DIYing a QRO cobwebb

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AJ5J, Feb 7, 2012.

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  1. G3TXQ

    G3TXQ Ham Member QRZ Page

    Christian,

    A few observations:

    Have you looked at the effect of that matching network on the other 5 bands?

    You'll find considerable interaction between the 15m and 40m elements on a 40m thru 10m version.

    I adopted the 1:4 balun approach because many people struggled to make the original T-match work!

    73,
    Steve G3TXQ
     
  2. DL1ELU

    DL1ELU Guest

    Hi Steve,
    nice to see you again!

    I have modelled a single 40m cobwebb with t-match so far, and yes I expect severe 40-15 interaction. Will see how it turns out at the end.
    Coming back to the parallel LC-network for broadening, I have calculated the reactance around 5 ohms each, which means the components will suffer high rf currents.

    I think I'll start with a large doorknob C and a coil wound from flat aluminium, any other recommendations welcome!


    73, Christian DL1ELU
     
  3. KC8VWM

    KC8VWM Ham Member QRZ Page

    I feel the "limited bandwidth" statement may be overstated.

    If you wan't a perfect "flat" VSWR over the entire10m band then yes, I suppose this is going to be an issue.

    Remember VSWR readings is not "bandwidth." The useable bandwidth of an antenna is just that. The objective should be, how far can we go up and down the band using this antenna regardless of a "flat" VSWR.

    My experience using the Cobwebb has been that it covers the portion of the 10m band that I am primarily interested in quite nicely and I have no regrets nor am I restricted in some way. Let me s'plain my reasoning why I feel this way.. :)

    For example on 28.000, the VSWR on my Cobwebb is 2:1:1 (Egads!!! I know!! ...lol) The main dip where it's "flat" is around 28.350. Then it rises again where the VSWR is about 2:1.1 at 28.550


    So what this means to me is you are not going to work the 29.600 FM portion of the band using this antenna. Heck, you aren't even going to be able to work on 29.000 either for that matter! But then again, who really cares?... After all, in the real world, the primary 10m activity is either on the CW portion of the 10m band up to around 28.600 on an extremely busy contesting weekend. The largest majority of contacts I ever made on 10m is always between 28.300-28.450. The only time I have heard stations higher in the band is during a world wide contest when the portion of the band below this is too crowded. Even then, I can work up to 28.650 without too much of a problem using this antenna even though I am probobly pushing it to the edges, but to my surprise it still works well anyways.

    Also in terms of actual "losses" we are talking about, ....a 1.5:1 VSWR compared to a "flat" 1.1:1 VSWR equates to 1/22nd of a single S-Unit on the other end. ...Do you really think anyone is even going to notice that?

    Tell you what.., if hams are up in arms fretting over such a monumenatal VSWR difference and such losses, perhaps they try to gain all these losses back by raising thier antenna up another 2 feet to make up the difference. lol

    Like I said before, low VSWR readings doesn't tell the entire story when it cpmes to actual real world usable bandwidth. The antenna works just fine, even with all it's perceived faults just the way it is in my opinion.

    The bottom line is, sometimes we have to try to ignore a certain percentage of what the graphs and charts are telling us about an antenna design, and we have to actually construct, install and use one to get the entire picture. :)
     
  4. KD5SPX

    KD5SPX Ham Member QRZ Page

    I have been reading with intrest the posts on cob webb antennas including the construction articles and have decided to build one myself.

    Today I purchased the 4 durango black widow 13' telescoping poles realizing I can extend less than 4 sections which provides a more sturdy rod for wire mounting while benefitting from the lightweight of the poles. Got these at Wally world for $10.66 each.

    I also purchased a 1/2" 5 ft length of pvc and a 1/2 to 3/4 adapater and a 3/4 pvc type L conduit also at a cost of $1.43 + $.46 + $3.06

    I found a 6 point GE electrical box ground strip and purchased 2 at $3.00 each.

    I am thinking of using THHN #14 and run from inside the L box ground strips out and into the 5ft conduit and all the way back to the base and connecting there with a 1:1 current balun which I have on hand and mount it on the mast soldering the 2 connections on the balun arms which has a so-239 connector at the bottom.

    This will allow me to minimize the weight on the arm and attach the coax to the bottom of the balun and run it down the mast.

    I have not yet decided on the wire I will use or the type of base I am going to use yet. I am thinking aluminum while durable is heavy and I was contemplating some type of a wood base reinforced with a very thin aluminum sheet on top of it.

    I was also contemplating that since I added about 4-5 ft of #14 thnn solid wire going back to the balun this will allow the actual cobwebb wires to be shorter at least by the same length overall. Maybe this will allow me to add another web for 40m band .... just thinking ...

