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

    Other snags so far.

    First antenna rotor wire batch works perfectly fine. Second batch seems to fray away from it's insulation. I had measured and cut 3 different antenna elements for the 10m band, and each wire length I cut kept fraying from it's insulation. *click photos to enlarge*

    IMAG0126.jpg

    IMAG0127.jpg

    If you look closely you can see the copper shining through the easily torn insulation.

    IMAG0128.jpg


    I still had some wire from another roll of rotor wire I was using and I went back to using that "wire batch" to finish the 10m antenna element.

    Apparently, the quality of the insulation seems to be different on different rolls, despite the idea they are the same brand of antenna rotor wire.

    Might be an idea to test your rotor wire and ensure it doesn't fray away from it's insulation before using it. The other antenna rotor wire I was using works perfectly fine with no such issues occuring at all.

    IMAG0109.jpg
     
  2. KC8VWM

    KC8VWM Ham Member QRZ Page

    Methods used to prepare antenna elements.

    Precise measuring was accomplished using a Lufkin tape measure, like this.


    [​IMG]



    This tape measure has a hook on the end which can be used in combination with a screwdriver to hold it in the ground. The rotor wire is punctured on the end with the screwdriver so both the tape measure and rotor wire are pinned into the ground.

    IMAG0105.jpg

    IMAG0106.jpg

    Below we are measuring and cutting the rotor wire for constructing two band elements at the same time.

    IMAG0108.jpg

    This is 5 conductor wire, so two wires are measured for the 26 feet needed for the 17m band element and cut.

    IMAG0110.jpg

    Next we peel back the 17m element (hanging down) but stop along the way, and cut another two wires here at the 22' mark for the 15m band element. This leaves one remaining wire left which we will not use.

    The antenna element pairs are separated from the ribbon cable.

    IMAG0109.jpg

    Fold each of the antenna elements exactly in half and slightly separate the two wires at the proposed feed point connection location in the middle.

    This can be accomplished by poking the end of a pair of scissors through the wire to separate them as shown below.

    IMAG0112.jpg

    Using your scissors cut though one of the wires and this connection will represent your feed point connection made to the feed point box shown below.

    IMAG0113.jpg

    The two sides / ends of the elements are stripped to expose about 1/8" of bare wire and the wires are twisted together. This forms the completed folded dipole loop.

    IMAG0111.jpg

    The antenna ends are soldered together and then a section of tape is folded over them to protect the soldered connections.

    IMAG0122.jpg

    Recap: Making the feedpoint connection:


    IMAG0113.jpg

    Connect the two open wires shown in photo above, on each side of this feed point box below:

    IMAG0100.jpg

    Tighten all connections using screws inside feed point box.
     
    Last edited: Nov 26, 2011
  3. KC8VWM

    KC8VWM Ham Member QRZ Page

    Rambling thoughts..

    Although all the elements are now installed, it got too dark to complete tying the antenna ends together with the proposed 3" gap spacing as the plans suggest. I thought I would do that part today, but it's raining and cold outside. <sigh>

    I found the plans for this variant design are all the same dimensions. They all specify the same wire lengths, same location installed on the spreaders and 3" gap spacing at the ends.

    Here's a few examples all specifying the same dimension information:

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

    http://homepage.ntlworld.com/andymuza/G7VOT/Cobweb/Cobweb1.html
    http://domain1809176.sites.streamlinedns.co.uk/cobwebb1_build.htm
    etc...

    It's amusing to read elsewhere the antenna requires a tapping point and yet many design variants are reported to be functioning without one.

    It's also interesting in my earlier 20m wire version testing, it seemed to work without any impedance transformation. I understand the theory of folded dipoles, but what I don't understand is why this design works in practice, (at least during initial testing) but in theory, the chosen design variant really shouldn't work at all.

    Seems these same exact dimensions have been repeated over and over again and have migrated into many other peoples web sites who have constructed this same antenna using these dimensions with reported success. ...But why does it work when theory tells us an impedance transformation is required?

    The feed point should exhibit an impedance of 280 ohms and I should be experiencing losses, electrical reactance, capacitance issues, a high VSWR and other issues etc..but it doesn't seem to be functioning that way, if that makes sense. I know it isn't magic, but what's going on here exactly and why are these construction plans repeated with success?

    Of course, more testing will be conducted when construction is complete, but so far the initial limited testing results seem quite favorable for some apparent reason.
     
    Last edited: Nov 26, 2011
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    Charles,

    Forgive me, but I believe you continue to misunderstand how this antenna works - there is no conflict between the theory and the practice!

    When you take a half-wave dipole and bend it almost into a square shape, its feedpoint impedance drops from the nominal 72 Ohms to around 12 Ohms. By constructing it as a 2-wire folded dipole the feedpoint impedance is quadrupled, bring it back very close to 50 Ohms. [I've no idea where you got your figure of 280 Ohms from.]

