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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

    The plan is to construct a portable cobwebby antenna.

    Looking over and attempting to construct plans found on the net.


    Constructed the hub. Have the fiberglass rods attached, so now to build something. :)


    ...Ok this looks feasible.

    http://mw0mxt.co.uk/lars/archive/cobweb.pdf

    Started by cutting the 20m antenna section. Plans state the "double" wire (folded dipole loop) should at be total of 34 feet long. Shorted at the ends, with exactly a 3 inch gap at the secured ends and one of the two wires at the feed point measured at the 17' mark (half of the total 34 feet) which is where you are supposed to attach the feedline. So I cut all the wire and make the wire exactly as stated. ...Sounds easy right?

    The drawing also states the wire position should be mounted 5'9" away from the center of the spreader. So I measure exactly 5'9" and attach clamps on each spreader to accommodate the wire according to the plans.

    Well I start hanging all the wire on the clamps at the 5'9" mark, and the wire sag horribly. The whole thing just sags like the antenna wire is cut 6 feet too long, ...but yet it isn't?!

    OK, so I take sure measurements, do some math and find they all fit nice at exactly 6.0" instead. ....But why do the drawing plans state the 34' of wire will fit exactly at 5'9" mark then, if they clearly don't? ...Not even close!?

    So I start comparing a few other articles and find other anomolties....

    Calculations tell me that a 34' folded dipole should resonate around 20m. (which is what SOME but not all of the plans seem to suggest)

    Incidentally 34' is 408 inches. The original cobweb design is here:

    Another cobweb design (the original) also uses a 2 wire setup for each dipole leg but the dimensions seems radically different though.. http://www.g3tpw.co.uk/

    The original design calls for the 20 meter section to be 5139mm or 201.9 inches..?!?! compared to the other design I tried which is 408 inches. So is that supposed to resonate on 20m ??

    ?!?!

    Seems none of the cobweb plan designs correlate in any way with one another. But they are all supposed to work ....right?

    Some contain erroneous information about measurements, and they all seem to lack the "correct" information about where to place wires on each of the spreaders exactly. Even though the wires when cut to the suggested dimensions, they don't fit on the spreaders at the point the plans indicate.

    Here: http://mw0mxt.co.uk/lars/archive/cobweb.pdf

    Other cobweb plan contradictions...

    Some plans suggest the ends should be shorted while other ends on the same antenna should be left open...... (How is it a folded dipole if the ends are left open?)
    10M = Open
    12M = Short
    15M = Open
    17M =- Short
    20M = Open

    See it here: http://domain1809176.sites.streamlinedns.co.uk/cobwebb1_build.htm

    Some plans suggest finding a center sweet spot point in the wire and shorting it out to obtain a good match and others say, with a folded dipole it isn't necessary to do that.

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

    Other designs have a folded dipole on one side of the antenna, and just a straight wire on the other side.. etc etc..

    ...If you're not confused yet, compare these following diagrams with one another for example:

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

    http://homepage.ntlworld.com/andymuza/G7VOT/Cobweb/Cobweb1.html

    http://homepage.ntlworld.com/andymuza/G7VOT/Cobweb/DIAG1.html

    ??

    So will the real Cobbwebb antenna please stand up...

    :D
     
    Last edited: Nov 14, 2011
  2. KC8VWM

    KC8VWM Ham Member QRZ Page

    Guess I should also mention, the antenna which requires (5139mm) 202 inches for the 20m wire section calls for the spreaders to accommodate this wire length to be 8.5 feet long, (http://www.g3tpw.co.uk/Page3CobWebbSpecification.htm ) This design calls for 9 8 feet (3m) http://domain1809176.sites.streamlinedns.co.uk/cobwebb1_build.htm

    While the much longer (34 feet) 408 inch, 20m antenna on this design only requires the spreaders to be just 5'9" long to hold the 20m band wire. (http://homepage.ntlworld.com/andymuza/G7VOT/Cobweb/Cobweb1.html)

    So I guess this means the longer the wire you are using, the shorter the spreaders need to be right?


