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Open Sleeve Dipole, Tribander Plus

Tribander Plus?  Plus what?  Friends with benefits maybe?

Back in 2002 or so, references to the “open sleeve dipole” sparked a curiosity in the topic.  Very few web references were available back then(in the internet dark ages).  The article that initially tweaked the curiosity was N6LF’s article “A Wideband 80-m Dipole”, which built upon an earlier article by K9AY from 1995. The open sleeve idea is also mentioned by Bill Orr, W6SAI.  Current ARRL Antenna books have information on this topic in Chapter 10, but that was not available in 2002.  Not much else, but there are a few engineering references going back at least as far as 1945.  None of which were readily available a decade ago.  Time marches…..

Recently Joel Hallas, W1ZR, ran a couple of articles on the subject in QST. This article on adding 6m to a tribander duplicated an idea I had myself, although I was more interested in the possibility of easily adding 17m to a tribander. W1ZR uses the term favored by K9AY, the “coupled-resonator”, from K9AY’s article “The Coupled-Resonator Principle: A Flexible Method for Multiband Antennas” in the ARRL Antenna Compendium #5.  K9AY’s “coupled-resonator” terminology is more precise and accurate as a general description of the principle for amateur uses.  Yet my own fuzzy gray matter remains wed to the term “open-sleeve”, which is really an example of the general principle in a narrow usage.

More references are available today: NQ6K with a well written treatment used as a sloper, and DK7ZB has a good page briefly explaining the concept. Plus a new ARRL Antenna book(Chapter 10.4-10.5) entry. This is the idea in general terms.  Placing another wire in parallel and “close” to another dipole allows you to feed the antenna at two different resonant frequencies.  The feedline is attached to the longer dipole only, the second(and/or third) wires are excited parasitically.  By carefully spacing the two wires, it is possible to get a 50 ohm match for both resonances.

Pretty handy idea….but….

O’course, the trick is in the details.  Getting the spacing correct can be tricky if you are intent on that 50 ohm match.  EZNEC modeling is simple enough for this gray cat, and it is easy enough to fiddle the dimensions of the second dipole and its length to get that 50 ohm match.  It turns out you can also add a third dipole too, but the spacing dimensions become a lot more critical.  Much like a fan dipole.

Generally, it seems that the higher frequency dipole winds up being slightly shorter than it would if it were a single dipole, and that is more pronounced as additional resonators are added.  Harmonic combinations also seem more sensitive to dimension changes affecting feed point impedance.  [NOTE: ARRL antenna book, chapter 10.5 contradicts my note of “slightly shorter”, indicating the dipoles might need to be slightly LONGER due to the capacitive coupling.  Hmmmmmmm.  Mayhaps wire spacing….?]

Outside the box of conventional

But what if we do NOT concentrate on the 50ohm match? What if we wish to use a balanced line feeder and a tuner, like for, oh, maybe a multi-band wire antenna for FIELD DAY?  Something with improved and predictable radiation patterns over a non-resonant doublet?

That seems to be a pretty good option.  The dipoles are resonant, even if they show an impedance that is not 50 ohms.  Resonance does not imply 50 ohms….that is just a happy convenient choice since radios are designed to have input impedances coincidental with the general impedance of a half-wave dipole fed in the center. But an end fed half wave is just as resonant even though the impedance is nowhere near 50ohms.

With resonance the radiation patterns are more predictable, and there are less likely to be odd lobes as might be the case with a generic doublet.  In the past we have used several different options for loading up a doublet on multiple bands, with mixed results.  So in April I decided to cobble an open sleeve antenna together and try it out for a few days.

Models?  We don’ neeed no steeeenkeeeng models…

No, but in this case the model was interesting.  Going for a decent compromise of performance versus convenience, a bit of EZNEC tinkering showed that a 40m/20m/10m  combination of elements might produce decent  radiation pattern results for four bands. Input impedance?  Maybe not so ideal.

The plan modeled was using 18ga ladderline for the mid-section, and attach legs on the ends for the 40m resonator.  Spacing the 10m wire at about 1.5 inches in the model produced a more stable impedance curve for 10m, so rather than just taping the 10m wire to the ladderline, small spacers are used.  Radiation patterns for 40m and 20m resemble normal dipole patterns.  The 15m pattern is normal for a 40m wire pressed into 15m service – a dipole pattern that is  breaking into two lobes with a nulled lobe perpendicular to the dipole.  The 10m pattern without the 10m element would show the same butterfly pattern.  Adding the 10m element makes a huge difference on both the radiation pattern and the feedpoint impedance in this instance.

