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. KI8W

    KI8W Ham Member QRZ Page

    Thinking about getting one of these but I would be a bit concerned what happens with an ice load. We get quite a few of those every winter. I also want to know if it needs any kind of rotator.
     
    Last edited: Nov 24, 2016
  2. K8JHR

    K8JHR Ham Member QRZ Page

    Nice video by an obviously nice guy, and yet...

    I do not believe this is a "loop" antenna, but a folded fan dipole, with five dipole wires connected at a common feed point. The far ends are spaced apart from each other with dielectric material and do not complete the loop.

    I do not believe this antenna provides "3-4 dbi gain" as claimed in the video. MFJ literature says it "packs a 3-4 dB gain advantage over ground-independent verticals" - which is a very different claim, because many ground-independent verticals show a substantial loss, and less than unity gain, compared with a dipole.

    Testing during a big domestic contest, when there are hundreds of high power stations with huge antennas on the air, does not provide a realistic indication of antenna performance. Almost any antenna will work during a big contest.

    I believe the tuning adjustment strips should be installed equally on both sides of each dipole - otherwise one side of the dipole will be longer than the other.

    Testing SWR on the rig by merely transmitting a sideband carrier is not a proper test protocol. It would be much better to show what happens when one transmits a 100 watt CW tone or at least have the technician speak. A sideband carrier with no voice data won't move the meter much.

    Just MY take... your mileage may vary ... I look forward to the direct A/B comparison with the vertical. / JR /
     
    KR3DX likes this.
  3. N7EKU

    N7EKU Ham Member QRZ Page

    Right,

    The cobbweb antenna is not a loop antenna, but a group of dipoles (fan dipole) with their wires bent into a square shape. There are questions on this forum almost every day about bending the wires of a dipole. This is just one that is really, really, bent! You get some advantages and disadvantages -- compared to a straight dipole -- when doing this:

    Cobbweb pluses:
    Omnidirectional so no worries about direction when errecting
    More gain when compared to the nulls of a dipole
    Takes one fourth of the linear space needed
    Needs only one vertical support

    Cobbweb minuses:
    Narrower bandwidth (covers less of the ham band with low SWR)
    Less gain compared to the main lobes of a dipole
    Ice/wind loading becomes more of an issue
    Must use a matching device due to low impedance at feedpoint (included in most designs)

    It's really a handy design for many small city lots. Not really anything unusual or strange, just what you get when you bend up a dipole. If I remember correctly the models I've seen for it, the heighth about ground issue (affecting angle of radiation) is still the same as for a regular dipole.

    73,


    Mark.
     
    KR3DX likes this.
  4. W8AAZ

    W8AAZ Ham Member QRZ Page

    Bet it would outperform a certain "half wave" mini vertical I had here once. Horizontal, probably less noise pickup. Well a square loop antenna worked well for me last summer on 6 meters so I guess I could carry over the same concept to the lower bands with this. No radials like a small vertical might need, single support and lightweight. Might finally be a practical way to get back onto the higher bands for me. Yea, ice and wind might be a factor, need to find more reviews. Does not cover the whole band, well a lot of the multiband verticals don't really, either. He did test it close to the ground. I bet that compared to a vertical, even moderate height increase might lower the pattern for better DX though. He needs to tell us how it works at say, 20 or 25 feet. Cause if you can afford a tower to put it up at a much higher height, you probably can afford a beam or something too.
     
  5. NX6ED

    NX6ED Ham Member QRZ Page

    My pet peave: The operator/assembler when testing the antenna neither listened first to the frequency before tuning nor did he identify himself (former is bad protocol and latter is a violation).
     
  6. 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.
  7. 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
  8. 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.
  9. 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
     
  10. 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 /
     

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