    ??
     
    Last edited: Nov 15, 2011
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Charles, I never built a Cobbwebb so can't speak from experience at all; however based on your findings, it sounds to me like a 160 meter version could fit in your pocket.
     
  4. KC9QVL

    KC9QVL Ham Member QRZ Page

  5. KC9QVL

    KC9QVL Ham Member QRZ Page

    (5139mm) 202 inches Thats each leg.

    Looks like that 5'9" is off. Even with the feed pointmore toward the center shortening the wire. The corner should be 6' 3 "
    All this (shorted, open, shorted at midpoint) is the wire used. Some plans are using speaker wire. Some are using 300 ohm window line
    Looks to the most consistent would be the window line. Even though that particular plan seems to have bad dimensions.

    Also take a look here goes into the design of the antenna pretty well.
    http://www.karinya.net/g3txq/cobweb/

    He also uses a 1:4 12.5 to 50 ohm current balun for matching
     
    Last edited: Nov 15, 2011
  6. KC8VWM

    KC8VWM Ham Member QRZ Page

    Yes 202" inches seem consistent for each side however if you look at the original G3TPW design "cobwebb revealed .pdf" on page 4 it indicates the length for the 20m wire is 5130mm and that's NOT each leg... but rather the diagram represents the entire antenna size! You will also note one of the wires in that diagram has a break in the center at the feed point in the photo. This break is normal, just not the dimensions.

    I agree the 20m wire position on the spreader (5'9" position) is not right as indicated here:

    [​IMG]



    I think the wire position for the 20m band on the spreaders indicated by G3TXQ is dead on (6.02 feet) and is actually consistent with my own findings in the real world when constructing it out of 34' of "double wire" shorted out at the ends in the following fashion:

    [​IMG]

    The 34' folded dipole loop (408") as shown above is electrically longer and shorted at the ends. This might end up too low for the 20m band, but I guess that might depend on the velocity factor of the wire I am using.

    You have suggested the total length is 404" in the original G3TPW design if we correct the diagram to represent a single side. However it is not shorted at the ends and requires "shorting" points in the wire. This presents a problem if you are not using the exact same wire they are using in the design. (Speaker wire in England might be different? 42 strands of uncertain wire gage with a PVC covering?)

    Incidentally, some are using TV window line and other are using speaker wire. I am making it very lightweight so I am trying antenna rotor ribbon cable. Speaker wire will probably provide more bandwidth, power handling and of course more weight.

    Hmmm.

    However, I am giving G3TXQ's design some thought. Not sure which design is superior (or accurate) at this point. I am thinking a folded dipole design is twice the size and length of the G3TXQ's single wire design. ( although they are physically around the same size) I wonder how this shorter wire length (1/2 the electrical size compared to a folded dipole) affects performance by comparison to one another. For example a folded dipole design might have better bandwidth because 2 wires are bigger than one?
     
    Last edited: Nov 15, 2011
  7. KC9QVL

    KC9QVL Ham Member QRZ Page

    Last edited: Nov 15, 2011
  8. KC8VWM

    KC8VWM Ham Member QRZ Page

    Yes!! Exactly.

    The diagram on page 4 indicates the whole thing should be 5130mm. (202") If you look at the center of the diagram, you will note it shows the correct "break" in the center at the feed point in the photo which suggests this must be the entire 20m antenna length.

    What it should indicate is to cut the whole thing 10260mm to avoid confusion. But it doesn't.

    5130 x 2 = 10260mm

    10260mm = 404"

    The folded dipole design I am working with (closed ends and no shorting points) specifies 408" (34')
     
    Last edited: Nov 15, 2011
  9. KC9QVL

    KC9QVL Ham Member QRZ Page

    Ok so 5130mm on the left. 0 in the middle. 5130 on the right. So 5130+5130=10260 mm or 404 inches or 33 feet.
     