Actual antenna

The antenna was constructed from a segment of 18ga 450 ohm ladder line and some scraps for 14ga thhn laying about.  The ladder line section was from a 20m folded dipole.  The parasitic radiators modeled best if the were slightly shorter than they would be in a single wire dipole.  So the ladderline was shortened to the model’s 20m dimensions.  Enough wire to complete the 40m legs to model dimensions was added to the fed wire. Dimensions were pulled directly from the model for all legs – no other trimming, and the pieces stitched together.  [No point in trimming since the intent is to feed the antenna with ladder line to a tuner, rather than obtain a 50 ohm match directly.] The 10m radiator was attached to the ladder line via a few sections of PVC pipe cut into 2 inch lengths for use as spacers.  These spacers were drilled and attached to the ladderline via cable ties and tape.  The 10m element then is attached to the outside of the spacers, away from the ladderline.

In hindsight, adding the 10m element with spacers is problematic for a “portable” antenna.  The spacers complicate storing and deploying the antenna.   Just taping the 10m element to the ladderline is probably worth the efficiency trade off – unless you have real reason to expect 10m to be significantly better than it has been of late.

The end result is a dipole that exhibits the radiation pattern of resonant dipoles on those three bands, and also is relatively easy to load on 15m.  The model show the 15m pattern is “butterfly” shaped, which is normal for a 40m dipole pressed into service on 15m.

Workee workee

It is now hung at almost exactly 30 feet height, and shows the expected performance on those four bands.  One of the major lobes on 15m must be favoring Europe, as the EU stations are better on this antenna than anything else in the yard.  20m and 40m are equivalent to the other dipoles.  Not much 10m activity heard yet, so no idea there.

I expect a similar fan dipole could be used in the same manner, where all of the dipoles are physically attached.  My experiments with fan dipoles was in an attempt to match to 50 ohms.  Very difficult to do for more than 2 bands.

Another caveat to be aware of is that combinations of dipoles at widely separated frequencies, i.e., more than a harmonic, will tend to have “unusual” patterns.

There were too many outside issues to allow much butt-in-chair time in one of my favorite contests, CQ WPX CW. But enough of a chance to try the antenna out, and I’ happy with the results.  The butterfly lobes on 15m must be in very favorable directions.  During the contest Europeans had huge signals, and I worked three JA stations on Sunday afternoon….which are certainly the only JA’s worked from this QTH this decade.

I’ll not leave it up permanently, because running the ladder line into the shack always raises RFI issues.  This antenna is RFI “cranky” on both 40m and 15m inside the shack, but thats probably more an issue related to the kludged feed-thru into  shack and the general in-shack rat’s nest of wiring rather than the antenna.  It is worth testing out for a bit longer though.

Maybe a few RFI issues will find resolutions during the testing.

Interesting open sleeve(coupled resonator!) ideas….

NQ6K  slopers….

N6LF Wideband 80m dipole:

Dan Levin, N6BZA and Marty Levin, W6BDN: Notes on phase delays when stacking:


9 comments to Open Sleeve Dipole, Tribander Plus

  • Earl

    Read your article re: open sleeve dipole with interest but a bit confused – how many total radiators did you use? Spacing? Length? Thanks, Earl

    • I used the normal dipole lengths you might use for any given band, sized to favor the CW segments. The ladder line section is the size of a 20m dipole, or about 33′ total length. The driven element has wire added at each end(Just one of the ladder line conductors…I used the “top”).about 17 feet of wire added to each end brings the driven conductor length to about 67 feet total. The bottom conductor of the ladder line becomes the open sleeve radiator for 20m. A third wire cut for 16.5 feet was taped to the ladder line via spaces.(468/28.2=16.59) That becomes the 10m element. For 15m no element was added. Three total elements, one driven(40m/15m) and two parasitic open sleeves(20m & 10m).

      The spacing is the space between 450 ohm ladderline, about 7/8 inch for 40 and 20 radiators. The 10m wire is spaced at about 1.2 inches since it is attached to standoffs cut from PVC pipe.

      Since the intent is to use a tuner, the lengths are not really critical, and it tunes easily on 40m/20m/15m/10m. I erred on the side of making the elements slightly long. The close spacing seems to make the load a bit capacitive according to the modeling. An ordinary MFJ tuner had no problems matching on any of those bands.

      Given our dismal results on 10m, that probably was not worth the extra effort. But better to be prepared.

      If you look up the FD results from 2012 for K4FQU, you will see that it “worked”.

      if that is still not clear, feel free to fire away with more questions.

