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…. http://findatlantis.com/wiki/index.php/20-15-10m_Triband_Sloper

N6LF Wideband 80m dipole:  http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf

Dan Levin, N6BZA and Marty Levin, W6BDN: Notes on phase delays when stacking: http://www.k6if.com/c3_stack_article.html