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The MFJ-1935 Cobweb antenna review and on-air test

Discussion in 'Amateur Radio News' started by KJ4YZI, Nov 21, 2016.

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  1. K5AX

    K5AX Premium Subscriber QRZ Page

    These are not 5 horizontal loops, they are 5 dipoles formed in a square. There is no gain, in fact a little less gain
    than a dipole that is stretched out. But it is a compact idea. But there is NO gain. Hopefully the narator is not trying
    to mislead anyone.
     
    KR3DX likes this.
  2. KM1H

    KM1H Ham Member QRZ Page

    No answer and no surprise either:rolleyes:
    Right from his web page as he pimps whatever he can get:

    Now IF instead of some recent Generic General Class ham playing at doing reviews QRZ could/would appoint a rotating group of hams with the necessary technical education, skills, plus operating skills to do the reviews justice then other non technical prospective buyers could make a good choice.

    Now for the best part he cant even read and follow the forums submission guidelines:eek:

    1. Every article must include a photo. We will typically use the first photo found in the article for our lead-in. If the subject is esoteric, such as an FCC action, then a copy of the FCC logo would be acceptable photo, however in all other cases we would ask that you avoid the use of text in the photo. This is also true for video submissions as well as Podcasts.
    2. Videos posted should include a separate photo as the video itself cannot be used as a lead-in picture. Lead-in photos should be in JPG format and sized to less than 640 pixels wide. The QRZ editors may, at their discretion, place or substitute a stock image for your thread lead-in if deemed appropriate.
     
    Last edited: Nov 27, 2016
  3. N6RGR

    N6RGR Premium Subscriber QRZ Page

    If you follow the link to the MFJ page you will find the same video that was posted here!!!

    Roger KK6IVD

    NOTE the following is also posted "All information, images, and documents on this website are the sole property of MFJ Enterprises, Inc."
     
    W5WAY likes this.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Exactly! Nice summary Mark. The Cobweb is a group of highly folded parallel dipoles and as such it has no gain over a standard dipole but it's pattern is also much closer to omnidirectional with just a few dB of front to side ratio.

    I've been running a Cobweb based on the G3TXQ design for the past six months. It's a very nice antenna in a compact form factor. The slightly lowered gain compared to a traditional dipole (as in 1-2 dB less peak gain) is offset by the near omni pattern so there's no deep pattern nulls off the ends as you'll see with a dipole. The SWR bandwidth is a bit less which is really noticeable on 10m and a bit on 15m and not an issue on the other supported bands. (12m and 17m are really narrow bands and the antenna covers the full 20m band at 2:1 SWR or less).

    I've had the antenna up through several Wyoming snowstorms including some wet and icy early season storms without mechanical problems. The antenna detunes a bit (resonant frequency on all bands shifts downward) during rain storms but not too badly as I see about a 50 kHz shift in resonant frequency on 20 meters during heavy rain which only impacts the high end of the SSB band. As the antenna dries out the normal tuning is restored. However some of those really wet early snowstorms that caked the antenna really shifted the resonant frequency by over 500 kHz on 20m and similar on other bands which basically rendered the antenna unusable until the sun came out and melted off the snow.

    OTOH, it's snowed for the past few days but the storms have been colder and the snow is coming in with much lower moisture content. I ran an SWR sweep of my Cobweb this morning after another cold low density snowstorm overnight and the resonant frequency was within a couple kHz of normal so almost no shift compared to rainy days or wet snow days. I can't say whether the MFJ version of this antenna demonstrates this same sensitivity to moisture but as I believe it's also based on Steve, G3TXQ's design I'd expect that it might. FWIW my other antennas like standard dipoles and ladder line fed Doublet don't show anywhere near the sensitivity to rain or wet snow that the Cobweb does. Still all in all it's a nice antenna to have and its near omni pattern is real handy for stations that are off the ends of my other wire antennas.

    I'd be curious to know what other hams experience in terms of Cobweb tuning on wet days and whether this has something to do with my particular build or the antenna design in general.

    -Dave
     
  5. K8JHR

    K8JHR Ham Member QRZ Page

    Nice summary of your experience, Dave. I appreciate all that, because I am in the process of building my own cob web antenna.

