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Xmit loop antenna for all bands.Dbl helix cont"d

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB9MZ, Nov 21, 2009.

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  1. KB9MZ

    KB9MZ Ham Member QRZ Page

    After reviewing a loop antenna design made of coax I decided to make a larger one for Top band. If you have something like a 2 el quad for say 20 M you can cover the space between the two el with window mesh using the 2 el as supports.
    This is basically the same as a loop or magnetic antenna except you have more than enough surface area of mesh that in effect, it also becomes a large Faraday shield relative to the two thin wires of a quad
    Feeding at one end and grounding the other makes it into a tubular style beam that will pick up all polarities thrown at it.
    Doesn't need a capacitor to tune as it is close to resonance on all bands!
    If you use aluminum window mesh then remove the coating first by soaking in Murriatic acid from Walmart then roll the aluminum weaving flat with a lawn roller.
    Put a light weight antenna like this on your tower and have fun on all top band frequencies. The current flow is only on the surface and follows the same paths as shown with the helix antenna on my home page Unwin antennas
    Give feed back as to how you get on so that others are encouraged to experiment.
     
  2. KB9MZ

    KB9MZ Ham Member QRZ Page

    As a postscript the antenna I have described is what the inventors of the EH
    antenna was trying to achieve but got lost in the proces!
    What you have is a Faraday shield that stops penetration of one field while allowing penetration of the other field. The fields that I am rfering to are the E and H fields. These two fields are separated by a boundary in equilibrium such that the electrons on both sides of the shield are equal and opposite to each other. Applying a time varying field to the outside of the faraday shield allows you to transmit, bearing in mind that the multitude of lines and holes provides a conductor for carrying the time varying current and where each associated hole provides the path for the displacement current where these two current flows interchange such that a static particle is accelerated away at the speed of light ( you can view the hole as a capacitor), the accelerated charge often referred to. When the time varying current is applied to the outside of the shield it keeps in equilibrium by following a path that is resonant from start to finish! It can even separate into separate paths of the same length where the ejected particles form a plasma cloud. Of course what can be transmitted can also be received!
    Thus the idea behind the EH antenna was sound. It was the method taken to acheive its aims that were misdirected such that the feed line did most of the radiating. Turning the above antenna on it's side and you have what the inventors were trying to achieve for broadcasting at a low height and without the need of a ground plane.
    Cheers
    Art
     
  3. K4SAV

    K4SAV Ham Member QRZ Page

    I put it on EZNEC. It was resonant on 129 MHz with an impedance of 64.6 ohms. There are no resonances lower than 129 MHz. The free space gain on 129 MHz was 1.7 dBi. The free space gain on 1.8 MHz was -38 dBi. The gain on 3.5 MHz was -28 dBi.

    What did you get?

    Jerry, K4SAV
     
  4. KB9MZ

    KB9MZ Ham Member QRZ Page

    I have modeled smaller ones with AO Pro but even that does not have enough segments to do the job properly. The actual antenna I have in the yard was made of two "turns" of 32" wide window mesh so that it formed a cube of approx 5 to 6 feet with 2 times 32inches on the side. A cube that just fits inside of a sphere always provides maximum efficiency.
    Making the antenna rather than relying on a mathematical with too few segments is much more satisfying. I stripped the coax back at the radio purely to ensure that the radiation pattern was not affected in any way by the transmission line.
     
  5. KB9MZ

    KB9MZ Ham Member QRZ Page

    The problem with trying to model a mesh is that the wires only radiate on one side instead of all sides of a wire! I know that a grid set up is sometimes used to mimic a flat sheet in some programs but I have no idea how the program adjusts to such. I would dearly love to see a colour depiction of current flow
    on a grid or plate but to date I have not seen one. Point to note is that when the hole is infitesimaly small is when maximum efficiency is obtained so using a grid to mimic a sheet seems reasonable except for radiation adjustment for one side of the shield only! Of course, when a perforation is not actually there then the true current flow becomes very debatable! Letting a physical antenna point the way is so much better than a questionable computer program.
     
  6. KB9MZ

    KB9MZ Ham Member QRZ Page

    A bit more on the perforation thing.Magnetic lines are literally that on the surface of sheet metal. Between those lines the gap is filled with circles or eddy currents with vortices. The hole therefore is not a consistent diameter
    which makes it difficult to calculate the cross sectional area of the "perforation.
    To get around this one has to provide a near zero resistance in the metal and maximum radiation outside on the surface which consists of tightly packed electrons. Ofcourse if we have all the current flowing outside then we have broken the laws of OHM. So when the 99.9% of the current flow is on the outside of the Faraday shield the vortices or perforations are essentially disapeared because eddy currents now fail to flow. In theory, the vortices has now moved fully into the encapsulating electron surface which is akin to perpetual motion as the electron skin cannot be driven alone! So in effect,
    programs CAN simulate the radiation of a flat sheet from one side which leaves open a final question for closure! Does a half wave radiator consists of two separate sides for current flow which would make the point of equilibrium or resonance equal to the period of a sine wave(Full Wave)? Or is the return path down the centre of the wire such that it cannot radiate, which then simulates a radiating shield. This also simulates a "tank circuit" where radiation only occures in one direction of exchange energy flow between inductance and the capacitance, or one time in a cycle or period of the time vary current applied ? Either way the Faraday shield DOES radiate
    and forms the maximum efficiency of radiation possible within the smallest volume known and where the thickness of the shield is determined by the current applied to the limit defined by the radius of the enclosing sphere
    for the thickness of the radiator.
    Cheers
    Art KB9MZ
     
  7. K4SAV

    K4SAV Ham Member QRZ Page

    You first described the antenna as a tube which implies that the ends are open. You said:
    "This is basically the same as a loop or magnetic antenna ..."

