SWR on Cobweb

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF7ES, Jun 13, 2019.

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  1. KF7ES

    KF7ES Ham Member QRZ Page

    Hello all,
    I've been tuning my 6-20 cobweb the best I can, and have the following results:

    Band, bottom of band swr, top of band swr

    20 = 2.9 / 3.8
    17 = 2.4 / 1.1
    15 = 5.2 / 4.2
    12 = 2.1 / 3.8
    10 = 6.5 / 2.3
    did not bother with 6

    I have the antenna at 35' (pic on qrz profile). I've heard from about everyone I talk to that other cobweb users have a 2:1 or better swr across the entire band, on all bands (except maybe 6 and 10). I'm curious what I might be doing wrong. Also with such a wide range of SWR across the bands, is my tuner really helping or just making me feel better?

    Thank you,
    Orion, KF7ES
     
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Commercial? Home Brew? Model?

    Have you tried adding a 1:1 Current Balun at the feed point?
     
  3. KF7ES

    KF7ES Ham Member QRZ Page

    It is the MFJ-1836H fed with 50' of RG213/u. No balun.
     
    KA0HCP likes this.
  4. KI8DJ

    KI8DJ Ham Member QRZ Page

    Looks to me like all elements except 12 are too long, swr better on low end. Try shortening them starting at 20 m and work your way up. Except for 12 m which likely is short.
     
  5. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    Element lengths may be the issue.

    I have a home brew model with an SWR of < 1.5:1 on every band. In fact, 1.5:1 is the highest reading for that matter.

    10m will not cover the entire band, you should have it tuned for an acceptable SWR in the area of interest on the 10m band. For example < 1.5:1 from 28.200 - 28.500 sort of thing etc.

    You may also have another issue such as a bad feed line problem, a poor connector issue or a short somewhere causing your SWR problems. This is where an antenna analyser could be connected directly to the antenna feed point to rule out the antenna as the problem. Similarly, a dummy load could be connected at the end of the feed line instead of the antenna to rule out the feed line as your possible problem.

    etc.
     
    Last edited: Jun 13, 2019
    KG7QJB, NH7RO, 2E0TWG and 1 other person like this.
  6. AI3V

    AI3V Ham Member QRZ Page

    How far is the antenna from your roof, and what is the roof made of?

    Also any metal under the roof, like foil backed insulation?

    Rege
     
    NH7RO and KC8VWM like this.
  7. SP3L

    SP3L Ham Member QRZ Page

    Could you write what type of coax you use and how long it is? If this is a short low loss coax cable, the results are quite normal for the Cobweb...
     
    AH7I likes this.
  8. KF7ES

    KF7ES Ham Member QRZ Page

    The Cobweb is fed with 50' of RG213/u. The roof is wood and it is mounted with a tripod and a 5' steel mast. No foil backed insulation.
     
  9. SP3L

    SP3L Ham Member QRZ Page

    That's what I thought. You have a low loss cable and it is quite short. Therefore the SWR at the TRX end is not so much lower than the SWR at the antenna feed point. The values are absolutely OK for the Cobweb. If you antenna tuner can tune your TX output with the current SWR values - no need to worry. You do not need to trim 20m, 17m and 15m bands. You may slightly improve the 12m bands by trimming it shorter by half an inch or so. As to the 10m band - do not expect that Cobweb can cover a big part of it. You have to decide which part of the band is most important for you and trim this radiator accordingly. For example, I would make it slightly longer to cover the CW part but your preferences may be different.

    If you have had long lossy cable, you would have had lower values of SWR at the TRX end of the coax. But this would not be anything to be happy about.

    So, the antenna and the feed line are OK. Just do the last trimmings and enjoy your Cobweb.

    73
    Jacek
     
    AH7I likes this.
  10. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    I respectfully disagree these antenna SWR readings are normal for the Cobweb.

    I have designed and constructed them, taking measurements with an antenna analyser directly at the antenna feed point with no feed line attached to the antenna whatsoever. Like any other antenna, the feed line should have very little or no effect on SWR readings and if it does, it suggests there's an issue occurring with something.

    So the issue here is either most likely related to the attached feed line which is causing a problem, or the antenna element lengths are in need of adjustment.

    Diagnostic approach:

    1. What is the SWR reading with a 50 ohm dummy load attached to the end of the feed line instead of the Cobweb antenna?
     
    Last edited: Jun 15, 2019
    K0UO, 2E0TWG and NH7RO like this.

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