Tribander Plus?\u00c2\u00a0 Plus what?\u00c2\u00a0 Friends with benefits maybe?<\/p>\n
Back in 2002 or so, references to the “open sleeve dipole”<\/a> sparked a curiosity in the topic.\u00c2\u00a0 Very few web references were available back then(in the internet dark ages<\/em>).\u00c2\u00a0 The article that initially tweaked the curiosity was N6LF’s article “A Wideband 80-m Dipole”<\/a>, which built upon an earlier article by K9AY from 1995. The open sleeve idea is also mentioned by Bill Orr, W6SAI.\u00c2\u00a0 Current ARRL Antenna books have information on this topic in Chapter 10, but that was not available in 2002.\u00c2\u00a0 Not much else, but there are a few engineering references going back at least as far as 1945.\u00c2\u00a0 None of which were readily available a decade ago.\u00c2\u00a0 Time marches…..<\/p>\n Recently Joel Hallas, W1ZR, ran a couple of articles on the subject in QST. This article on adding 6m to a tribander<\/a> 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 \u00e2\u20ac\u0153The Coupled-Resonator Principle: A Flexible Method for Multiband Antennas\u00e2\u20ac\u009d in the ARRL Antenna Compendium #5.\u00c2\u00a0 K9AY’s “coupled-resonator” terminology is more precise and accurate as a general description of the principle for amateur uses.\u00c2\u00a0 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.<\/p>\n More references are available today: NQ6K with a well written treatment used as a sloper<\/a>, and DK7ZB has a good page briefly explaining the concept.<\/a> Plus a new ARRL Antenna book(Chapter 10.4-10.5) entry. This is the idea in general terms.\u00c2\u00a0 Placing another wire in parallel and “close” to another dipole allows you to feed the antenna at two different resonant frequencies.\u00c2\u00a0 The feedline is attached to the longer dipole only, the second(and\/or third) wires are excited parasitically.\u00c2\u00a0 By carefully spacing the two wires, it is possible to get a 50 ohm match for both resonances.<\/p>\n O’course, the trick is in the details.\u00c2\u00a0 Getting the spacing correct can be tricky if you are intent on that 50 ohm match.\u00c2\u00a0 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.\u00c2\u00a0 It turns out you can also add a third dipole too, but the spacing dimensions become a lot more critical.\u00c2\u00a0 Much like a fan dipole.<\/p>\n 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.\u00c2\u00a0 Harmonic combinations also seem more sensitive to dimension changes affecting feed point impedance.\u00c2\u00a0 [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.\u00c2\u00a0 Hmmmmmmm.\u00c2\u00a0 Mayhaps wire spacing….?]<\/p>\n 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?\u00c2\u00a0 Something with improved and predictable radiation patterns over a non-resonant doublet?<\/p>\n That seems to be a pretty good option.\u00c2\u00a0 The dipoles are resonant, even if they show an impedance that is not 50 ohms.\u00c2\u00a0 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.<\/p>\n 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.\u00c2\u00a0 In the past we have used several different options for loading up a doublet on multiple bands, with mixed results.\u00c2\u00a0 So in April I decided to cobble an open sleeve antenna together and try it out for a few days.<\/p>\n No, but in this case the model was interesting.\u00c2\u00a0 Going for a decent compromise of performance versus convenience, a bit of EZNEC tinkering showed that a 40m\/20m\/10m\u00c2\u00a0 combination of elements might produce decent\u00c2\u00a0 radiation pattern results for four bands. Input impedance?\u00c2\u00a0 Maybe not so ideal.<\/p>\n The plan modeled was using 18ga ladderline for the mid-section, and attach legs on the ends for the 40m resonator.\u00c2\u00a0 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.\u00c2\u00a0 Radiation patterns for 40m and 20m resemble normal dipole patterns.\u00c2\u00a0 The 15m pattern is normal for a 40m wire pressed into 15m service – a dipole pattern that is\u00c2\u00a0 breaking into two lobes with a nulled lobe perpendicular to the dipole.\u00c2\u00a0 The 10m pattern without the 10m element would show the same butterfly pattern.\u00c2\u00a0 Adding the 10m element makes a huge difference on both the radiation pattern and the feedpoint impedance in this instance.<\/p>\n The antenna was constructed from a segment of 18ga 450 ohm ladder line and some scraps for 14ga thhn laying about.\u00c2\u00a0 The ladder line section was from a 20m folded dipole.\u00c2\u00a0 The parasitic radiators modeled best if the were slightly shorter than they would be in a single wire dipole.\u00c2\u00a0 So the ladderline was shortened to the model’s 20m dimensions.\u00c2\u00a0 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.\u00c2\u00a0 [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.]<\/em> 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.\u00c2\u00a0 These spacers were drilled and attached to the ladderline via cable ties and tape.\u00c2\u00a0 The 10m element then is attached to the outside of the spacers, away from the ladderline.<\/p>\n In hindsight, adding the 10m element with spacers is problematic for a “portable” antenna.\u00c2\u00a0 The spacers complicate storing and deploying the antenna.\u00c2\u00a0\u00c2\u00a0 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.<\/p>\n 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.\u00c2\u00a0 The model show the 15m pattern is “butterfly” shaped, which is normal for a 40m dipole pressed into service on 15m.<\/p>\n It is now hung at almost exactly 30 feet height, and shows the expected performance on those four bands.\u00c2\u00a0 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.\u00c2\u00a0 20m and 40m are equivalent to the other dipoles.\u00c2\u00a0 Not much 10m activity heard yet, so no idea there.<\/p>\n I expect a similar fan dipole could be used in the same manner, where all of the dipoles are physically attached.\u00c2\u00a0 My experiments with fan dipoles was in an attempt to match to 50 ohms.\u00c2\u00a0 Very difficult to do for more than 2 bands.<\/p>\n 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.<\/p>\n 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.\u00c2\u00a0 The butterfly lobes on 15m must be in very favorable directions.\u00c2\u00a0 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.<\/p>\n I’ll not leave it up permanently, because running the ladder line into the shack always raises RFI issues.\u00c2\u00a0 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\u00c2\u00a0 shack and the general in-shack rat’s nest of wiring rather than the antenna.\u00c2\u00a0 It is worth testing out for a bit longer though.<\/p>\n Maybe a few RFI issues will find resolutions during the testing.<\/p>\n Interesting open sleeve(coupled resonator!) ideas….<\/p>\n NQ6K\u00c2\u00a0 slopers…. http:\/\/findatlantis.com\/wiki\/index.php\/20-15-10m_Triband_Sloper<\/a><\/p>\n N6LF Wideband 80m dipole:\u00c2\u00a0 http:\/\/rudys.typepad.com\/ant\/files\/antenna_broadband_dipole.pdf<\/a><\/p>\nPretty handy idea….but….<\/h5>\n
Outside the box of conventional<\/h5>\n
Models?\u00c2\u00a0 We don’ neeed no steeeenkeeeng models…<\/h5>\n
Actual antenna<\/h5>\n
Workee workee<\/h5>\n