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CB Radio - Antenna Basics: Part 1

Discussion in 'CB Antennas' started by Robb, Jan 5, 2010.

  1. Robb Yup

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    What is an antenna? A length of wire is a basic antenna; but it is often associated in being a metal rod placed in an open area to receive and sometimes transmit as well. That antenna is making the receiver sensitive to micro-voltages at various frequencies that have been transmitted from various locations. Sometimes it is receiving commercial broadcast, or internet data, or even television broadcasts. But what they all share in common is the ability to receive and/or transmit information by weak electrical power over large distances at a given frequency.



    An antenna is nothing more than a given length of conductive material that takes these weak electrical impulses and conducts them down into the radio. It is the radio's purpose to sample a given frequency by discriminating against all other electrical impulses/frequencies that are also present. After it picks out any one frequency, it then decodes it electrically back into sound waves thru a series of special circuits that can be understood by us. Of course it doesn't have to be a radio - a television or computer modem is doing much the same thing. But for the purpose of this article, we will consider radios as the basic medium.

    It is one thing to have an antenna that receives, and another to have one both receive and transmit from the same antenna. The requirements are different. Just about any length of wire can pick up many different and random frequencies. But to transmit on that same wire or rod; it needs to be a resonant length of that frequency. Any one 'cycle' mentioned is the full distance that it takes for any given frequency to go electrically from "+" to "-" just once!
    "Frequency' and 'cycle' are the same thing.

    Resonant Peaks...
    The most commonly used lengths are 1/4, 1/2, 5/8, 3/4, 7/8, or even one full wavelength of any given frequency. Any frequency has a wavelength in which there is a sharp resonant peaks associated with it. A given frequency - such as 28 megahertz - has a known wavelength of 10 meters in length. Which is roughly 33 ft for a full cycle. So, a 10 meter antenna will need to be 1/4 to 7/8 in length of 33 feet. A 1/2 wave antenna will be 1/2 of that 33 feet - which is approximately 16 1/2 feet in length - as an example. A 1/4 wave antenna will be 1/4 of that 33 ft - which is 8 1/4 feet in length. And so on. You can figure out the most commonly used resonant points of any antenna - by knowing the wave length being used and the desired wave interval (1/4, 1/2, 5/8, and so on). For the purposes of transmitting properly, the antenna must be resonant on one of those previously above mentioned fractions - to transmit the energy away from the radio and into the antenna to be heard miles away effectively - for any given frequency.

    'Resonant Antenna' definition: "An antenna for which there is a sharp peak in the power radiated or intercepted by the antenna at a certain frequency, at which electric currents in the antenna form a standing-wave pattern." (Thanks 'Doc'!)

    As frequency changes; then so does the length of that wave.
    As the frequency increases; the wave length becomes shorter and shorter.
    As the frequency decreases; the wavelength becomes longer and longer.
    Examples:
    At 144 mhz - the length is 2 meters for a single cycle.
    At 3.2 mhz - the length is 80 meters for a single cycle.

    Let's say that we are using a CB radio. Let's say that it is on channel 20 - which is 27.205 megahertz. The radio can both receive and transmit on that frequency. "Megahertz' in this case - means that the frequency is changing polarity ("+ to -") 27,205,000 times a second as it travels at the speed of light. When we transmit; it will need to do this efficiently so that all of the electrical power that has been converted into electrical frequency out of the radio and on to the antenna. It is the lack of efficiency that makes any antenna a poor transmitting candidate.

    The antenna - more than any other component - is most responsible for making the radio effective. Not the other way around. Even a poor quality radio transmitter can be maximized by having an antenna if it is very efficient and resonant. If the radio does not have this efficiency when transmitting, it can also damage the radio. The electrical energy will be reflected back into the radio's components and cause damage because it cannot find the antenna resonant enough. So, we must maximize the antenna's efficiency and be sure that it is tuned correctly.

    Both an antennas resistance to flow frequency dependant energy on to it, and its resonance at a given frequency are the two most important factors in determining any antennas efficiency.