  • Earl

    KAZ – upon more careful reading I see the answers to my questions were pretty much in the original article. I work night shift – my graveyard brain must have been engaged at the time of the initial reading. Thanks for your patience. I hope to get some time to experiment with your ideas soon. Earl – KD7HXN

  • onno de pa1ap

    Hi Keith,

    Interesting experiment you have performed and thanks very much for documenting it. Have been working with open line feed combined with 2 x 50 feet dipole and it has never been a super antenna on the higher bands. Decided to pick up 2 x 16 feet wire and installed it as a sleeve to the existing dipole. Space is about 3 inch. It might be conditions being different than last 6 years but now I can hear VK every morning… So something really changed in the good direction I think. Thanks for sharing and hpe to contact you some day on the bands. Regards, Onno

    • Thanks for stopping by Onno. In your specific case the extra sleeve may well have changed the pattern from your antenna in a large way. With your original doublet sized as a G5RV, it probably had a narrow EDZ style pattern on 20m before the mods. Adding a sleeve, I would expect that to become more like an ordinary dipole on 20m. So the orientation of the legs might be more critical in its original configuration.

      I now need to model it. Interesting idea.

      Anyway, I hope you wanted to hear those VK’s. Work them all and have fun.

      73 de w4kaz

  • AB0TO

    I have been using an open sleeve dipole I adapted from the W6SAI HF Antenna Handbook for about eleven or twelve years at my location with good results. It is fairly widebanded for a multiband antenna because there are no traps or coils, and it is full size on it’s resonant bands.

    The antenna is made of 16 gauge insulated stranded wire, egg insulators, nylon cord (being changed to Dacron during maintenance), lexan spacers, and small diameter plastic water pipe, and the primary bands it is cut for are 20, 30, and 40 meters. The 40 meter dipole is fed with coax (not the most efficient feedline but it works)with no balun. 4NEC2 set the wire spacing from the driven 40 meter dipole to the other elements to about 1 7/8″ for the 20 meter wire and 2 1/8″ for the 30 meter wire. The 20 and 30 meter wires are each just a single wire cut for resonance. The wires are attached to a bridle at each end with 1/8″ Nylon or Dacron cord. I use heavier rope to support the antenna. During construction the wire was cut extra long and the excess looped back through the insulators and wrapped around itself so frequency adjustments could be made easily. Adjustment on one band has minor effect on the others because the wires don’t touch.

    The antenna hangs by pulleys from trees by the ends at about 35 feet height with no center support and with a lot of sag that brings the feed point down to about 25 feet of height. A weight made of two 3ft X 2 1/2in sections of angle iron nested together on one end keeps it taut. This antenna has survived ice and extremely high winds that have done considerable damage to trees and structures. The trees are about 80 to 90 feet apart with my house in the middle so it fits the space nicely. I can let the antenna down by uncoiling the extra rope at each end.

    The antenna works with a tuner on the other bands 80 meters through 6 meters, and I run an SB220 on 80 meters for the Missouri Traffic Net. I have even made contacts on 160 meters with this antenna. I have made DX contacts around the world with it with 100 watts and even made contacts to Asiatic Russia using it and my Elecraft KX1 QRP rig.

    • Thanks for the comments.

      The W6SAI reference was the first that I found, and at about the same time frame you mention, 2000 thru 2003. I was a little disappointed the ARRL Antenna book had so little to add. More recent articles in QST have added to the documentation, and I had only recently found a copy of K9AY’s article.

      It is a great concept. We only waited a lot longer to try it out. Too many years wasted, because it worked well for us at K4FQU [FD score in 2012 rank #10-of-135 class 1A].

      In my own implementation the wire spacing is inadequate. EZNEC shows there will be losses, but those losses are a lot less than those we faced forcing a single element doublet onto multiple bands. So as a multi-band antenna the sleeve dipole can play a very useful role, and its another great tool for the ‘Bag-O-Antennas’.

      Planning for K4FQU 2013 FD currently has us as class 3A, and we may fly more single band antennas this year. We are at the same site, which lends itself well to extra antennas. Lots of space and two well located lines of trees.

      Good luck working the DX, es 73

  • If you require more information on the open-sleeve antenna, please refer to
    the ARRL Antenna Book, 16th or 17th edition. In the 16th edition published
    1991, on pages 7-4 through 7-8, I describe the basic principles of the
    open-sleeve monopole as published by Dr. J.T. Bolljahn of Stanford Research
    Institute. The open-sleeve dipole is derived from the open-sleeve

    Also, I published an article, “The Open-Sleeve Antenna” in CQ Magazine,
    Vol. 39, No. 8, August 1983, pages 13-19. This may be the earliest “Ham Radio” reference to the open sleeve dipole, although the professional antenna people have known and used it in designs much earlier.

    73 Roger WB0DGF

  • Mr Cox, thanks for stopping by. It took a while but I finally found someone who was able to provide a copy of your article only recently. I expect to add links and attribution to the archive for those who subscribe or are willing to pay the fee for a one day pass. It is only recently I found that CQ archives are now available for a small fee.

    I am surprised that the open-sleeve technique remained relatively obscure for so long. It is a useful tool to have in the toolkit, and one of those things that make antenna construction so interesting for me.

    Thanks for your own contribution, and thanks for the reminder.

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