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

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

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  1. VK6FLAB

    VK6FLAB Ham Member QRZ Page

    foundations-of-amateur-radio_300.jpg
    Foundations of Amateur Radio

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

    The other day I did an experiment. I searched for "dipole calculator" and using the first 20 results I calculated the length of a dipole suitable for 7.130 MHz. I chose the frequency for no other reason that there is a 7130 DX net every Monday, Wednesday and Friday and for the longest time I've been unable to participate due to the lack of a HF antenna in my new shack.

    So here's some things I learnt from doing this experiment.

    Depending on which calculator I use, the length of my dipole can vary by over a meter from longest to shortest result.

    Depending on my desire to use metric or imperial measurements, my dipole will be a different length, because of course electrons move at a different speed if you're not using the metric system.

    In case you're wondering, 1 inch is defined as being exactly 2.54 cm, so there's plenty of opportunity to vary that.

    Speaking of standards, we all agree that the speed of light is a constant, right? Turns out that for some calculators, you can change the speed of light.

    I'll skip over the notion that none of the calculators actually show what they're using as the speed of light and move on to other interesting discoveries.

    Apparently you can determine the length of a dipole down to the sub-atomic length, with one calculator going down to the size of an electron to indicate how much wire you should cut from a spool.

    There are forms that make doing the calculation really easy, single box to type in the frequency, so the answer must be right.

    There are some that use random standard numbers, even a text book example that uses some number, but no indication where it comes from. For example, the number 486 features regularly, but so does 150 and 5905.

    There are forms that provide you with several boxes, but no indication which box needs what value, so your answer may or may not indicate the number of eggs per chicken per parsec.

    One dipole calculator result is actually for a vertical, so your search engine helping you might not actually give you the calculator you expect.

    There are percentage correction factors. 5% seems to be a favourite number, but no indication as to what the origin of that number is.

    There's a calculator that allows you to specify the feed point impedance, not sure how that works, but it's a nice feature to have when you're calculating the length of your dipole. Not.

    One regular instruction is to cut long, that is, measure your wire and cut it longer than the calculator states. How much longer is left as an exercise to the reader. Should it be 1 mm longer, 1 cm longer, or should it be 1 m longer and how much should that change if the frequency changes?

    Let's move on. The word ground features heavily in these calculators. The phrase "average ground" does too. No indication as to what makes an average ground, or how to go about determining what changes if your ground isn't average.

    We all agree that the dipole should be half a wave-length above the ground, right?

    How much is that?

    The same wave length as the length of the dipole we've just calculated, or a different one?

    How does the length of the dipole vary if the height varies? While we're looking at variation, how much variation is there depending on how thick the wire you're using is and what about insulation? None of those things are even mentioned in any of these calculators.

    Dipole calculators, wonderful invention, shame about the implementation.

    I'm Onno VK6FLAB

    To listen to the podcast, visit the website: http://podcasts.itmaze.com.au/foundations/ and scroll to the bottom for the latest episode. You can also use your podcast tool of choice and search for my callsign, VK6FLAB.

    All podcast transcipts are collated and edited in an annual volume which you can find by searching for my callsign on your local Amazon store, or visit my author page: http://amazon.com/author/owh.

    Foundations of Amateur Radio Volume 7 is out now - with chapters on digital modes, coax connector loss, waterfalls, station performance and more.

    Feel free to get in touch directly via email: onno@itmaze.com.au, or follow on twitter: @VK6FLAB (http://twitter.com/vk6flab/)

    If you'd like to join a weekly net for new and returning amateurs, check out the details at http://ftroop.vk6.net, the net runs every week on Saturday, from 00:00 to 01:00 UTC on Echolink, IRLP, AllStar Link and 2m FM via various repeaters.
     

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    KE0SZD, F6HLQ, CT1EIU and 11 others like this.
  2. SA0PAL

    SA0PAL XML Subscriber QRZ Page

  3. NF6E

    NF6E Ham Member QRZ Page

    Nice read, but not news. Why are these non-news items continually being spotlighted as news?

