Jump to content
Ham Community
RF Connection is proud to co-sponsor Ham Community and its members – keeping it free of advertising clutter – right up to the end of June 2021.

Say thank you by visiting their website or special Ham Community page.

Why do we have ¼ ⅜ ½ ¾ wavelength antennas?


Recommended Posts

Hello guys,

I have some questions about antennas that probably some of you who know more than me might be able to speak on.  I guarantee you know more about antennas than I do.

If my basic understanding of antennas is right, an antenna's length is based on the wavelength of whatever frequency you're using it for.  Specifically I'm talking about my 2-meter antenna as that is about all I'm familiar with.  So if I understand, the 2-meter wavelength is 2-meters, or aprox 78 inches.  What they call my "quarter wave antenna" means it's cut to 1/4 the wavelength, or aprox. 19 1/2 inches.  (39x2)/4  And if I understand, a "half wave antenna" would be cut to half the wavelength, aprox. 39 inches.

So my dumb newbie question, if my understanding of antenna/wavelenghts is right,  using my 1/4 wavelength antenna as the example, What happens to the other 3/4 of the electromagnetic wave?  Does the antenna TX and RX 1/4 of the wavelength and discard the other 3/4?  Or does something happen that the antenna compensates for the difference, or it just "knows" what the other 3/4 of the wavelength is or what?  The reason I ask is I'm told by other Hams that a 5/8 wave antenna would give me better performance, so I assume there must be something going on such as the 5/8 wave antenna is using more of the wavelength or something?

Is it like geometry where if I have 1/4th of a circle I can know the diameter of the circle?  Is the radio able to take 1/4 of the wavelength from the received signal from the antenna and figure out the other 3/4 with math somehow?  Is that why a 5/8 wavelength is "better" than a 1/4 wavelength, becuase the math might be more accurate with a larger part of the wavelength?

Thanks!
kt40bx

Link to post
Share on other sites
  • Administrators
ELMER

Hey Anthony.

First a link to a book I love:

http://www.arrl.org/shop/Antenna-Physics-An-Introduction-2nd-Edition/

There are hundreds of books on antennas but what I love about this one is that it approaches it from the purest physics standpoint.

Now for the answer(s).

Option 1: I bore you with the physics of electrical length, impedance, etc. all the things that 'generate' the wave or signal from the antenna.

Option 2: I use the metaphor my dad used when I asked him the same question in ±when I was was having my crisis transition from Fruit Loops to Captain Crunch.

Here's what I'll do. I'll start with option 2 and if it unsatisfactory, I'll bore you with a very long post on option 1 😛

the wave.jpgMost hams make the mistake of not visualizing what Alan @W4DOI loves to make fun of, my WAVE (I bore him to death with my wave talk). A wave is a wave is a wave. It's a physical reality with measurements. It has amplitude, frequency, speed. Most people, I assure you, forget this. I live by it. Now, imagine that wave, that nicely formed wave. Imagine it as a rope used during a boot camp or boxing workout. The rope is your wave. Now, instead of grabbing the end of the rope with your hand, grab it with a stick. In the first instance, grab it with a 2 meter stick. So the rope is attached at one end of the stick and your hand is holding the other end. Now imagine the same scenario with a stick that is ¼ meters long. Which of the two sticks will allow you to most easily generate a wave that is two meters in length and of equal amplitude? Obviously, the longer stick will do so more easily. The shorter stick will still do it, but you'll have to wave your hand harder and higher/lower. In essence, the closer the antenna is to the full wave, or any harmonic thereof, the more efficiently it will generate the wave.

Basically, in all cases, the antenna will generate the same wave, but it will do so with different electrical effort. So why do we bother having ⅜ or ⅝...? Well, that's for another post 😇

Do you want the option 1 explanation? Will gladly do so. Or... you could buy the book, read it voraciously and become our new resident antenna guru 💪

  • Like | Congratulations 1
Link to post
Share on other sites
  • K3MRI changed the title to Why do we have ¼ ⅜ ½ ¾ wavelength antennas?

The Capn Crunch move was definately the right sction, Brother.  :)

I think I understand it better now.  Makes a lot of sense explained that way.  So to the receiving antenna, the wave is the same regardless of what Antenna transmitts it?  It doesn't know the difference.
That was where I was going off track, the "quarter wave" and "five-eights wave" description gives the impression to us ignorant of the physics that only 1/4 or 5/8 of the wavelength is sent.  Thus my question of "What happens to the remainder of the wave."  So nothing happens to it, it's still there.  It's just the method the TX anrenna uses to send it.

Thanks for the simple and relevant explanation VS the "geek explanation" option.  Now I can mess my hair all up and look like Einstein.  :)

 

  • Funny 1
Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...