    Now you say it is a cube, which implies the ends are closed. No matter, that's simple to model too. Assuming the cube is fed at a slit across one side, it's resonant on 205 MHz with no resonant points lower than that. Free space gain at 205 MHz is 0.76 dBi. Gain at 1.8 MHz is -44.5 dBi and gain at 3.5 MHz is -34 dBi.

    Your description of the feed point is not very clear:

    "Feeding at one end and grounding the other makes it into a tubular style beam that will pick up all polarities thrown at it."

    I'm not sure how you feed a cube (or a tube) at one end and ground the other. You could do that with a wire inside a cube but in that case you would really have a Faraday shield and the gain on all frequencies would probably be in the negative hundred dBs range.

    On 1.8 MHz the radiation resistance of the cube antenna is 5.4 e-7 ohms. The feedpoint impedance at 1.8 MHz is approximately 0.02 +j4 ohms. The 0.02 number will vary a little depending on the size of the wire used in the mesh. Just the resistive loss in the antenna is 37,000 times as much as the radiation resistance, so the efficiency will be pretty close to zero.

    None of the numbers in the EZNEC analysis are realizable. The feedline will have radiation far in excess of the antenna gain. I don't think it is possible to create a choke with enough impedance to prevent that.

    Oh you could build this and feed it directly with coax (no choke) and you could probably work stations with it, but that is because the feedline radiation will be very high. This is a classic configuration for large amounts of feedline radiation (unbalanced feedline feeding a very low impedance balanced antenna).


    Do you mean you disconnected the shield at the rig and fed it with a single wire?

    Jerry, K4SAV
     
  8. KB9MZ

    KB9MZ Ham Member QRZ Page

    The shape of the radiater is like a tunnel with the ends open. For max efficiency
    it must fit nicely inside a sphere without penatrating the boundary. This is standard physics teachings. Look up Poincare and Pointings vector.
    Hopefully things are becoming clearer.
    With respect to the feeder. It is generally known that the presence of a feeder
    within the radiated pattern can affect and modify the radiation pattern.
    To remove this we have only one wire going to the transmitter and the other is a ground loop. This provides equal currents in the coax and shield thus isolating it from the radiator where the shielding siphons off to the ground any interferences.
    Now you don't have to ISOLATE the feed line but if one wants to measure the radiation field one should. On the other side of the coin it really doesn't matter if the feeder radiates if you are succesfull in your communications on all bands!
    Hope that makes it clearer.
    With respect to your comment regarding the perforation. Normally for a capacitor there is a formula depending on the respective areas and the distance apart. In the case of a mesh the capacitance depends on a network as well as proximetry effects of other associated wires. It is no where as simple as that depicted for capacitance in the books! Ofcourse this section of static particles must be present such that the intersection of the main current carrir field is allowed to intersect. This same action is shown in a scope where the particle is accelerated as well as in salvage sorting yards
    where items are elevated and thrown into different bins depending on the metalloic constituents.
     
  9. K4SAV

    K4SAV Ham Member QRZ Page

    I was afraid that was what you did. Well that will make the antenna system perform a lot better but the performance will have little to do with the thing you are calling the antenna. The "mesh loop" will simply serve as some end loading for the random length wire antenna which is provided by the shield of the coax. All significant radiation will be from the coax shield and almost nothing from the "mesh".

    Opening the coax shield will not prevent coax shield radiation. It will GUARANTEE coax radiation.

    Jerry, K4SAV
     
  10. KB9MZ

    KB9MZ Ham Member QRZ Page

    Well I understand your personal view but what interest me is your logic or references that lead you to such statements. My great grand daughter can refuted anything that is stated but can never apply logic to that same statement because of lack of education. Since it is well known in science that a Faraday shield does indeed radiate, one would first endevour to refute the truthness of that statement which is prolific in the science books.
    My opinion of antenna programs is that they are proving worthy of comparison
    to physicsl But in all cases in physics a radiator takes precidence over a mathematical tool until the differences attained are fully understood.
    Worst of all is the avoidance of "change" where the loudest voice takes presidence over the findings of science.
    I look forward to your reasoning why a collection of conductive wires cannot or will not radiate which would then apply some logic to what is just,.... well,....
    .....something I cannot express.
    Maybe you can supply a reason that grids
    are used in programing to mimic the radiation from a flat pattern, when proper account is made of close spaced conducters!
     
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