An efficient, shortened dipole - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: July 28, 2025, 01:11:25 pm
content: Let me start from the page which gave me the ideahttps://www.qsl.net/dk7zb/portabel/inverted_u.htmnow, that inverted U is nice, and allows to somewhat reduce the real estate needed for the dipole, but the author didn't consider/solve a couple of pitfalls which reduce the gain, I didFirst of all, if we look at the distribution of currents along a straight dipole we'll see that about the 60% of the arms, starting from the center feedpoint, is where most current is, that is such area contributes the most to the dipole radiation (btw reception too in case of doubt) and in turn, to gainso let's redesign the inverted U dipole so that the straight portion is 60% of the arm lenght (the "dangling" one will then be the 40%) ... now there's another issue, the distance between the (dangling) ends of the arms and ground will be reduced and may (will) increase losses, so.. instead of letting that 40% just "dangle down" why don't we fold it toward the center ? We won't be "closing" the dipole btw and we'll also get another advantage, that is, if we choose an appropriate spacing we'll obtain a better match to 50 Ohms!!but... enough chatter, let me show you some data, the graphs below come from a model I realized usinng 4NEC2, in this case the dipole was calculated for the 20 meters band and placed at an height of 0.60 wavelenght to maximize gain (seeHEREfor details about optimal height and gain) - also notice the gain at 11° above horizon, not badyou can clearly see the structure described above, the overall span of the dipole is 8.17 meters (26.8ft) while the gain (7.77dBi) is only slightly (some tenth of dB) lower than the gain from a straight (full lenght) dipole at the same height, PLUS the impedance match is almost perfect (51 + j0.36) with an SWR of 1.02 and an almost flat SWR curve over the whole band (same goes on 10 meters)not bad imVVHo, thoughts ?Darn me, forgot the 4NEC2 model, here it isCode:CM 20m band shortened dipoleCM feed it using a guanella 1:1CM or an "ugly balun"CE' design frequencySY freq=14.175SY wire=0.0075SY wave=300/freq' height from groundSY hfac=0.60' adjust these for best impedance matchSY lfac=1.2872 ' lenght adjSY sfac=0.0815 ' spacing' wave fractionSY frac=0.50' startSY tlen=(wave*frac)*lfacSY arms=tlen/2SY hght=wave*hfac' fed wiresSY front=(arms*0.60)SY other=arms-front' sides and finsl stubsSY sides=other*sfacSY stub=other-sidesSY hlow=hght-sidesSY segs=3SY segm=21SY segl=51GW 1 segl 0 0 hght 0 -front hght wireGW 2 segl 0 0 hght 0 front hght wireGW 3 segs 0 -front hght 0 -front hlow wireGW 4 segs 0 front hght 0 front hlow wireGW 5 segm 0 -front hlow 0 -stub hlow wireGW 6 segm 0 front hlow 0 stub hlow wireGE 1GN 2 0 0 0 13 0.005EKEX 6 1 1 0 1 0FR 0 0 0 0 freq 0ENfeel free to fiddle with the model and recalculate the dipole for other bands

author: A.Z.
author_rank: Super Contributor
timestamp: July 28, 2025, 02:22:34 pm
content: for the ones willing to go directly to the build here are the sizes for the 20m bandthe height of the upper wire is 0.6 Lambda from ground, that is about 13m (42ft)and the wire in the model is stranded copper with teflon insulation and a diameterof 2.5mm (AWG #14)

author: iMo
author_rank: Super Contributor
timestamp: July 28, 2025, 03:03:41 pm
content: Did you try with the optimizer in the 4NEC2 (or is it in Mmana?) - to let the sw fiddling with the end parts?It seems to me the end parts should be tuned somehow for optimal performance.And I will definitely use a simple balun there in the feed-point.

author: A.Z.
author_rank: Super Contributor
timestamp: July 28, 2025, 03:26:18 pm
content: Quote from: iMo on July 28, 2025, 03:03:41 pmYES I DIDIn fact, willing to adapt the NEC model to a different band my suggestion is the following (with reference to the model I posted)1. change "freq" from 14.175 to whatever other design frequency, for example 28.800 or 7.1002. use the optimizer to adjust "lfac" for min SWR (lenght)3. use the optimizer to adjust "sfac" for min SWR (spacing)4. repeat from 2 until satisfieddo not optimize both "lfac" and "sfac" at once, it may seem easier/faster but won't give you the best result, andthe sw is 4NEC2 I prefer it over MMANA and ezNECregarding the balun and all the other considerations, I believeI already covered them, just a matter of reading

