Is a "Full Wave" dipole a good idea or not?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC0MS, Oct 20, 2008.

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  1. W5DXP

    W5DXP Ham Member QRZ Page

    On the contrary, a large percentage of hams, including me, are using full-wave dipoles. A 130 foot dipole (plus or minus) for instance, is a very popular antenna that is 1/2WL on the lowest frequency of operation (3.6 MHz). When used on 40m, it becomes a one-wavelength dipole with some gain over a 1/2WL dipole. If one feeds this antenna with 450 ohm ladder-line through a good balanced tuner, it will work well on all HF bands. Here's how I do it without a tuner:

    http://www.w5dxp.com/notuner.htm

    On 75m, I feed the 1/2WL dipole with 1/2WL of ladder-line. The result is an impedance between 33 and 80 ohms looking into the ladder-line. If the antenna feedpoint impedance is 50 ohms, the impedance looking into the ladder-line will be close to 50 ohms.

    On 40m, I feed the 1WL dipole with 3/4WL of ladder-line. The result is an impedance between 33 and 80 ohms looking into the ladder-line. If the antenna feedpoint impedance is 5000 ohms, the impedance looking into the ladder-line will be close to 45 ohms. To understand how that is possible, take a look at 1/4WL matching sections in The ARRL Handbook or The ARRL Antenna Book.

    It all has to do with the low-loss impedance transforming properties of mismatched ladder-line. It works like a charm when one takes advantage of an understanding of this phenomenon.
     
  2. M0UKD

    M0UKD Ham Member QRZ Page

    Contrary to popular belief, the dipole is so named because it has two electrical poles, not two physical poles. Wouldn't that be a di-element!? Just like a magnet has two magnetic poles, a North and a South, we have two electrical poles, a Positive and a Negative. Being a half wave, there is always two opposite poles on the tips at each half cycle. Any half wave antenna is actually a dipole.

    There is no such thing as a full wave dipole. It would be a quad pole :) Just call it a centre fed full wave.
     
    Last edited: Nov 1, 2008
    MM0HVU likes this.
  3. AC0FP

    AC0FP Ham Member QRZ Page

    If you have the room I like Steve's idea about the EDZ doublet antenna made up of two 5/8 wave elements. I beleive the October or November 2006 QST magazine ran a real nice article on those including the ladder line match length to get to 50 ohms for each band. Of course I have window line to a tuner so I can use it on multiple bands.

    I used my 15 meter EDZ for my VP6DX contact on 15 meters, using 100 watts he answered me back on the first call. :)

    73,

    Frank:)
     
  4. W8JI

    W8JI Ham Member QRZ Page

    Gosh, we better call McGraw-Hill and other publishers of EE books and tell them the old experts like Kraus and Jasik were wrong about dipoles. After all, they have text that shows a "full-wave dipole" in peer reviewed engineering textbooks.

    Seriously, the confusion comes from physics where a dipole is a two pole charge or field, like a "magnetic dipole" (N and S) or an "electric dipole" (+ and -).

    In antennas, it has always been understood a dipole is an antenna split in the center with two equal or mirrored halves. Jasik, one of the leading electromagnetics antenna physicists the world has ever known, and dozens of others besides him have no problem with the concept of full wave dipoles.

    As a matter of fact Jasik's popular Antenna Engineering Handbook, a standard reference book, has a whole section under the heading "full-wave dipole" listig advantages of this antenna over a "half-wave dipole" and a "Collinear dipole".


    Don't fear people. It is all good and proper to have Collinear Dipoles, full wave dipoles, 1/8th wave dipoles, fan dipoles, multimode dipoles, and many other dipoles that have more than one current maxima and more than two voltage maximums. If John Kraus, Jordan, Bailman, Jasik, Termin, and others use the term full wave dipole in engineering texts, it is probably good enough for a bunch of hams.

    73 Tom
     
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  5. K7FE

    K7FE QRZ Lifetime Member #1 Platinum Subscriber Life Member QRZ Page

  6. N9DSJ

    N9DSJ Ham Member QRZ Page

    Thanks for that link, I had not seen it before - an interesting read!