    The only thing slightly unusual about your build is that you are seeing a low SWR without using inboard shorting links. As I mentioned earlier, and explain in detail on my web site, the need for those links depend on the characteristics of the particular twin lead you are using; some types of lead - including yours apparently - work satisfactorily with the links right at the tips.

    73,
    Steve G3TXQ
     
  5. KC8VWM

    KC8VWM Ham Member QRZ Page

    Last edited: Nov 26, 2011
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Charles,

    Yes, when it's in a straight line; but when you bend it into a square shape the feedpoint impedance tumbles by a factor 4 or more, just like the single-wire dipole impedance drops from the nominal 73 Ohms to about 12 Ohms.

    And just to pick up on some of your links:

    1) The MW0MXT link shows a design which uses wide-spaced 300 Ohm ribbon as the twin-lead; theory tells us that wont require inboard shorting links

    2) If you read the third link fully you'll find that the builder used Figure-8 twin-lead and had some pretty poor SWR figures without the inboard shorting links. Then he built a MK2 version with the inboard shorting links and achieved SWRs in the range 1.1:1 to 1.3:1
    http://domain1809176.sites.streamlinedns.co.uk/cobwebb2_build.htm

    73,
    Steve G3TXQ
     
  7. KC8VWM

    KC8VWM Ham Member QRZ Page

    Do you think twin rotor wire is close to 300 ohms?
     
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    Charles,

    Take a look at the second chart here:
    http://www.karinya.net/g3txq/folded_dipole/

    That shows the best SWR you might expect to achieve on a folded dipole, for a range of different wire characteristics, when you don't use the inboard shorting link. Look carefully and you'll see that it's a combination of wire characteristic impedance and velocity factor which cause the problem, not just the characteristic impedance.

    You could have a wire with a characteristic impedance as low as 100 Ohms (the solid red line) and still not have a problem, provided its velocity factor was higher than about 0.85. But if you have a characteristic impedance of 100 Ohms and a Vf lower than about 0.8, the minimum SWR is beginning to climb rapidly.

    The wire which the builder in your third link used typically has a Zo of about 100 and Vf of 0.7 or lower, so he could have expected SWRs in excess of 2:1 if he'd looked at that chart.

    I don't know the characteristics of your rotor wire, but I would expect a Zo in the range 100 Ohms -150 Ohms. The fact that you don't need inboard shorting links suggests its Vf is quite high.

    It's this variability in wire characteristics, and the lack of understanding about their implications, which led me to the single-wire design.

    Edit: graph included here to avoid any ambiguity:
    [​IMG]

    73,
    Steve G3TXQ
     
  9. KC8VWM

    KC8VWM Ham Member QRZ Page

    I know I mentioned it earlier in the thread, but the fact this must work must be due to the combination of Vf of the rotor wire I am using, in combination with the shorting links I have on the ends of the wire. Seems we both agree on this result.

    I have no idea why I was thinking (rambling about) it should be 280 ohms at the feed point. I forgot all about winding it into a square... I guess I am thinking about it too much. :)

    It's not really that I am misunderstanding it so much as I am somehow anticipating it's not going to work as I might think it will... I guess we will just have to see what happens when the feedline is connected to the rig.

    Looking at your chart supports the theory regarding the possible velocity factor of the wire I am using. However, I am not sure if this wire is controlled or maintained during the manufacturing process from one batch of rotor wire to another in actual practice. Guess the same thing could be said for any other type of wire used for that matter.

    I was thinking about designing an adjustable/variable shorting link. That is to say, the design suggests a shorting link should be placed in the vicinity of 40 - 60% distance away from the feed point depending on the wire used I guess. I know we could approximate the location using calculations if the wire variables are known, however what if the variables are not known? This is where such a designed device would really serve beneficial.

    "What if" we designed a set of exposed aluminum rails at this location with a mechanical sliding adjustable link between the two? A type of adjustable hairpin / shorting link matching device. Another rambling thought I was thinking about to confuse myself even further was to construct it using your single wire design using a matching transformer at the feed point, but installing two conductor elements anyways for the extended bandwidth. Not sure how this approach might diminish the efficiency factor exactly, but I suspect the costs might be worth it especially on some of the wider bands like 10m. "What if" one of the conductors was slightly longer than the other conductor on the same band? A mini fan dipole scenerio... but designed for one band, but each conductor could be individually designed for the upper and lower portions of the same band... Just thinking... or confusing myself.. not sure.. :)
     
    Last edited: Nov 27, 2011
  10. KC8VWM

    KC8VWM Ham Member QRZ Page

    Somewhere I was reading this antenna design also works on 6m and 2m as well. ...Can anyone else confirm this observation?
     
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