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how does an ISOPOLE work?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC9ZHR, Jun 11, 2019.

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  1. KC9ZHR

    KC9ZHR Ham Member QRZ Page

    correct me if I am wrong (I am a young 'un) but from what I can see from what little info is left on the web it looks like a sleeve antenna with a couple cone shaped decoupling stubs.
    N1VAU and NH7RO like this.
  2. NH7RO

    NH7RO Ham Member QRZ Page

    You just answered your own question; also known as a coaxial dipole.

    I just found out that a company named Spectral makes them nowadays for public service and amateur bands (a lot like AEA did when they were around). Search "isolpole" here and a link will turn up quickly.
  3. KC9ZHR

    KC9ZHR Ham Member QRZ Page

    AI3V likes this.
  4. ND5Y

    ND5Y Ham Member QRZ Page

  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    NO, they’re definitely not snake oil.

    They are the lower active sleeve of the center-fed double CF Zepp design and work better than a straight-sided cylinder; you can even model that to see why. It was a brilliant design, and I believe patented by AEA at least 30 years ago. The only drawback is slightly more wind loading, but if the support can handle that, it works better than using a straight tube.
    Last edited: Jun 12, 2019
    KB0MNM, NH7RO and N1VAU like this.
  6. AI3V

    AI3V Ham Member QRZ Page

    Here is the most basic of antennas: the 1/4 wavelength ground plane.


    Because most so239 connectors have a square flange, and a hole in each corner, it's customary to use 4 radials.


    Now, the radials do several things,

    First and foremost they are "the other half " of the antenna.

    Now a quarter wavelength of wire has a neat property, if the far end is not connected to anything, the transmitter end will have a low impedance, about 35 ohms.

    When connected to a 50 ohm coax, this allows for efficient transfer of power (current flow)

    They also form a virtual ground- hence the name ground plane. What this means, is any conductors on the side of the radials away from the vertical wire are invisable to the antenna.

    In other words, the radials "decouple" the antenna from the outside side of the coax shield.

    Also, bending the wires down, away from the vertical causes the impedance to rise a bit, close to 50 ohms to improve the vswr on a 50 ohm coax.

    Note that as few as 2 radial wires will provide these effects, most ground plane antennas use either 3 or 4, it could be argued mostly for aesthetics.

    Now consider what happens as you add radial wires.


    Pretty soon they touch, and you have a solid sheet (cone)

    That's what the cones are.

    Last edited: Jun 12, 2019
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  7. NH7RO

    NH7RO Ham Member QRZ Page

    ...and by adding a second cone (think infinite number of radials added as a second set) you will decouple the feedline even more than one set can, too---hence the fact that isopoles have two cones instead of one straight sleeve or cone and is therefore superior to a standard coaxial sleeve dipole.
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    It also keeps the “high voltage” ends of the sleeve(s) further isolated from the mounting mast, so there’s about zero chance of arching over that path — the path is just too long.
  9. NH7RO

    NH7RO Ham Member QRZ Page

    Not always the case if one is in Utah:
    Widespread arching occurred there millions of years before AEA came up with their non-arcing Isopole...:p
    WR2E likes this.
  10. AI3V

    AI3V Ham Member QRZ Page

    Its a nice theory.

    And sammy hammy $pends a lot of ca$h on "common mode"

    Strange that in the commercial radio world, you know, the folks that actually make money with radio "common mode" is unheard of.

    You will look a long time for a "second set of radials" on a commercial site.

    Things that make you go hmmmm.... :)


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