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. KC0MS

    KC0MS Ham Member QRZ Page

    I read all about half-wave dipoles, but what about a FULL-wave dipole?

    Are there any pro's or con's to putting one of those up if space permits? Any advantage over a half-wave? If I get the green light on this, is the same freg-div-into-468 formula what I would use for dimensions?
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    If you're serious:

    1. No of course the formula would be 936/f for a "full wavelength" dipole.

    2. No, the reason people don't use these is because they're very difficult to match: Impossible to use coax altogether; you can use open wire balance line to match a full-wave dipole but it will still be a 5 or 10:1 VSWR...it just won't matter much because the line loss will still be reasonably low. Still, the risk of arching stuff is much higher with a voltage-fed antenna (like a full-wavelength dipole), and there's about nothing to gain overall.

    A better idea is the "double extended Zepp" which is fairly commonly used and about 0.64 wavelengths per leg, or 1.28 wavelengths overall. Its formula is 1198/f and it's a better choice than a full-wavelength doublet.

    WB2WIK/6
     
  3. W2VW

    W2VW Ham Member QRZ Page

    A full wave dipole will show some gain in the broadside if it is installed high enough.

    A full wave dipole by definition is resonant. It cannot be fed directly with coax though. Lots of people substitute the term resonant for any antenna load that has a low SWR when fed with a hunk of 50 ohm coax. Good quality balanced line feed with a balanced output antenna "tuner" is a common way to have success.

    Google: pair of half waves in phase.
     
  4. WZ0O

    WZ0O Ham Member QRZ Page

    Check the last Bill Oar book on antennas. AES has it its $19.95. I built the off center fed full wave for 17M and like it. Has a off center figure 8 pattern that runs the same direction as the antenna is strung. 4 lobes. My antenna runs north and south. T he lobes run 380 to 340, 20 to 80, 11o to 170 and 200 to 260. Major nulls at 90, 180, 270 and 360. Hope this helps if you need more about it pm me.
    Brad
    WZ0O
     
  5. KC0MS

    KC0MS Ham Member QRZ Page


    I was serious in asking the question alright, but now after reading the responses, I see why no one does it.

    Gotta ask, ya know...it's the only way to ever find out....

    Besides wasting the time it would take to put one up and then find out the hard way it won't tune.

    Thanks
     
  6. K3WRV

    K3WRV Guest

    I actually use something like that - about 550 Feet long, fed in the center with 450 Ohm ladder lind and fed thru a Johnson Matchbox(tm). On 40 meters and up, it's great. Ok on 160 and 80. I've also fed it with TV foam twinlead and an MFJ 941 tuner.

    In theory, it shows a lot of lobes and tends to be directional off the ends., but you can develop some gain Mine runs NE>SW and pretty much covers the world, but is weak towards South America. The lobes haven't been much of a problem. On 80, a 1/2 wave dipole beats it for the most part, but neither is very high - 30-50 feet.

    Wire antennas are easy and cheap to make, and if you don't like it, recycle the wire into something else and try again.

    But as WIK has pointed out, you'll see some pretty strange impedences (I've never tried to measure them), but if you can match them, go for it.

    A half wave dipole provides a good match for 70 Ohm Coax, which is one reason it's popular. And it has a pretty easy to understand radiation pattern (like an infinity or a donunut). But there's nothing magic about it. A very old "all band" antenna is a half wave dipole for the lowest band, fed with open wire, so an 80- meter dipole run on 10 will five the same idea. Try it, you MIGHT like it. Else try something else.

    de Bob
     
  7. WZ0O

    WZ0O Ham Member QRZ Page

    The OCF will tune to about 1.6 to 1 and mine works much better than the 1/2 wave it replaced. Went together in about an hr. and is fed with RG8. I did use a current balun at the antenna to cut down on feed line trouble.
    73
    Brad
     
  8. KA4DPO

    KA4DPO Ham Member QRZ Page

    A half wave dipole is doable but requires a quarter wave matching section (tuning stub) to match to coax. Think about a J pole antenna, same thing. The biggest problem you run into at HF is that the matching section is usually only good over a narrow frequency range and adjustment of the stub can be a royal pain. Even then, you would still want to use open wire line or ladder line as a feeder.

    As WIK pointed out there are easier ways to get gain, and a good match with a wire antenna than a full wave dipole. I just didn't want you to think it couldn't be done. It can, but not as easily as some other antenna configurations.
     
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    ::That was a typo! I'm sure you meant "a full wave dipole....":p

    Anyway, "everything works," it's just a matter of how well. If you have a terrific antenna but can't transfer power into it because it's hugely mismatched, then it's not so great.

    A narrowband matching network can always be built to transfer power from a source to a load even if the natural mismatch is terrible....but then you have a narrowband matching network, which works at one frequency and not anywhere else. This is why most hams don't bother -- however it's a pretty acceptable solution for non-hams in the commercial/broadcast/aerospace/etc industries where a single-frequency match is just fine.

    WB2WIK/6
     
  10. KC4RAN

    KC4RAN Ham Member QRZ Page

    A fullwave loop, on the other hand, is one of those antennas that many inexperienced hams dismiss as boring, but don't realize the potential they hold, even as multibanders...
     
  11. 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.
     
  12. 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
  13. 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:)
     
  14. 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
     
  15. K7FE

    K7FE QRZ Lifetime Member #1 QRZ Page

  16. N9DSJ

    N9DSJ Subscriber QRZ Page

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

    73,

    Bill N9DSJ
     
  17. 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
     
  18. 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
  19. 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.
     
  20. 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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