  10. KC8VWM

    KC8VWM Ham Member QRZ Page

    Gee, 33ft... that's about right for the 40m band isn't it? lol

    Ok, so 16.5' x 2 is a halfwave dipole on 20m. Sounds about right then.

    468/14.200 = 32.957 or thereabouts

    My current design is 408" or 34 feet:

    468/13.750 34.03 feet.

    13.750 MHz? .. hmm wonder if the velocity factor of the wire I am using will change things and what direction?

    I should mention the majority of the designs are specifying 34' Whether they are right or not will be seen when I connect the contraption to the rig tomorrow and report my findings.
     
    Last edited: Nov 15, 2011
  11. G3TXQ

    G3TXQ Ham Member QRZ Page

    Perhaps I could just clarify some things about the 2 alternative versions of the cobweb antenna.

    When you take a centre-fed half-wave dipole and bend it into (nearly) a square, the feedpoint impedance drops to around 12 Ohms. Steve Webb - the original designer of the CobWebb - solved the matching problem by making each dipole a twin-wire "folded dipole" which raises the feedpoint impedance by a factor of 4, to 48 Ohms.

    However, when you use twin "figure-8" wire for the folded dipoles you have to take into account the wire's differential-mode characteristic impedance and velocity factor - in other words, how it behaves as a transmission line! I explain the reasons why, here:
    http://www.karinya.net/g3txq/folded_dipole/

    With typical Figure-8 wire you need to place a shorting link some way from the ends of each element, otherwise you can never get a decent match. Put simply: the overall length of the wires determines what frequency you will get minimum SWR, and the position of the short determines how low the SWR will be. Unless your wire has the same dielectric characteristics as another designer, you shouldn't expect to have the same shorting positions as he does.

    My alternative design tackles the matching problem differently by using single-wire dipoles with a 1:4 current balun at the feedpoint to raise the impedance to 48 Ohms. I went that route because I didn't like having to "tweak" the shorting positions on the folded dipole version.

    Don't worry too much about where the wires sit on the support frame, nor what the end gaps are - they are not critical; just do what you have to do to tune the dipoles to your preferred frequencies and get the wires moderately tight.

    Details of my alternative design here:
    http://www.karinya.net/g3txq/cobweb/

    Steve G3TXQ
     
  12. MI6KAK

    MI6KAK Ham Member QRZ Page

    I was down this road a while back deciding which design to go with. I went with Steves design in the end as it was easy to follow and no tapping points to try and get right. You mentioned portable, this design has only single wire per band much easier to take apart and put together. I have to add tuning was easy and it works great for my needs. It has so far survived 70mph winds and no damage. Pic on my page.

    73 Carl
     
  13. KC8VWM

    KC8VWM Ham Member QRZ Page

    Thanks guys for dropping in and thanks for your input.

    Well here's the grand idea. :) This thread will document my progress as I am constructing this thing.

    Basically, I want to use it on the roof of my RV when I am parked. The design I am making (or at least trying to) is very lightweight, yet rugged and I have a storage bag the entire antenna will be stored inside when it's not being used.

    For a quick test run, I assembled the framework with fiberglass fishing poles and installed it up on the roof of the RV just to see what it might look like. I guess my wife thinks it could be used as a neat way to dry our laundry while camping. :) What do you think?

    IMAG0026.jpg IMAG0025.jpg


    I have a portable folding tripod and some lightweight aluminum poles and it will get the antenna about 23 feet in the air when installed on top of my RV. This antenna is intended for portable operating during field day, camping and when parked on top of a mountain sort of thing. So far all this lightweight portable antenna framework, goes up and comes down pretty quickly. About 10-15 minutes.