    Part 2 tomorrow...
    http://www.worldwidedx.com/cb-antennas/39893-antenna-basics-part-2-a.html

    This is meant for the beginner; if you found this page while browsing on the internet - save this page!
    For more information, please read this:
    The Ultimate Guide to 11 Meter CB Antennas
     
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  2. W5LZ Crotchety Old Bastard

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    Correct this part first.
    Resonance is not defined by length. Certain fractional wave lengths certainly can be resonant, but that's only circumstantially a usable characteristic, not a definition. Otherwise, why would those 2, 3, or 5 foot antennas be resonant??
    - 'Doc
     
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  3. bejo member From Indonesia

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    thanks for part one is usefull to for me nit just for beginer .... is use refresh to... and i hope posting for transmission to .... thanks
     
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  4. wavrider W9WDX Amateur Radio Club Member

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    Rob you may have opened a can of worms here, Think about the OCF dipole or Windom, it operates on Current nulls not on length for frequency.


    Now if the topic is limited to a particular antenna, The ground plane per say then it takes all the other type of antennas out of the topic.

    I admire and congragulate you on the attempt to explain, but antenna's are such a broad topic with so many different types may be better to discuss specific types to eliminate any confusion.

    Good luck and look forward to "The rest of the story" as Paul Harvey used to say.
     
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  5. Robb Yup

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    Let's keep in mind that I am spoon-feeding those who know nothing or little about antennas. It is intended/meant for beginners that need a basic understanding - who need to have a working knowledge for future use.

    Your criticisms are both correct and noted. Changes have been made to the script.
    I also encourage those who posted to write another thread after I write the second one. Teaching this subject in a way that makes better radio operators out of CBers and possibly new Hams is the goal. To pick up where I leave off and go into details fuller - with explinations that would make clear to someone that knows nothing or little - should be the guideline.
    Have at it guys...I will be looking forward to reading them...
    (y)
     
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  6. wavrider W9WDX Amateur Radio Club Member

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    Rob, yes completely understand, hard to explain everything on a forum, sometimes hard to explain in real life:D

    I do the same thing here, got a teenager who built a full wave loop vertically suspended fed at the bottom for Horizontal polarization, he enjoyed the dx, (11 meter of course), then helped him build another one and phase them for the 3db added gain.

    He is now studying for his tech test in Feb at the Orlando hamfest, and it all started from hanging wires from trees.

    Another friend 61 years old just passed his tech exam at the Silver Springs hamfest, he built his first all band doublet including homebrew 4:1 Balun. Lookng for his General ticket in Feb at Orlando.

    Antennas are fun, and can be very confusing, especially to operators just starting out.

    Formula's are great 468/FREQ is a wonderful place to start.

    There are many great site's dedicated to antennas and experimenting, hamuniverse is one I especially enjoy as some antennas are so unorthodox that they seem crazy, yet they work rather well.

    This hobby is for fun and enjoyment, it seems that most forums whoever posts an instructional thread will get ripped apart by armchair engineers.

    Visit QRZ'd sometime and read some of the replies, seems like some of the members try to drive perspective new hams away before they ever get started.

    Good luck on the informational/educational threads you are posting, they are good info for all.
     
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  7. CDX-007 Transducer Fanatic

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    Excellent post / thread!

    Waaaaaay back when I was not a Ham nor understood theory as well as I sorta-kinda do now, I decided to build a 3 element Quad for 11m.

    I didn't know much of what I was doing but I decided to try and ended up with an antenna which poked holes (with 1/2 the power!) in local 5 element Yagi signals into Australia.

    Yay for me? - hah, who cares - what I'm really getting at is I decided to TRY. Fail or succeed, I went ahead and DID IT, and that's at least 1/2 the battle I run into when trying to help another Radio Operator either understand or build an antenna for her/himself, the ""OH NO, I COULD NEVER UNDERSTAND OR DO THAT!!!" factor, which is by far the biggest impass to implementing theory in such a way as to bring it to life.

    First they have to get past the fear of the unknown, THEN they can begin to see it come together.
    Just my .02 worth.
     
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  8. wavrider W9WDX Amateur Radio Club Member

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    Good .02 cents,

    cubical quads, built my share, and had alot of fun with them, really enjoy experimenting with wires.

    yes if can get an operator to start building something that is the had part, and then explain how it works is sometimes harder than building it.
     