    73,

    Bill N9DSJ
     
  7. W0LMD

    W0LMD Ham Member QRZ Page

    Full wave dipole as cloud warmer.

    50 years ago, I built a full wave dipole for 80M Teletype operation from my QTH North of Detroit. I had a number of friends around Southern Michigan, and Ohio and Indiana on RTTY, so I wanted a very strong signal by using a short range 80M antenna with minimal selective fading and other QSB effects.

    I built a 270' long antenna strung between the tops of two 50' high Rohn telescoping tubular TV masts. Next, I strung a 283.5' reflector 30' below the full wave dipole.

    Before putting up the dipole, I had soldered on 68' of 600 ohm open wire line, that I made, to the center of the dipole, and attached the other end of this open wire line to the house's 8' high roof soffit with insulators.

    I made a temporary 7" long shorting bar with an alligator clip at each end, and attached an alligator clip to a the center conductor and another alligator clip to the braid of a short piece of 50 ohm RG-8 coax wandering though the roof soffit into the basement where my 813 RTTY rig was located.

    Since this antenna was built BAAM (Before Antenna Analyzes and Microsoft), in the days when the air was clean but SEX was dirty, I clipped the sorting bar across the house ends of the open line feeder and then attached the coax clips onto each side of the open wire line about 7' towards the antenna from the shorting bar to start.

    I knew the antenna feed line was probably too long but the points where the shorting bar was clipped onto this 1/4 wavelength long piece of open wire feed line would set the antenna's resonant frequency. The house end of the feed line, being shorted,would be 0 ohms impedance and the other end be some high impedance. So, after about a half day's worth of alligator clipping, then running down the basement to test on my primitive SWR Bridge and then 813 in tune position to determine what to move and which way, and running back upstairs to clip, I had an almost perfect match. I could have taken a week to learn Smith charting etc. and found the best match quicker, but I always prefer quick way to success. Some might argue that I should use a 1 to 1 balun to avoid radiation, but I have always been under the mistaken opinion which says that is what an antenna is supposed to do, radiate!

    And radiate it did. It was one of my best designs of all time, and until its demise years later due to a Michigan freezing rain storm, it worked out like a tall dog.

    I am going to make another one of these soon for my Colorado mountaintop QTH, only this time, it now being SAAM (Since Antenna Analyzers and Microsoft) when the air is dirty but Sex is clean, I have modeled the antenna and found that as I move the reflector off sideways about 30' so that the wire structure from the ends looks like a 2 element wire beam pointing upwards at a 45 degree angle, I will have a broad front lobe in a pattern perpendicular to the line of the wires, and an elevation pattern that extends from straight up to about 30 degrees high. Putting this on a NW to SE line from my house in Conifer, Colorado, I should be able to put out a very big signal from Denver to Europe on 80M with a gain of around 7 or 8 db.


    Robert W0LMD
     
  8. VK3NEA

    VK3NEA Ham Member QRZ Page

    Odd multiples of a quarter wave should work
    so try 1 and a 1/4 wave length each leg of the dipole.

    But as WB2WIK said a better choice of antenna would be an extended double zepp
    and put it up as high as you can.


    Alan.
     
    Last edited: Nov 30, 2008
  9. AE5JU

    AE5JU Ham Member QRZ Page

    Would not a centerfed full wave dipole for 80 meters also be the same length as a centerfed halfwave dipole for 160 meters?

    And the problem is, for that full wave dipole at the specified frequency, there is a node (very high impedance) right at the center feed point, which is why an 80 meter halfwave dipole does not work well at 40 meters, 20 meters, or any other even numbered harmonic.
     
  10. W4INF

    W4INF Ham Member QRZ Page

    Also, the different lengths will effect the lobes, and also change how the lobes develop at varying heights above ground.

    As simple as a dipole looks, its a little more complex at the heart of it.

    Have fun-
    Andrew
     
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