    I am also using some Dacron line to function as guy wires to secure it all to the rack on top of the RV as seen in these photos:

    IMAG0027.jpg
     
    Last edited: Nov 15, 2011
  14. MI6KAK

    MI6KAK Ham Member QRZ Page

    Looks fine how are you attaching the feed point? also laundry should be removed when adjusting SWR ;)
     
  15. KC8VWM

    KC8VWM Ham Member QRZ Page

    Day 2. :)

    Installing and testing the 20m wire portion of the antenna in the backyard antenna range.

    The design chosen so far is a folded dipole contructed out of twin lead rotor cable wire. Rotor cable wire was chosen because:

    It is very UV resistant in the outdoors.

    It is lighter than TV twin lead or even twin conductor speaker wire.

    Contruction:

    The twin wire rotor wire loop is contructed to a length of 34 feet and it's shorted at the ends. There is a gap of exactly 3" at the end. 2" of the wire is folded over itself and secured with a ziptie to form a small loop to accomodate a piece of UV resistant dacron rope to hold it together at the ends.


    IMAG0030.jpg

    The wire is held to the fishing rod sections at exactly 6' from the center of the hub to each fiberglass pole with these beefy cable clamps. I decided these are much stronger than the idea of holding the wire there using cable zip ties.

    IMAG0031.jpg

    For testing this antenna we are going to need to install some sort of SO 239 connector to attach to the feedpoint. This is just temporary until I contruct the proper feedpoint for the antenna at a later time and will be adequate for testing purposes. So, where do I find an S0-239 for this anyways?

    Oh.. I know just the place to find one... :)

    Everyone should have a well organized toolbox just like mine for such antenna projects.... lol

    IMAG0028.jpg

    Well soldering a wire to an SO 239 conector to serve as a temporary antenna feedpoint sounds easy enough doesn't it?

    IMAG0037.jpg

    Ok let's try again... attempt # 2

    IMAG0036.jpg

    Alright seem solder never melts quite right in the outdoors for some reason.. So let's just go ahead and put a nasty big glob on there instead. lol

    IMAG0035.jpg

    Phew.. well that should do it. Now we can figure out what this wire is doing...

    Incidently, this is the rotor cable wire I am using for the project. I suspect you will have to travel in time back to 1974 to get some though. :)

    IMAG0029.jpg
     
  16. KC8VWM

    KC8VWM Ham Member QRZ Page

    Day 2 and a half...

    Before I post my test measurements, I will highlight some of the construction of this antenna along with some observations.

    The first subject is my apparent obsession with regard to where the antenna wire is located on the spreaders. Mine is located at 6 feet on each side. The plans call for 5'9"

    Could it be this is the correct location based on the idea the feedpoint location (when installed) is going to pull the wire in toward the center?

    Hmmm.. Well anyways. This part of the antenna hasn't been contructed yet but I am betting I am going to have to make adjustments whent it is.

    So here's the main, but sleek lightweight aluminum center hub design I have chosen for this project.

    IMAG0032.jpg

    The rods fit perfectly and are held inside using the eye screws attached on the bottom of the tubes. These eye screws also serve double duty as guy wire support points. (Using UV resistant dacron rope when installed on top of the RV or any other location for that matter in a portable operation scenerio.)

    IMAG0033.jpg

    The ends of the fiberglass rods have been taped to help protect them from damage and helps them to snug up nicely inside the tubes and to keep them from vibrating in the wind. A section of shrink tubing was also installed on the fishing rod and it is too big to fit inside the aluminum tubes. What this does exactly, is it ensures the rods are installed at exactly the right position when fitting them inside the tubes. The rods can't be installed too far inside the tubes or too far out. They will be inserted at the perfect spot everytime. This also demonstrates how the antenna will quickly break down and disassemble for storage.

    The top aluminum pole is only around a foot long or so and it remains permanently attached to the hub assembly. The short mast is simply slid into a longer lightweight mast and so no clamps on the hub are ever required to be removed, disassembled or reinstalled during setup or takedown operation.

    This shows the antenna in its assembly position with the double rotor wire attached and the short mast section which is to remain permanently attached to the hub.