    What is the difference between using just a single wire for each band or using a twin in a folded dipole configuration other the reduction in resistance of 4x? Is there any benefit?

    I was just pondering trying to get as close to 50 ohm as possible without using anything other than the 1:1 balun.

    Your thoughts are welcome.

    Thanks
    Wayne
    Kd5spx.
     
  5. OM5GT

    OM5GT QRZ Member

    My antenna Cobweb (G3TXQ) PSW completely lost when it rains...
    What is your PSV in the rain?
     
  6. AJ5J

    AJ5J Ham Member QRZ Page

    Wayne; I just noticed your post of a week ago and glad I did. You DO NOT want to use a 1:1 balun for a single-wire elements cobweb as the impedance drops to around 12 ohms at the feedpoint with that configuration. Use a 1:4 Guanella (current) balun as outlined in detail on G3TXQ's great website. The first balun I ever homebrewed was one of these except that I made a beefier version for QRO (FT-240-61 cores and milspec teflon RG316) and it was basically easy to construct.

    The advantage of this version over the folded dipole is ease of tuning and overall simplicity (IMO). The folded dipole version will be a bit lighter in weight as it does not require a balun (I would think you could use your 1:1 balun if you wanted, though). I never considered building a cobweb until I discovered the single-wire version of Steve's, then I was determined to build one!

    If you haven't been to Steve's website yet check it out for all the details . Search G3TXQ on QRZ and a link to his site is there.

    OM5GT; I haven't tried my cobweb in the rain here in New Mexico as we hardly ever have rain (worst drought on record, too). I am not QRV much these days but I imagine it would be a minimal change now that I have the balun box well sealed.

    73, Jeff
     
  7. M0PZT

    M0PZT Ham Member QRZ Page

    I recently build a G3TXQ version CobWebb and am rather impressed. It was a fun project to build and as far as bandwidth goes - I'm seeing pretty-much the same as that stated by Steve on his website. 10m is a tricky one - I like to operate PSK as well as SSB, but with only about 330KHz of 3:1 swing, it's tricky without additional "help". Rain/water-droplets will affect the resonance, just like a conventional dipole - It'll probably "drop" by anything up to 100KHz!

    I built mine for just over £50UK. I used black PVC conduit (dum-dum-dumm!) rather than fibreglass - I've yet to see any ill-effects, plus the colour blends in nicely with the roof so doesn't cause too much visual-QRM.

    http://www.m0pzt.com/?cobweb
     
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    Charlie,

    That's a nice write-up!

    73,
    Steve G3TXQ
     
  9. KD5SPX

    KD5SPX Ham Member QRZ Page

    I have gathered everything I need now. I am using a 5" x 8" stud protector plate from home depot for the base it is 20ga galvanized steel strong yet light in weight and am using an EMT connector into the center of the plate with a weatherproof cap sealed with silicone to keep water out of the EMT conduit pole which the base will be mounted on. All 4 poles I got at wally world are super light shakesphere for about $10 each and are 13' long I am thinking taking out 2 sections since they are 4 sections and I end up with a pole about 3/8 to 1/2 inch at the ends very sturdy. I am going to use the twin lead I have and make the folded dipole version and the box and run the wires back to the base and connect to my 1:1 balun mounted under the base on the conduit. From there I will connect with RG8 50ft back to the shack. I mainly operate the phone bands so I will use my MFJ259B to check the tune at about 8 ft when I string the wires.

    Waiting on good weather.

    Thanks!

    KD5SPX
     
  10. AJ5J

    AJ5J Ham Member QRZ Page

    Charlie, M0PZT; Kudos seconded on your great write-up! I really like your elements mounting method and the use of PVC affords easy and safe drilling. I wanted to do the same with my fibreglass spreaders but as we know, that is a no-no.

    Thank you also for the link to Aerial Parts of Colchester; looks like an excellent resource for mounting bits and DIY antenna items, too.

    Wayne, KD5SPX; Sounds like you are all good to go but I am only wondering how you plan to tune the twinlead folded dipole elements. I know a lot of builders use push-pins into closely spaced speaker cable but that won't work with twin lead. Maybe just try shorting the ends and working your way back from there? I suspect it will be a slow and laborious process as the tuning is somewhat affected by the adjacent elements and you have to devise a way to cut away the insulation at the right points so you can short them. At least you have an MFJ to aid you. If I can think of an easy way to tune the twinlead I'll post a followup (remember with your folded dipole version the ends are usually NOT the shorting point so it can get complicated in a hurry---why I went with the TXQ version).

    Btw, when I was tuning my cobweb about 9' above ground I was hearing all kinds of DX; I bet you will be pleased once you have yous all ready to go. Please take some pictures along the way if you can; we'd love to see how it goes.

    73, Jeff
     
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