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  9. Rob is this something youre copying from somewhere else or are you putting it together yourself as you go along ? just curious ..

    good thread though ;)
     
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  10. Lost Ram Member

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    Anyone have any spec's on an open air coil 5/8's or .64 base antenna. I have mocked up on like booty's but I have been trying to get an open air coil to work. I can get as good as 1.7 SWR with radials at 90 degrees, I think I am needing to go 30-45 degrees on the radials. Anyway I am looking for any info or spec's on the open air coil. I have a crude arrangement for testing right now.
    Thanks, Kerry
     
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  11. W5LZ Crotchety Old Bastard

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    Lost Ram,
    Just a couple of things to think about.
    Radials position/angle.
    The 'angle' of a radial can certainly change/adjust the antenna's characteristics, such as input impedance, but they have no affect on the shape of the antennas radiation pattern, that's a function of the antenna's length (1/4w, 1/2w, etc.) and height above ground.
    If you start at your last posted low SWR point, and start 'drooping' those radials, you'll see that SWR go down. No idea how much lower it would get, or the angle the radials would end up at, but things will get 'better'. (It's like 'drooping' the legs of a dipole antenna to lower input impedance.)
    What size coil, or coils, to make the thing work? One way of finding that out is by starting with a coil arrangement that you know works, and figuring things 'backwards' from there. If you know the dimensions of the coil, you can calculate it's inductance. If there are two coils (like one above, and one below the 'tap') figure each separately by their dimensions. The ratio between the two is important, you wanna keep that. It's been too long since I messed with that formula, I don't remember it off hand. It's included in most of the 'Hand Books' though, shouldn't be hard to find. Once you know the inductances of those 'two' coils, you'll also find that you can get the same values of inductance by varying the dimensions of those coils. You can make them larger, smaller, change their 'shapes', etc. Some 'shapes' of coils are easier to make than others for any number of reasons. Pick the one easiest for you, or the one you 'like'. (How about one large ring and a shorting bar, sound familiar?)
    If you use one size of coil and jsut 'tap' it somewhere for the center conductor of the feed line, it'll work, right? Okay, how about instead of adjusting the vertial thingy's length, you make the top of that coil also serve as a 'loading' coil? Add another turn or two, 'tap' is where you find resonance.
    All kinds of variables there for different reasons/purposes. More power handling capability? Slightly larger sized wire to make up the coil, then adjust the spacing so it doesn't arc. Can't make it any 'taller'? Okay, make it 'wider'.
    See where all of that's going? Until it really gets ridiculous, changing one dimension to compensate for some other dimension is vry common. Knock yourself out with it! You get to the same place, just by a different 'route'.
    - 'Doc
     
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  12. Lost Ram Member

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    Is this the same Doc from CB radio talk???? Thanks for the info Sir. I have a coil of 3/8 copper tube but will be using #10 copper wire for the final build. This weekend I am going to be making my coil out of #10 copper wire. The 3/8 tube is much larger so I cant really copy it for number of turns and diameter. I figured maybe someone had a good starting point. If not I was going to go with about 50" of #10 copper around a 3" diameter. I have a MFJ analyzer so that takes out some of the hassle. :D
    Thanks Doc,
    Kerry
     
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  13. Robb Yup

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    Thanks BootyMon!

    Maybe you can post some thoughts of what you have learned in building your 5/8 wave antenna - here. AS in specifically - those things that you learned that you didn't know before while in the process of putting it together and making it work correctly.

    I would be interested...
     
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  14. W5LZ Crotchety Old Bastard

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    Lost Ram,
    Am I the same one as on CBRT? Probably, I go to several sites that are radio related. If it's me, I always sign my user name the same, it's gotten to be a habit. If it's about something that I may have said -right-, then naturally it was me! If it was something I said that wasn't very well liked, it could certainly have been me too. Me and old Cassius Clay (know who that is??) have a lot in common, we are both the 'Greatest', dance like a butter fly, sting like a bee! Give me just little bit and I'll think of some more BS like that. Now you know it's me, right?
    - 'Doc
     
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  15. Nursecosmo Member

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    Radials shouldn't affect the circular pattern much, but they definitely change the take off angles of the pattern and can dramatically change the shape of the radiation pattern by either flattening it out (lowering the radial angles) or rounding it out (radials at strait angles or lowered two far past optimum).
     
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