Everything you know about dipole (calculators) is wrong ...

Discussion in 'Amateur Radio News' started by VK6FLAB, Oct 12, 2018.

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  1. WA5VGO

    WA5VGO Ham Member QRZ Page

    Anyone that can’t calculate a 1/2 wave antenna on a scrap of paper and then constuct it isn’t worthy of being called an amateur radio operator. It just doesn’t get any more fundamental than this.
     
    K0PV, W9SUN, N5RFX and 7 others like this.
  2. K3RLD

    K3RLD Ham Member QRZ Page

    Velocity factor? Certainly matters if you are building a phasing line.....
     
    KM4WRQ and VE7JBX like this.
  3. SA6CKE

    SA6CKE Premium Subscriber QRZ Page

    Whenever I have the need to make sure an antenna is correct I use the RigExpert AA-600 I have. The theoretical dipole size is just a starting point.

    A well tuned antenna is the best you can have - tuners is in reality not a substitute for a good well-tuned antenna.
     
    KC4V, KB5ZJJ, KG5THG and 2 others like this.
  4. KG5THG

    KG5THG XML Subscriber QRZ Page

    While we're kicking around dipoles, here's a good read: https://www.eham.net/articles/23802

    I fall into the let's see what my AA-230 says about my piece of wire and I'll go from there group.
     
    WN1MB likes this.
  5. 9A5AEU

    9A5AEU Ham Member QRZ Page

    You're*
    73 de 9A5AEU :)
     
    WB5WPA and WN1MB like this.
  6. K9CTB

    K9CTB Ham Member QRZ Page

    Whenever I come across an article that has in the title "you're doing xxx wrong" I read. It's the non-picture equivalent of a "...and then THIS happened" click trap, I know ... but I am compelled to do so. The calculation for the length of a dipole hasn't changed, and 468/F(Mhz) is still the formula. As for "ground", you're kinda stuck with what you have. Install the antenna a half-wave or higher above ground if you want DX .... install it ~.2 wavelengths above ground if you want "NVIS" operation. Even if you calculate to the millimeter, when you're done with your "calculator", you're still gonna add some length to your answer so you can trim until you get the match you want with your transceiver. Making simple things complicated is the job of academia ... it's not supposed to be what we do. That's why we're called "amateurs". :)
     
    KB5ZJJ and WN1MB like this.
  7. W5JCK

    W5JCK Ham Member QRZ Page

    Okay, there are shortcut, ballpark formulas that any ham should be able to use with some degree of accuracy. Then there is the actual formulas based on physics that we should know, even if we don't use them. Which ever you use, be sure to cut the sides of the dipole extra long and fold them over at the ends so you can use the extra length for fine tuning in the field, if necessary.

    If you want the physics formula, then here it is. BTW, scientific and engineering formulas are calculated in metric, not American Standard, unless you want to crash your billion dollar space craft into the surface of Mars...

    Calculate a one wavelength antenna with this formula: Wavelength = Speed of Light in meters/sec * Velocity Factor / Frequency in Hz

    Speed of Light in vacuum of space
    = 299,792,458 meters/sec

    Velocity Factor: Unless you are building an antenna in the vacuum of space, you will need to adjust for the rate of speed in which an electromagnetic field can travel through a medium, like a dipole copper wire. Yes the speed of light is 299,792,458 meters/sec in the vacuum of space, but it is going to travel through a 14 gauge stranded copper wire on Earth at a different rate than it would travel in the vacuum of space. Therefore you need to know the velocity factor ratio (VF) for the medium you are using. For example, for #12 or #14 insulated wire the VF = around 0.94, but for RG-58U coax the VF = around 0.66. So look it up for the medium you will use.

    Tuned Frequency of Antenna: If you are going to build an antenna you obviously need to know which frequency you want to have it tuned for. For example, let us say you want to have your antenna tuned for 7.200 MHz in the 40m band. For the formula, use hertz rather than megaherts, so 7.200 MHz = 7,200,000 Hz.

    Example: Let us calculate a full wavelength for a wire antenna using #14 stranded copper wire tuned to 7.200 MHz (40m band).

    Wavelength = Speed of Light in meters/sec * Velocity Factor / Frequency in Hz

    Wavelength = 299,792,458 meters/sec * 0.94 / 7,200,000 Hz = 39.13957091 meters

    Remember that this is for a full wavelength. If we are going to build a 1/2 wavelength standard dipole, then we need each side of the dipole to be 1/4 wavelength, so the length of each 1/4 wavelength side needs to be 39.13957091 meters / 4 = 9.784892726 meters. That converts to 32' 1.23". (One meter = 39.370079 inches or 3.28084 feet.) This of course assumes the 1/2 wavelength standard dipole will be erected horizontally at 1/2 wavelength above the ground. There are also other factors such as ground/earth type below the antenna and height of antenna, and angle of antenna in reference to the ground/earth, etc. You can make it as complicated as you want, or as simple as you want. Bottom line though is always make it longer than your calculations so you can adjust it if necessary.
     
    VK4CH, N6ITE and WN1MB like this.
  8. KC8VWM

    KC8VWM Moderator Volunteer Moderator QRZ Page

    I seen the intended humor in the article.

    Thanks. :)
     
  9. WN1MB

    WN1MB Ham Member QRZ Page

    Whew!

    My turn:

    234 / 7.2 = 32' 6"

    Moral?

    KISS, silly.
     
  10. K9WW

    K9WW Ham Member QRZ Page

    If you want real confusion, try looking for calculations for an EDZ. Every source I've seen uses a different "magic number". I simply found a bunch of those "magic numbers and averaged them out to come up with 1234. Easy to remember. I've enjoyed reading the article and especially the replies.

    73, Kirk K9WW
     
    KC8VWM and VK6APZ like this.

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