Dipole SWR readings, trim antenna or call it good?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N6WZQ, Apr 14, 2019.

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  1. N6WZQ

    N6WZQ Ham Member QRZ Page

    Finally got my OCF Dipole antenna up this afternoon and managed not to drop anything on my head. (BTW, extending an old galvanized steel mast by hand is a real pain!)
    On top of the mast is a 2m/70cm copper pipe J pole and a couple feet below that is the apex for the OCF Dipole I made based on measurements of the 6m-40m dipole by NI4L. One side is 23', the other side is 45'. I added a little extra to each end to allow for trimming if needed.
    Here is the average SWR across each band as assessed by an MFJ antenna analyer:
    6m-3.0
    10m-2.8
    12m-3.1
    15m-12
    17m-10
    20m-2
    30m-6
    40m-2
    60m-1.8

    For reference I got a reading of 1.0SWR at 5.1Mhz if that helps in assisting with the question of trim or leave it alone.
    So given the above readings and knowing that I will be using an external antenna tuner, should I leave it alone or try cutting and see what I end up with? I would just hate to start trimming and then find out I need to add wire.
    This antenna will be used with an Icom IC-706MKII with an external MFJ antenna tuner.
     
  2. NH7RO

    NH7RO Ham Member QRZ Page

    Uh, oh. Why don't you put it up first and see how it is out of the box? Then start messing with it if not satisfactory...
     
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  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Single-point measurements don't reveal much.

    Better to "sweep" the entire frequency range from 5 to 54 MHz (60m through 6m) and plot impedance vs. frequency over the entire range to see what's really going on.

    Also: If you are measuring this at the end of a length of coax, it's really important to know the "type" (to determine VF) and length of the coax, as any time an antenna doesn't have a native impedance of 50 Ohms with X=0, the coax acts as a conjugate matching transformer which literally changes everything about the antenna; in addition to that, any time the VSWR(50) gets really high like 10:1 or more, coax loss improves the measured SWR at the "shack" end of the line -- and that can be by quite a lot, especially at higher frequencies (12m/10m/6m) since the line loss X2 increases (improves) the measured return loss, which is all the analyzer is measuring.
     
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  4. K7MEM

    K7MEM Ham Member QRZ Page

    How much extra did you add?
    What do you get if you removed the extra you added?
    Are you using the 4:1 balun described in NI4L's antenna?

    Trimming a OCF dipole is not as simple as tuning a fan dipole or trap dipole. If you change the dimensions of a OCF antenna, you also change how each and every band responds.

    Your best bet is to adjust the lengths to correct values, string it up, and just use it. No matter what, you will probably need that tuner.
     
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  5. N6WZQ

    N6WZQ Ham Member QRZ Page

    This was a homebrew antenna. So that I would not have to add additional wire after it was up, I added an extra 1.5' or so to each end. I figured better to start out with too much and need to trim vs needing to add wire and create a weakpoint in the antenna wire.
     
  6. N6WZQ

    N6WZQ Ham Member QRZ Page

    I took measurements at the low end, middle, and high end of each band. I included the average readings to make my initial post more condensed.

    Complete SWR readings are:
    6 meters-low 4, mid 3, high 3
    10 meters-low 2.4, mid 2.8, high4.8
    12 meters-3.2 throughout
    15 meters-low 12.4, mid 12, high 12
    17 meters-10 throughout
    20 meters-low 1.6, mid 2, high 2.5
    30 meters-6 throughout
    40 meters-low 2.4, mid 2, high 1.6
    60 meters-1.8 throughout
     
  7. N6WZQ

    N6WZQ Ham Member QRZ Page

    Yes, using a 4:1 balun. As you stated, in tuning this type of antenna, it will change performance on each band. This is why I was asking this tuning question of people that know a whole lot more about antennas than I do. I'd hate to trim it too much (making performance worse) then need to add wire back.
    I guess what I am wanting to know is, considering this is the first wire antenna I have made, how do the numbers look? Should I just call it good or under the circumstances does this antenna have a lot to be desired and does it really need to be trimmed?
     
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd call it good under the circumstances.

    I'm assuming you plan to use a shack tuner to match into this antenna. You'll need to if you're running a rig with solid state finals on most bands except 60m and portions of 20m and 40m. If so you're in pretty good shape on all bands except 17m and 15m and even those aren't the end of the world depending on what coax you're running and how long that coax run is.

    An OCFD is a pretty ambitious first antenna unless you use a tried and true design and even then only if it's mounted up high and clear from surrounding objects that can impact tuning. Tuning an OCFD to improve one or more bands without worsening other bands is a tricky proposition and usually requires a combo of modeling and field measurements and even then it's hard to get every band matched as well as we'd generally like.

    I'd run with what you've got as long as you have a decent tuner and don't need to run a tunerless rig with solid state finals straight into the antenna. Even a tube rig with a pi output matching network would load on most of those bands without too much trouble though 17m and 15m would still be problematic which doesn't matter much at this point in the sunspot cycle where those bands aren't exactly jumping.
     
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  9. N6WZQ

    N6WZQ Ham Member QRZ Page

    I have 3 tuners at the moment. A new MFJ 949E Versa Tuner II, an MFJ 941B Versa TunerII and another MFJ that I can't seem to find at the moment. My coax run is 70' of LMR400 for this antenna.The peak of the antenna is about 35' off the ground. The ends are about 10' off the ground at roof level.
     
    Last edited: Apr 14, 2019
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  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You should be fine and coax losses will be very low, even on 17m and 15m if your tuner can find a good matching solution which is likely given the 10:1 and 12:1 SWRs you posted.

    On all the other bands the tuner will clean up the SWR a bit but coax losses will be negligible as in a fraction of a dB higher than perfect 1:1 matches on those bands. Even on 30m with a 6:1 SWR you should have a bit less than 1dB total transmission line loss and the other bands are better.

    I wouldn't mess with the antenna any more and just run with it.
     
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