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. G3TXQ

    G3TXQ Ham Member QRZ Page

    I would expect the batch to batch variation of Zo and Vf to be small enough not to matter in this application. That makes it an attractive design option for an antenna supplier who only needs to characterise the wire once; but it becomes problematic when individual self-builders choose their own wire types.

    If you have an analyser it's very easy to measure the wire Zo and Vf

    Sounds like a lot of trouble, and it would introduce an impedance discontinuity over a significant length of the antenna. If you can't characterise the wire a simpler approach is to use a pin pressed through the insulation to short the pair; keep moving the pin(s) until you get a good match.

    Sounds like a workable idea - if I get a chance I'll try that on my version.

    It was once a popular antenna for VHF, nearly always built from self-supporting aluminium tube for the folded dipole elements and in the form of a circle; often referred to as a "Halo" for obvious reasons. When it was bent into a square rather than a circle it was referred to as a "Squalo". Performance is exactly what you would expect from a simple horizontally polarised, omnidirectional antenna: about 1dB down on a half-wave dipole.

    73,
    Steve G3TXQ
     
  2. KF7NUA

    KF7NUA Ham Member QRZ Page

    Charles - did you make the center hub or purchase it?
     
  3. KC8VWM

    KC8VWM Ham Member QRZ Page

    The hub was purchased at a local farm supply store.

    I am not sure of the correct "name" this hub is called.

    Here's exactly what I am using:

    [​IMG]

    http://www.canopies4less.com/servlet/the-139/1"-PREMIUM-FLAT-CANOPY/Detail

    The hub I am using is basically originally intended to hold 1" diameter poles together to construct a frame. The frame, when constructed can hold a tarp, green house or similar temporary shelter of some kind.

    They are very sleek and lightweight, but yet very strong and rigid. They are well designed and originally engineered and intended for erecting structures involving weight bearing structural stress. They are also well suited for long term use in outdoor enviroments.

    They also happen to hold 12' fishing poles for constructing antenna's quite nicely too. :)


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    Last edited: Nov 27, 2011
  4. KC8VWM

    KC8VWM Ham Member QRZ Page

    Understood, but I guess I wasn't clear. :) I was wondering if people are using "this" particular antenna design on 6m and 2m. I was reading, it is also resonant on those bands without adding any other additional elements for those bands to this existing design. In other words, is this antenna capable of functioning as a 7 band antenna.
     
  5. G3TXQ

    G3TXQ Ham Member QRZ Page

    Charles,

    I've read reports that folk see SWR "dips" on other bands, but that doesn't necessarily mean it's an effective antenna.

    I suspect that some of those extra "dips" arise from CM current on the feedline - a choke that's effective on 20m is unlikely to be very effective on 2m. Even if it IS the main array that is being excited rather than the feedline, I would expect significant deviation from an omnidirectional pattern; there may also be a loss issue at the higher frequencies because the quarter-wave twin-lead stubs across the feedpoint are likely to be lossy.

    Could you make contacts on those bands? Sure!

    73,
    Steve G3TXQ
     
  6. KC8VWM

    KC8VWM Ham Member QRZ Page

    Still some wire spacing on spreader measurement issues to resolve but still... :)

    IMAG0151.jpg

    IMAG0154.jpg

    IMAG0153.jpg

    IMAG0152.jpg

    IMAG0150.jpg
     
  7. KC8VWM

    KC8VWM Ham Member QRZ Page

    [​IMG]

    The most common cobweb variant.

    Observations and problems encountered with the above design.

    You will notice the wire spacing for the gap is specified at 3" on every wire.

    Here's what my wire spacing gaps looks like on my antenna:

    IMAG0146.jpg

    Following the common design plans above, it is impossible to place the wire on the spreaders in their specified locations and somehow have sufficient wire to create a 3" gap between each element.

    The gap spacings seen in the photo above make more sense.

    These measurements are from a modified cobwebb variant version seen here:

    http://domain1809176.sites.streamlinedns.co.uk/cobwebb2_build.htm

    Gap spacing seen in the photo above.

    10m = 2.75"
    12m = 5.11"
    15m = 7.87"
    17m = 11.02"
    20m = 15.74"

    These gap spacings as specified make more sense and are "somewhat" compatible with these following wire locations on the spreader:

    IMAG0149.jpg

    Measured from the center of the hub to the wire located on the spreaders as seen in the photo above:

    10m = 34.4"
    12m = 40.7"
    15m = 47.4"
    17m = 56.8"
    20m = 72.9"

    I say "somewhat" because it seems you need to make slight adjustments in order for the wire to end up taught which deviates from the measurements. That is to say, you are not likely going to end up having the wire located on the same spot on each one of the spreaders in the same exact way.

