Ape Answers 012: Avoid Mistakes, Designing Ham Radio Antennas with Math

Want to build an effective ham radio antenna but aren’t sure how to size it correctly? In this video, we’ll guide you through the process of sizing an antenna to match your desired frequency, helping you achieve optimal performance for your ham radio setup. From basic calculations to practical installation tips, this guide covers everything you need to know.
Ape Answers is a short format video series where we answer Ham Radio related questions from viewers. We cover questions that were asked on my videos to get you that answers you are looking for. Whether you're a beginner or an experienced user, we've got you covered with detailed explanations and solutions to common issues. Tune in to get your questions answered!
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[@todop]

Its easy to make the rough estimate for an antenna if you are in europe or any metric country

[@jean-lucaudoin8538]

I agree. Why start in metric then finish in medieval (sorry, imperial) system, adding a layer of complexity of risk of error? It's fascinating to see the US being alone using this outdated method, after officially converting to the metric system in... 1965.

[@donaldjones1759]

I did cut a piece of coax RG9/U using this formally. Then I had an indicted me to terminate on end with a PL-259. I snipped a little off at a time until I reached 0 impedance. I then installed a 50 0hm feed line, but I was unable to get the SWR lower than 2.4. I am using RG-9/U Beldinn. Cut at 7.154 Mhz. I have built two of these antennas in my lifetime, each lasting 15 years. I always use the Double Bazooka MIT design. I have also always used RG-6 or thebonevthatblastest the longest RG-11. I wanted more broadband and low noise. I successfully built an 80-meter out of RG-232, and it still works and easily handles full PEP. Suggestions are most welcome. I used NFJ 269 advanalyser. Other than sticking with what worked. Where did I go wrong?

[@Mary-wk6of]

Thank you so much The Smokin Ape!☺️

[@TheSmokinApe]

Any time!

[@Mandarin110631]

thanks Ape! always learning up in here...

[@TheSmokinApe]

Glad it was helpful 👍

[@Joe_KS1I]

Excellent explanation. Every newbie should be taught this.
This is how I teach it:
300 / 2 (for half-wave dipole antenna in Mhz) = 150
150m x 3.28 ft per m = 492 ft
492 ft x .95vf = 467.4 (we’ll call it 468)

[@TheSmokinApe]

Thanks for the info Joe 👍

[@robertmeyer4744]

Very good APE. This is why the metric system is better ! the math lines up better. The outher thing that changes is a dipole at correct height will be 72 ohm impedance. A quarter wave vertical is 36 ohms. Our feedline is mostly 50 ohms and modern transmitter is a 50 ohm standard . So we just correct for that in adjusting the length to get Low SWR to transceiver. SWR is standing wave ratio . this does not mean the antenna is resident. which may be like 1.6 SWR on some dipoles. This is why a smith chart would be better. SWR is the happiness of the transmitter. A dummy load can be 1:1 SWR and transmitter happy and will be a bad antenna . The math gives os a starting point . we have to adjust for the real world . And that never the same. #1 rule of antenna's everything affects everything . #2 all antennas are a compromise . The best thing is just try it ! 73

[@KS0JD]

Nice explanation Ape! 73

[@TheSmokinApe]

Thanks Jeff 👍

[@threadripper979]

Nit picking a slight internal inconsistency: You said that going from 492 to 468 includes compensation for velocity factor, but you also used .95 velocity factor to further reduce the line length. Isn't that using velocity factor twice?

[@nrgxpert]

I thought the same thing. At 6:58 the VF of 0.95 is used a second time and 15 ft per dipole leg is the described result. I believe this will be too short for 20m FT8.

[@cybersean3000]

The speed of light is 299,792,458 meters per second. When calculating antenna length, this is rounded up to 300.000.000. When we divide this by one million, this is where the 300 comes from. Instead of talking about frequencies in the millions, we divide them by one million and call them megahertz. Calculating antenna lengths base on 300 will give the full wave result.
The speed of light, in US imperial feet, is 983,571,056 feet per second. But dividing it by two does not equal 468. So where does it come from and why is 936? The short story is about 100 years ago, antennas were measured by wavelength, not frequency. 468 was used to account for losses such as the velocity factor in wire, environmental factors, end insulators, etc. Calculating in metric, with the constant of 300 does provide more accuracy, and allows you to enter the velocity factor of the specific wire you are using. Using feet as the constant may provide a value which may be to short or too long. It is easier to trim wire than to lengthen it. "I keep cutting the wire but it just keeps getting shorter!"
That is my $.02 worth of unrequited babbling.

[@TheSmokinApe]

Thanks for the info CS 👍

[@Swamp-Fox]

My takeaway.....a little math on the front side saves me a little time and frustration on the back side!

[@TheSmokinApe]

That is true 👍

[@Krispin291]

Sensei

[@TheSmokinApe]

Thanks for watching Krispin 👍

[@hazer72]

Freedom Units!!

[@TheSmokinApe]

Right

[@WECB640]

468/number of turns in the spring x number of times you broke it + times you cussed. 😆