DIYing a QRO cobwebb

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AK5B, Feb 7, 2012.

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  1. AK5B

    AK5B Ham Member QRZ Page

    Exactly, Charles; it's like trying to devise a formula for a 5-band fan dipole (it doesn't exist, at least in the real world).

    Btw, my cobweb survived another night of very high gusty winds here; suppose that bodes well for it's future as long as I keep the guy ropes taught.

    73, Jeff
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    Actually, 475/F would have been a pretty accurate cutting formula for my final dimensions on every band. Here are the final dimensions vs the dimensions predicted by that formula for the wire length to each side of the feedpoint:

    20m: 201" vs 201.4"
    17m: 157.5" vs 157.5"
    15m: 135" vs 134.4"
    12m: 114" vs 114.3"
    10m: 100.5" vs 100.4"

    But actual dimensions - and therefore cutting formulas -will always depend on many factors unique to a particular build, not least wire insulation thickness and type.

    Steve G3TXQ
  3. W7KKK

    W7KKK Ham Member QRZ Page

    But look at it this way, if it worked the first time just using what we know, or think we know, it would be much fun anymore.
    I think building things is part of what we are as a hobbyist. And those that are just operators just don't get it.
  4. KC8VWM

    KC8VWM Ham Member QRZ Page

    Interesting Steve.... Just for fun, here's my own notes for comparison:

    Resonant @ 22.100 therefore x = antenna formula

    x = 22 feet X 22.100 Mhz

    x= 486.2 length.....frequency..actual observed VSWR

    10m = 17' 486.2/freq = 28.600 1.3:1
    12m = 19' 486.2/freq = 25.590 1.2:1
    15m = 22' 486.2/freq = 22.100 1.1:1
    17m = 26' 486.2/freq = 18.700 1.2:1
    20m = 34' 486.2/freq = 14.300 1.1:1

    The above reflects the "plans" actual measurements for constructing the antenna and actual VSWR readings.

    Based on this formula my design should be as follows:

    band.. design frequency...rotor wire length

    10m = 28.450 205.1" or 17.09'
    12m = 24.960 233.7" or 19.47'
    15m = 21.362 273.1" or 22.75'
    17m = 18.140 321.6" or 26.8'
    20m = 14.285 408.4" or 34.03'

    The following reflects the changes required based on this "new formula" vs. the original "trial and error" design:

    10m = add 1.1" (optional)
    12m = add 5.7"
    15m = add 9.1"
    17m = add 9.6"
    20m = add 0.4" (optional)

    ...or at least so it goes with the rotor wire folded dipole version anyways. :)

    I think some factors come into play such as wire gap spacing between the element ends, Vf of the wire used and spacing of the elements and how they interact with one another is most likely some of the reasons I could not hit the moving target when attempting to devise a formula for it's design in my case anyways.

    So that's basically how I came up with this 486.2/F figure, but I agree every build is somewhat unique in this respect.
    Last edited: Sep 18, 2012
  5. KC8VWM

    KC8VWM Ham Member QRZ Page

    Well let me be the first to tell you it isn't going anywhere. I left my 40m verison cobwebb (that great big thing the neighbors keep gawking at and asking questions like do I belong to the government lol :)) and I am pleased to inform you that it just survived a very,very bad wind storm here.

    The storm had very strong sustaining winds. Sometimes the winds reached in excess of 60 mph. It took out a large tree limb in my backyard ( pretty much half of an entire tree I have yet to cut apart with a chainsaw and remove from the yard.)

    ...and yet my antenna survived just fine. I thought it was a gonner for sure and I kind of squinted a little expecting the worst when I looked up at it.

    ...But when I looked up and behold to my surprise, the huge 40m band cobwebb contraption was actually still up there doing fine. Needless to say a smaller cobwebb should survive on the roof of your car while driving 85 mph on the interstate, but don't quote me on that ok? lol

    Last edited: Sep 18, 2012
  6. AK5B

    AK5B Ham Member QRZ Page

    If there are ever any videos turning up on You Tube with a guy hauling down the higway with a cobweb stuck on the roof I'm sure they will show a certain fellow by the name of Cobweb Charley behind the wheel...
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    I would expect the L*F relationship to be much more complex if you use a folded dipole without moving the shorting links inwards, which I think is still your configuration? See Section Bi on tuning, here:

    Steve G3TXQ
  8. OM5GT

    OM5GT QRZ Member

    How could for the 10 m band put two dipoles one for CW and one for SSB ?
    They would be so good. I use it on 80 meters CW and SSB band and have PSV OK 2 times. Regards Robert OM5GT
  9. AK5B

    AK5B Ham Member QRZ Page

    Not so sure that two 10m elements would play well in a cobweb due to the interaction with the other bands. Still, it's certainly worth a try.

    Might work if you eliminated the 12m elements altogether.

    In my opinion, though, a 10m Moxon will work much better and tune a lot wider. That is what I'm using now for 10m and usually use the cobweb for 12 through 20.

    73, Jeff
  10. DL1ELU

    DL1ELU Guest

    G'day everybody,
    just found this interesting thread when searching for cobwebb designs. I'm Christian DL1ELU and manufacture folding hex beam antennas, maybe some of you already are aware of... :eek:

    Some potential customers asked for a small nondirectional multiband antenna atop a single pole, covering 40-10. I think the cobwebb principle works for this, and it would fit the fiberglass frame I already use for the hex.
    I did some modelling with mmana and found the single wire dipoles could be matched to 50 ohms with a t-match, overcoming the ferrite balun. The only drawback of the cobwebb is the narrow bandwidth, which some of you have already experienced.

    Simulating around with L/C matching circuits, I found that with a L and a C across the feedpoint and extreme L/C ratio (e.g. ~4700 pF and ~0.1 uH for 40m) the bandwidth can be extremely enhanced, as the parallel resonance circuit just equals out the antenna's reactance. Now I have the slight feeling that in reality this won't work because of high losses in the LC circuit. Am I right or is it worth a try?

    Best 73, Christian DL1ELU
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