    With regard to gain... the reviewer merely repeats MFJ's claim in its instruction manual. MFJ expressly states this antenna has a "3-4 dB gain advantage over ground-independent verticals" - which, I suppose, says more about the relative weakness of "ground-less" vertical antennas, than it does about the strengths of the cob web. ;-) / JR /
     
  6. NN6EE

    NN6EE Premium Subscriber QRZ Page

    AH FORGET ABOUT THE ANTENNA GUYS!!! THE VIDEO PROVES THERE'S ALOTTA "CUTIE YLs" IN OUR "BOYZ" HOBBY!!! :)
     
  7. SV2EVS

    SV2EVS Ham Member QRZ Page

    Hi everyone,

    Whould this antenna outperforms a homemade dipole with a 1:1 balun ?
     
  8. NV2K

    NV2K Ham Member QRZ Page

    Not in terms of gain. The advantages of the Cobweb design over a basic dipole include:
    1. Multiband (resonance in the five highest ham HF bands)
    2. Compact (fits into area less than 3m x 3m)
    3. Relatively omnidirectional (also a liability; less gain; less quiet)
    A dipole can outperform this antenna on its resonant band in its directions of maximum gain. It is something to consider if you want a relatively compact antenna with a near-omnidirectional pattern that is resonant on these five bands.
     
    K7TRF likes this.
  9. ON6AB

    ON6AB XML Subscriber QRZ Page

    I did some comparison between an AV620 vertical mounted 6m AGL and a cobweb at 10m AGL.
    I compared a lot of stations on FT8 reported signal strenght and carrier strenght on a FLEX3000 panadapter.
    It tuned out that on average, the cobweb has a 4dB gain advantage over the vertical on regular DX (because of ground gain).
    For extreme DX, (Europe to West Coast , VK and ZL) they are on par but only on 20m.
    On the higher bands, the cobweb always has the advantage (because the radiation angle becomes lower).
    Tuning a cobweb is very critical and detuning because of rain can be severe. In my case it was almost 250Khz which rendered the cobweb practically useless.
     
  10. N7TEI

    N7TEI Platinum Subscriber Platinum Subscriber QRZ Page

    Been using the MFJ-1835 for 6 months now. i work exclusively CW and SSB. Spent two days assembling the antenna and over a week tuning each band right where I wanted it. Easy to install and has already survived the Utah weather and a few moderate wind storms without a glitch. Its mounted on a pipe in the back yard at 28 feet without any nearby antennas or obstructions. Overall, it has poor performance on almost every band when compared to a Carolina Windom at 34 feet. Worked several stations and nets with both antennas and am consistently getting better reports when I am on the wire antenna.
     
  11. KK6QMS

    KK6QMS XML Subscriber QRZ Page

    I would love to see bandwidth specs in antenna reviews. A wire dipole can be <1:1 across the band on most bands but anything with a coil etc bandwidth can be very narrow. Bandwidth is a critical data point not sure why it is not included in more reviews. Here is an example from a 4 band vert by Mosely. I chose to show bandwidth below 1.5:1
    [​IMG]
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    [​IMG]
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    [​IMG]
     
  12. KD2JF

    KD2JF Ham Member QRZ Page

    I've been using a 7 band Cobweb made in Poland for a year now. First off they a playing a magic numbers game with the gain figure. A bent dipole cant have 4-5db gain. But it can perform better than an antenna which is lossy to begin with. When compared to my 40M end fed inverted V there are a lot of situations where the V performs better. Esp on the higher bands where the V starts to have gain lobes. The omnidirectional characteristic turns out to be a minus on receive as noise is picked up from all directions. Its seems to work the best on the two lowest bands (20/17) and not so well on the highest. The one thing that drives me crazy about it is the wet. Its SWR is completely useless in the rain. When I drive it with a kilowatt it does well on transmit and I've worked a ton of DX from Japan to Russia. I am looking for a lower noise alternative now. An antenna that becomes a brick in the rain doesn't interest me.
     
  13. K9UR

    K9UR Premium Subscriber QRZ Page

    Since the cobweb uses a 1:4 balun / transformer at the feed point to change the impedance from 12.5 ohms up to 50 ohms I wonder if the solution to a noisy antenna might be to place a home made 1:1 isolation choke using a mix 31 toroid just below the 1:4 feed point. Seems to me all antennas benefit from a 1:1 choke. Including dipoles and this modified cobweb multi band dipole. It might help tuning too.


    I’m considering one to try.
     
  14. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    To be clear this antenna has no gain over a dipole for each band.

    My grandson has one up at about 35 ft and it does quite well, however recent ice storm did break a couple of elements

    73 from,
    The K0UO " Rhombic Antenna Farm" miles of wire in the Air & On the AIR daily
     

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