    Article posted here:
    upload_2018-10-12_3-16-2.png

    More appropriate location here:
    upload_2018-10-12_3-15-48.png

    P.S. The dipole calculation formula I learned when I was 12 still works today. Half wave in feet = 468 / Freq. in Mhz. (80 meter example: 468/3.7 = 126.5 feet)
    <sigh>
     

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    W1TRY, N8RJP, WA4IPE and 22 others like this.
  4. W9AAM

    W9AAM Ham Member QRZ Page

    ...then the antenna is winding through trees and over the cars in the driveway...
    Of course, the calculations are approximate, some final length adjustment is always necessary.
    Adjustment and developing the skills to do it and getting the station to sing is what amateur radio is all about.

    The West Mountain Radio site has one of the best antenna calculators.
    http://www.westmountainradio.com/antenna_calculator.php
     
    CT1EIU, K9FV, G1HYD and 6 others like this.
  5. M0CUS

    M0CUS XML Subscriber QRZ Page

    Simple for me. EG for A 20m dipole. I know I need a half wave dipole so I measure out 10m. I then lay out some wire over where I have measured out and cut it to that length plus 2m. Then I fold it in half and cut it again so I now have 2 x 6m lengths of wire. Connect feed line and haul it up. Check for resonance as I know it will be wrong for 20m and trim as necessary. No matter what calculation you use, you will have to trim for resonance unless you are very lucky or just dont care.
     
    GM4JPZ, G1HYD, K9WWW and 4 others like this.
  6. NU4R

    NU4R Ham Member QRZ Page

    Well...I found it best to use metric measurements on one half of the dipole and imperial on the other BUT...point the metric end towards the direction you most want to collect DX from.
     
    M6TUE, K4KP, AH6BI and 11 others like this.
  7. VK6APZ

    VK6APZ Premium Subscriber QRZ Page

    Your not from around here are you, no one else seems to have a problem making a simple dipole ho hum.
    Been doing this antenna stuff for 40 years now and still learning.

    73 VK6APZ.
     
    OH2YV, WN1MB, G1HYD and 6 others like this.
  8. WN1MB

    WN1MB Ham Member QRZ Page

    A sign of the times: the need of a computer, monitor, keyboard, mouse, Internet connection ... all to calculate the length of a 40 meter 1/2 wave dipole. Pretty sad.

    It's amazing any of us were able to make antennas, let alone contacts, back in those glorious prehistoric days before Al Gore invented the Internet.
     
    K6AEA, WB5KDZ, K8AI and 46 others like this.
  9. K4TO

    K4TO Ham Member QRZ Page

    Ease up! This is an article for beginners. Our beginning hams mostly come from the computer driven generation. We old curmudgeons must realize that the future of ham radio lies with these who are the computer generation. the article is great in that it emphasizes that the info from the internet is not only plentiful, but varies widely in accuracy and must be taken with a grain of salt. Personally, I have noted that my time operating on the air started a downward trend from the moment I bought my first computer in 1992. I now spend as much time keeping my computer going as I do my radio. It is a love/hate relationship. I love to hate computers. But, they have become almost indispensable. All I have to say to Mr. Bill Gates is that he better have his life together when he dies, or the first words he will hear are, "Welcome to Hell. Here is your Macintosh computer."

    I am licensed and active on the air for 61 years. The comment about learning continues is absolutely true. I have just spent my Summer building and installing four BIG antennas on a 155 foot rotating tower. It was the best Summer I can remember in a long time. 73. Dave, K4TO
     
    NA4RA, KB0LFL, G8KVM and 17 others like this.
  10. WN1MB

    WN1MB Ham Member QRZ Page

    Fine. But if these beginners can't compute the length of a 40 meter 1/2 wave dipole with a paper and pencil, then they're as good as cripples if they have to figure something out when away from a computer.

    Coincidentally and distantly parallel/related: a 40 year old co-worker showed up to work last night bitching about "blowing out a tire" on the way to work. He was all set to call a tow truck to haul his car to the shop because "my care doesn't have a spare tire." After our shift ended this morning, a couple of us went out to check his car. As expected, the car did have a spare and jack. Surprise, surprise.

    However, the 40 year old co-worker - a male - didn't know how to use the scissor jack, where to place it, or how to otherwise change the tire.

    I'm still shaking my head in disbelief.

    Which is exactly why I migrated to Linux in 2005, or thereabouts. Never looked back...
     
    K6AEA, N8VIL, AC0XR and 18 others like this.

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