

    Perfect, NO. Works well, Yes...
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Unsupported opinion, AGAIN! Get a life, Rege...

    A real antenna, measured with a real RigExpert AA-600.
     

Share This Page