    For example, the 20m wire spreaders located directly adjacent to each side of the feed point will require the wire to be slightly extended closer in than the actual measurements indicate. This is due to the "V" shape of the 20m element.

    Study the photo:

    IMAG0152.jpg

    Part of the problem seems to be related to the idea the feed point itself pulling the wire toward the center does not permit everything to be equally square or symmetrical for some of the wires placed on some of the spreaders.

    On the other hand, the rear spreaders are more or less just a straight line of wire, therefore the rear spreaders will be "closer" according to the actual measurements.

    IMAG0153.jpg

    Similar issues occur with other bands related to the measurement location indicated for the wire placement on the spreaders.

    It is not clear how these variations of wire placement on the spreaders is critical or not in terms of performance, if at all. However it appears none of the plans seem to explain this issue very well.
     
    Last edited: Nov 28, 2011
  8. KC8VWM

    KC8VWM Ham Member QRZ Page

    The antenna is very compact, takes up little space, but yet each element is a full sized dipole antenna.

    This photo is intended to provide a sense of scale comparing it's size in relationship to a typical bbq propane tank.

    IMAG0157.jpg

    IMAG0154.jpg
     
  9. KC8VWM

    KC8VWM Ham Member QRZ Page

    IMAG0161.jpg

    Please remind me not to drive around with the antenna on there...:)

    Some preliminary results. Without making any adjustments at all after freshly constructed according to the plans.

    ...Works great on 10m and 20m

    20m

    The VSWR on 20m shifted upwards in the band slightly from an earlier test which is fine with me, so I am leaving it alone for now.

    1.1:1 from approx. 14.235 - 14.325 - 1.5:1 on 14.100 and 1.5:1 on 14.400

    17m Antenna too short - Analyzer shows resonance around 18.800
    15m Antenna too short - Analyzer shows resonance around 22.100
    12m Antenna too short - Analyzer shows resonance around 25.300

    10m results

    1.1:1 from 28.200 - 28.400 - VSWR edges are 1.5:1 at 28.550 and 1.5:1 at 28.150


    So.. Did some more playing around..

    I tried adjusting the height of the poles (removed sections) to make sure there wasn't any interaction with the metal roof. It didn't matter where the antenna was located above the roof, the VSWR always remained the same.

    I changed the feedline length. That didn't make any difference either.

    The wires are not in their proper locations on the spreaders. I will adjust them and see if it changes anything at a later time.

    Other observations:

    I noticed the antenna is very frequency selective during receiving. For example if you tune too far beyond the range of its resonate design frequency, the background noise level drops out and it goes deaf so to speak. It could not hear well in the SW broadcast band below 10.000 mhz but yet it could hear very well on the 40m band. However generally speaking, this is not a very good general purpose SW listening antenna at all lol.

    Tested the 20m antenna against a 20m dipole located at the approx. same height. The cobweb is definitely a much quieter antenna. Heard a few DX stations and switched between the cobweb and the dipole. Most of the time, I could hear the stations better on the cobweb but it was primarily due to the fact it was a quieter antenna on receive. As far as most signals go, the cobweb seemed to beat the dipole but other times the dipole beat the cobweb. I think it largely depends on where the stations are located with respect to the orientation of the dipole. Sometimes the dipole would hear a US station at 5-7 s-units and the cobweb would hear the same station at s-9 to 10 over 9. Other times the dipole seemed better, although it was a noisier signal. Sometimes I could hear stations on the cobweb and they would disappear completely when switching over to the dipole. Just for fun, I turned the "direction" of cobweb antenna to see if made any difference in signals. Nope...It's omni directional alright! :)

    I also compared the cobweb to a full sized 20m vertical I stuck in the ground with a few random radials placed around it. Simply put, the cobweb beat the pants off the ground mounted vertical.

    It's difficult to draw any final conclusions. The cobweb is an omni directional antenna and the dipole I was comparing it against has nulls in certain directions. I would say the cobweb performs as expected. Of course I will have to play around with the other bands to bring things in resonance to test those bands further. I did hear some activity on 15m,17m and 12m. Similar to the 20m band, the 10m band was buzzing with loud signals and working very well if that means anything. :)
     
  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    Charles,

    Good stuff!

    Here's a comparison between a half-wave dipole and a cobweb at 10 degrees elevation angle; both antennas at 30ft over average ground:

    [​IMG]

    For 60% of the azimuth the cobweb beats a non-rotatable dipole. Perhaps more importantly, the cobweb is never more than a dB below the dipole but at times is 10dB better.

    Looks like yours is still following the theory ;)

    Steve G3TXQ
     
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