Antenna plans that don't make any sense.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC8VWM, Nov 14, 2011.

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  1. KC8VWM

    KC8VWM Ham Member QRZ Page

    Q:

    Why is it always really windy outside when you are outside building antennas?
    :p
     
  2. M0GVZ

    M0GVZ Ham Member QRZ Page

    G3TPW, Steve Webb, is the original designer of the antenna. He lives 20 miles up the road from me. Nice chap, well retired now but still making them. I have one up here.

    This is the same as my genuine from the designer original right down to the way to put the cables on the spreaders:
    http://test.g0mtd.co.uk/CB1.pdf

    OK, The G3TPW design is the original. It uses twin speaker wire with a short part way down each "leg" of each dipole to act as a gamma T match to raise the feedpoint impedance to roughly 50 Ohms. Without that, the feedpoint impedance would be about 12 Ohms. Now the thing is that the shorting point can change dependent on the diameter of cable, insulation, velocity factor. So not only do you have to get the length right, the short needs to be in the right place.

    Steve Hunt, G3TXQ, modified the design. It now uses single core cable making it a whole lot easier to tune because you only need to alter the length. In Steve G3TXQ's design, because you are using single wires the feedpoint impedance is 12 Ohms. So in order to match it to a more radio friendly impedance, he uses a 1:4 Guanella Current Balun. This brings the impedance up to ~48 Ohms and also makes the design slightly more broadband than the original.

    If I were you, I'd build the G3TXQ variant - it is a whole lot easier to tune and will give you more coverage of 20m, 15m and 10m band below 2:1 SWR. He includes a full description of how the original and his works, gives dimensions and instructions on how to build the Balun. More here: http://karinya.net/g3txq/cobweb/
     
    Last edited: Nov 18, 2011
  3. M0GVZ

    M0GVZ Ham Member QRZ Page

    Doesn't work like that. The only difference is one, the original, uses a gamma T match to raise the feedpoint impedance which is why you use speaker wire, the second G3TXQ one uses a 1:4 Guanella current balun instead so you only need single wire. There is no advantage in performance to either. G3TXQ's is a whole lot easier to tune though.
     
  4. KC8VWM

    KC8VWM Ham Member QRZ Page

    The purpose of this thread is to examine the cobweb antenna variants published on the internet, and to determine if there is any validity to their designs.

    Of course, if none of these variants floating around on the internet work as they claim to be, (Hence the thread title, "Antenna Plans That Don't Make Sense") I will be reconstructing it using a known and proven design such as the G3TXQ antenna for example.

    This is "one" of the most common cobweb design variants floating around and it's currently in progress.

    [​IMG]

    Here's another cobweb variant of almost identical construction:


    http://2.bp.blogspot.com/_ZSFuWiwjvpo/SCURg4d3_pI/AAAAAAAABDo/hhxWq26z-a0/s1600/Diagram.jpg


    ...So the question is, "do these actually work?"


    So far the results suggest the wire dimensions indicated for the 20m band folded dipole described in the diagram does actually work however, the results are not yet conclusive because the entire antenna is not yet constructed.

    Another purpose of the thread is to show others the specific details of how to construct such an antenna, including G3TXQ's design or any other similar design they might choose.

    This thread is more or less intended as an online version of an actual amateur radio antenna experiment in progress. Everyone is invited to participate, share their thoughts, personal experiences, comments, and suggestions.

    My Best,
     

    Attached Files:

    Last edited: Nov 18, 2011
  5. KC8VWM

    KC8VWM Ham Member QRZ Page

    Now we will detail the feedpoint pole construction. The orginal cobwebb design calls for a pole with a 1000mm distance however, the position of the feedpoint is mostly determined by where the wires all pull in together and where the actual feedpoint ends up resting in it's final position. So therefore, I am not actually going to cut the feed point pole to any particular final resting place dimension just yet.

    However we can prepare it for final assembly and actually cut the pole in the right place after we locate and determine the actual position it will sit after all the antenna wires are installed.

    First, we need to remove the end cap from the fiberglass rod using a pipe cutter.


    IMAG0050.jpg

    IMAG0052.jpg

    We will now install some shrink wrap on the end. This will protect the end from repeated use.

    IMAG0053.jpg

    The shrink wrap also helps to keep the end fitting snugly inside the hub coupling we are going to use. This is just a 3/4" PVC pipe coupling from Home Depot.

    IMAG0056.jpg

    Next we will start constructing the feed point "box" - antenna connection section.

    IMAG0054.jpg
     
    Last edited: Nov 18, 2011
  6. KC8VWM

    KC8VWM Ham Member QRZ Page

    As mentioned earlier I will be installing grounding bars inside this electrical box outlet to construct the feedpoint.

    The electrical box is lightweight, extremely durable and well suited for outdoor use. The door is sealed with a rubber seal to protect the inside connections from the elements.

    IMAG0059.jpg

    We will need to accomodate 5 antenna positions and 1 feed line on this ground bar arrangement, therefore I purchased a third one just for the extra bus bar screws.

    IMAG0058.jpg

    The photo above shows the 6 screw arrangement (5 for ant. 1 for feedline inside) we need and the longer stainless steel screw is used to attach the bus bar to the box at the last position on the ground bar where the drill bit is positioned below.