    IMAG0034.jpg

    To raise the antenna higher, we simply add the desire amount of lightweight aluminum mast sections.

    IMAG0038.jpg

    IMAG0039.jpg

    Ok the camera angle makes it look like it's 100' in the air..lol

    The antenna will be tested at this position which is around 15 feet off the ground.
     
    Last edited: Nov 15, 2011
  17. KC8VWM

    KC8VWM Ham Member QRZ Page

    [​IMG]

    So this is the basic design using 34 feet of rotor wire. No shorting anywhere along the wire.

    Results:

    Works everywhere from 160- 2 meters with a 1.1:0 VSWR and I made a 20 over S9 contact with a station located in Alaska.

    ... Well ok, mabey not.. :p

    Anyways, I am happy to report I am pleasantly surprised at the initial results:

    14.005 = 1.5:1
    14.020 = 1.3:1
    14.045 = 1.2:1

    14.050 - 14.170 flat as the top of a proverbial doornail 1.1:1

    14.190 = 1.2:1
    14.225 = 1.5:1
    14.255 = 1.7:1
    14.285 = 1.8:1
    14.315 = 2.0:1
    14.345 = 2.5:1

    Observations:

    I connected the antenna to my FT817 after testing the VSWR with an MFJ antenna analyser.

    My first impression was the antenna was recieving quite well for what it is. I kind of expected much less performance than what I was actually hearing.

    With the antenna located at just 15 feet, I was hearing stations all over the 20m band. The antenna seems quiet and noise levels were almost non existant. I heard a weak CQ registering around 2 s units on my FT817 on 14. 285. It was station KJ4ISH. I have no idea what he was using on his end exactly, (100 watts?) His profile suggests he's a mobile station. I responded back to him using just 5 watts and this antenna. He did respond on my second attempt although, I couldn't complete the QSO. Not really all that surprising or unusual considering the setup I was usingon my end. I was kinda surprised he even responded at all considering his weak 2 s unit signal.

    I was also hearing the usual foreign language stations operating at the bottom end of 20. Mostly at around S-5 to S-7.

    CW and digital stations signals were coming in quite nicely too. S9 + on occasion. I could hear a lot of activity everywhere on 20m. Seemed like the band was quite busy today. Perhaps propogation was just exceptional not sure, but it does seem the antenna is working.

    I never adjusted or pruned the antenna at all. It also seems to exhibit fairly good bandwidth, and I am a bit surprised it resonates where it does considering the rotor wire folded dipole loop is cut to 34 feet.

    ...What are your thoughts?
     
    Last edited: Nov 15, 2011
  18. G3TXQ

    G3TXQ Ham Member QRZ Page

    Charles,

    If it helps - the usual feed position for a 20m thru 10m 5-band version is where the 17m wires are still in the form of a square; in other words the 20m feedpoint gets pulled towards the centre, and the 15m,12m and 10m feedpoints get pushed out away from the centre.

    73,
    Steve G3TXQ
     
  19. KC8VWM

    KC8VWM Ham Member QRZ Page

    Absolutely it helps. It will make the feedpoint construction process a much easier endeavour. I sure do appreciate your expertise and input on this project.

    All comments, suggestions and even criticisms about what I am doing are most welcomed. :)

    On another note, I am expecting the VSWR readings I have recorded to change once some other bands are added but so far, so good. :)

    My Best,
     
  20. MI6KAK

    MI6KAK Ham Member QRZ Page

    Im not sure how your getting the SWR results. The origional Cobwebb has to have a T match (Tapping point) on each folded dipole to meet the 50 Ohm requirement using twin wire due to the shape. If it was a case of just shaping a folded dipole into a square you have to ask why the T match and having to be very accurate about it? or using single wire with a 4:1 balun.

    Not a criticisim and Im no expert, I just cant see how dipoles in a square can not generate a mismatch. I have to say the antenna structure is looking good and good photos this will be a well looked at thread keep it going!
     
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