How to calculate "barber pole" antenna lenght?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K2STP, Jun 12, 2021.

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  1. K2STP

    K2STP Ham Member QRZ Page

    Ok, I'm about to start experimenting with a few "barber pole" design antenna's. That is, wrapping 1" copper tape around PVC pipes of varying diameters, and also for varying bands (2 meters, 6 meters, and 20 meters).

    I'm wondering if anyone has a good method or formula for calculating the estimated required lengths of both the pvc pipe and copper tape, based on the band of course AND the diameter of the pipe. For example, if I use a wider pipe (10" vs 3" pipe), how do I easily calculate the difference in length to be used without having to lay out the tape or string or something... which can then be fine adjusted later on using swr... after built by cutting the tape?

    There must be a wiz-bang math equation to automate calculating the lengths? The known inputs would be:
    -Pipe diameter in inches (OD)
    -Frequency in MHz
    -Tape width and/or spacing ? (my case, 1" wide tape, wrapped with a 1" spacing between each wrap)

    Any math geniuses out there have any ideas? I'm not one! ;)
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    NEC-based antenna modelers have a hard time with this. It would take a "EM field modeler" to do it justice.
     
    K0UO likes this.
  3. AK5B

    AK5B Ham Member QRZ Page

    Why?
     
  4. W9KEY

    W9KEY XML Subscriber QRZ Page

    Sounds like an interesting project. I've read that helical wound vertical HF antennas were popular with the Brits (in their small gardens) to get more wire packed into a shorter linear length. I also recall reading a "rule of thumb" stating you may need almost twice as much conductor length (wire) as would commonly be expected for a normal 1/2 wavelength dipole - presumably due to coil winding interaction / coupling. I also suspect they (helical verticals) didn't work all that well, or would be more popular today - but that's just a guess.
    • But I'm curious - antennas for 2, 6, and 20 meters are already pretty "short". What do you gain by winding the conductor into a helix (spiral)?
    • Why use 1" copper tape? Wouldn't 1/4" wide tape (with adhesive backing) be less expensive and allow for tighter winding (more turns per foot)?
    • Or why not simply wind wire around a PVC pipe with some regular spacing?
    • Here is one explanation of how to calculate the length of a helix: https://sciencing.com/calculate-helical-length-7808380.html
     
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

  6. AC6LA

    AC6LA Ham Member QRZ Page

    More info here and here.

    Dan, AC6LA
     
  7. K2STP

    K2STP Ham Member QRZ Page

    Why? I read an article last year by W6NBC in QST (link below), and have wanted to experiment with them a bit since.

    https://w6nbc.com/articles/20xx-QSTcopperhelixantenna.pdf

    Already short? Really? Well that is a matter of opinion and frame of reference I would think. A 60' long antenna I would not call "short". But anyway, haven't hams (and everyone else for that matter!) been trying to shorten antenna's since the beginning of time?

    I've discussed this with a few people that have done it, and apparently they show very favorable performance. The wide copper element also makes for a wider than normal bandwidth.

    PVC pipes cost close to nothing, 50 feet of 1" wide 1.5MIL thick copper tape is $10 on Amazon, add a connector and 2 stainless screws, and you have a compact high performance home brew antenna, what's not to like? And all the while, I actually learn something!!!! ;-)

    Lastly, what ham wouldn't want a barber pole on top of their house??? If I put a rotater on it, I can spin it around and maybe somebody will stop by for a haircut!! :D
     
    Last edited: Jun 13, 2021
    2E0CIT likes this.
  8. AK5B

    AK5B Ham Member QRZ Page

    :D Ham haircuts, 73 cents here! (Must be licensed for that price) :D

    I am leery of any W6NBC design to begin with (long story but refers to his "Double Delta Skeleton Slot' for starters).

    "High performance?" Really not trying to be argumentative here, but I rather doubt any shortened antenna could truly be considered that. Close to standard performance in a smaller package might be a better way to describe what you want to attempt. Remember, helical winding introduces a lot of inductance which may need a good dose of capacitance to counter it...

    Carry on and have fun experimenting; it's the best way to learn antennas.

    73,

    Jeff
     
  9. W6MK

    W6MK Ham Member QRZ Page

    I agree. It seems to be possible to load antennas in various ways in order to shorten them, but, as in
    every other aspect of circuit design, there are compromises.

    There is no magic. Regarding antennas the usual compromises are: length, efficiency, bandwidth--pick two.

    In addition to minimizing antenna length, loading can move the high current (maximum radiation) area of the antenna to a preferred position.

    Last, keep in mind that antenna design also includes providing an appropriate impedance match
    at the intended feed point. With the proper antenna feed system, an antenna for a given band can be
    reduced significantly in length (30%) with little, if any, significant loss of efficiency.
     
    SV2HZF, M0AGP and AK5B like this.
  10. K2STP

    K2STP Ham Member QRZ Page

    When I said high performance, I meant with respect to this type of antenna, not that it will be the magic mother of all antennas! ;)

    Thanks for the info guys…..!
     

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