    IMAG0059.jpg

    Position the ground bar on the left side of the box and drill a hole through the last position on the ground bar at the rear of the box as illistrated above. The longer stainless steel screw will be used to secure the ground bar in this position.

    IMAG0060.jpg

    The long stainless steel screws are cut using a bolt cutter and were filed down for a custom fit.

    IMAG0061.jpg

    Next we will drill holes in the side of the box to accomodate the antenna wire.

    We can use the third unused ground bar as our guide for doing this part.

    This will ensure the holes drilled on the outside of the box will line up properly with the holes on the ground bar inside. Start with the center and install a screw in this hole as shown below. Drill out the remainder of the holes "except" for the last hole in the rear and the first hole located in the front.

    Remember, there are a total of 7 holes in the ground bar, but all we need to drill are 5 holes for 5 antenna wires.

    IMAG0066.jpg

    IMAG0067.jpg

    Remove the center screw from the template ground bar and this should be your result shown above. Repeat on both sides of the feedpoint box.

    Now remove the ground bars from the box and prepare the feedline by ensuring you feed it though the reducer and the box before finalizing your connection to the ground bar.

    IMAG0072.jpg

    Place a section of electrical tape over the end and poke it through the tape like this and then fold it over. This prevents short circuits from occuring at the feedpoint connection.
    IMAG0073.jpg

    IMAG0075.jpg

    We are now prepared to connect our feedline to the ground bar connection.
     
    Last edited: Nov 18, 2011
  7. KC8VWM

    KC8VWM Ham Member QRZ Page

    **Remember, you can click on any pictue to enlarge***

    Ensure the feedline is routed through the box, before making the feed point connection.

    IMAG0077.jpg

    The feed point connection is made on the opposite side where the stainless steel screws hold the ground bars inside the box.

    IMAG0076.jpg

    The connection is made with the ground bars removed from the box as shown above. Insert each side of the feedline, in each side of the ground bar. Tighten the screws for the feedline connection. Pull the feedline until the ground bars are back in thier respective positions inside the box and reisntall the stainless steel screws to secure them back in place.


    IMAG0069.jpg

    Reinstall the bars back inside the box with the feedline attached and fold over a small piece of left over shrink tubing.

    IMAG0070.jpg

    Insert the shrink tubing between the ground bars inside the box. This prevents any wire pushed through the holes on the outside of the box from reaching the opposite ground bar and shorting out the connection.

    IMAG0078.jpg

    Use a multimeter to check your connections. You can use the outside screws which are attached to the ground bars inside.

    IMAG0079.jpg

    The stainless steel screws located on the outside of the box are useful for diagnosing potential problems with your antenna system.

    IMAG0080.jpg

    Here we are checking to ensure the PL259 and feedline is making proper electrical contact with the grounding bars.

    IMAG0071.jpg

    The feed point box is now completed. Assemble and secure the rubber seal and outside cover.
     
    Last edited: Nov 19, 2011
  8. KC8VWM

    KC8VWM Ham Member QRZ Page

    The balun specifications for the original G3TPW Cobwebb antenna design indicates effective choking when wound into a 4" diameter circle form consisting of 6 turns of coaxial cable and installed inside the box at the feed point.

    This design will consist of the 1:1 air wound balun according to these specifications, but it's placement will be located outside the feed point box instead of inside the box as shown in the G3TPW design.

    Of course, many questions remain in terms of how effective such a balun design is exactly for the intended project and there are certainly better methods which could be utilized to more effectively choke common mode currents from the feedline.

    However, we will proceed with the original balun design as intended, although discussion is most welcomed. :)

    IMAG0081.jpg

    To construct the balun, we will need to raid the cupboards for an object that is 4" in diameter such as this fine tin of chicken chunks. This is why we will be calling this design a chicken chunk balun. :)

    IMAG0082.jpg

    Tightly wind 6 turns of the coaxial cable feedline around the form ensuring you position the appropriate cable length in such a manner from the feed point that it will enable us to secure the finished balun to the feed point pole at a later time.

    When the coaxial cable windings are tightly wound, we will slip zip ties behind the cable to help secure the balun form as illustrated above. Place the cable ties about every 3" as you go.

    IMAG0083.jpg

    When the cable ties are installed cut off the ends and wrap electrical tape around the entire form before removing the chicken chunk can.

    IMAG0084.jpg

    When the can is removed your 1:1 balun should resemble the photo above.

    IMAG0085.jpg

    As mentioned earlier, there should be enough cable length to permit the balun to be mounted directly on the feed point pole using zip ties as shown above. Ensure you create a drip loop as shown, from the feed point location as it leads to the balun.

    This concludes the construction of the 1:1 air would balun for the Cobwebb antenna design.
     
    Last edited: Nov 19, 2011
  9. BX2ABT

    BX2ABT Ham Member QRZ Page

    Excellent couple of postings. Well done and I hope it works first time.
     
  10. KC8VWM

    KC8VWM Ham Member QRZ Page

    I hope so too. We will soon find out. :)

    This lightweight, but yet more durable feed point design arrangement I have constructed for the cobweb antenna, will appear to look something like this when it's attached to the rest of the antenna.

    IMAG0089.jpg

    I admit, the whole thing kinda reminds me of a futuristic looking Klingon Bird of Prey as seen on Star Trek though. Hopefully this antenna doesn't shoot any laser beams.:p :)

    [​IMG]
     
    Last edited: Nov 19, 2011
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