author: A.Z.
author_rank: Super Contributor
timestamp: July 29, 2025, 04:12:59 pm
content: note: modeled a straight dipole in NEC with the same parameters, gain is 7.83 dBi and radiation is the same (same for flat SWR curve) so the shortened one is definitely worth

author: A.Z.
author_rank: Super Contributor
timestamp: July 29, 2025, 05:21:18 pm
content: then if someone wants more gain (almost 11dBi)https://qrper.net/viewtopic.php?t=84the above is ok from 21 to 144 MHz

author: radiolistener
author_rank: Super Contributor
timestamp: July 30, 2025, 04:25:56 am
content: I think such an antenna may not be particularly efficient. Since the ends of the folded structure are left open, the folded-back sections form closely spaced parallel conductors carrying equal and opposite currents - which leads to near-complete cancellation of their far-field radiation, much like what occurs in a parallel transmission line. As a result, these segments contribute very little to radiation and behave more like reactive resonant elements, potentially dissipating some power as heat in these sections due to conductor losses rather than radiating it.I haven’t done a detailed analysis, so the actual losses might not be significant, but at first glance the configuration seems not optimal in terms of efficiency and antenna size. This appears to be due to a portion of the antenna’s physical length being used ineffectively as a reactive matching section rather than for radiation. I think, it would likely be more advantageous to eliminate those folded-back segments altogether, treat the structure as a conventional dipole and use an external matching network at the feed point - resulting in a physically smaller antenna that, with a well-designed matching circuit, could potentially be significantly more efficient.

author: Bud
author_rank: Super Contributor
timestamp: July 30, 2025, 04:49:21 am
content: It is not equal opposite currents. Not equal, since current distribution drops to zero toward dipole open ends.

author: Co6aka
author_rank: Unknown
timestamp: July 30, 2025, 05:00:28 am
content: Quote from: radiolistener on July 30, 2025, 04:25:56 amIn general I concur, except current/radiation maximums along the elements must be considered. Try simply bending the ends 90-degrees instead. Capacitance "hats" at the ends with preceding inductance would probably be better overall. Keep in mind that sidelobes will be a function of current distribution, and nearby objects and support structures happily mess up your beautiful simulations.

author: Co6aka
author_rank: Unknown
timestamp: July 30, 2025, 05:14:56 am
content: Quote from: Bud on July 30, 2025, 04:49:21 amDistribution at the ends will be almost-flat, so if the bent-back length is short current should be almost-equal.If the objective is to shorten the length, using capacitance and inductance at the ends would probably be better.From my experience with 3/8 wavelength verticals, it might be an interesting exercise to experiment with "shortened" 3/8 wavelength dipole elements.

author: A.Z.
author_rank: Super Contributor
timestamp: July 30, 2025, 08:19:02 am
content: Quote from: Bud on July 30, 2025, 04:49:21 amnot just thatbut no sweat, already explained it andtesteditjust look at current distribution and phase

author: A.Z.
author_rank: Super Contributor
timestamp: July 30, 2025, 08:27:04 am
content: Quote from: Co6aka on July 30, 2025, 05:14:56 amshortenandobtain a better impedance match with little gain difference

author: A.Z.
author_rank: Super Contributor
timestamp: July 30, 2025, 12:55:02 pm
content: to be even more clearthe fold is at 60% from center (feedpoint)1. current distribution shows that 60% of a straight dipole, starting from center contributes to radiation (and reception)A straight dipole impedance is around 60..70 Ohm2. using linear loading impedance dropsjust connect the dots

author: A.Z.
author_rank: Super Contributor
timestamp: July 30, 2025, 01:11:10 pm
content: the image below shows the data from a regular, straight dipole with the same parameters as the shortened one, judge by yourselfhere is the NEC model in case someone needs to further fiddleCode:CM dipoleCESY freq=14.175SY wire=0.0075SY wave=300/freqSY hfac=0.60SY lfac=0.9559SY frac=0.50SY tlen=(wave*frac)*lfacSY arms=tlen/2SY hght=wave*hfacSY segs=3SY segm=21SY segl=51GW1segl00hght0-armshghtwireGW2segl00hght0armshghtwireGE1GN2000130.005EKEX611010FR0000freq0EN

author: A.Z.
author_rank: Super Contributor
timestamp: August 02, 2025, 11:22:17 am
content: Quote from: Co6aka on July 30, 2025, 05:14:56 amthe objective is to shorten the dipole, improve the match to 50 Ohm and mantain a gain comparable to the full size dipole and the approach touches all points

author: A.Z.
author_rank: Super Contributor
timestamp: August 02, 2025, 11:25:03 am
content: Quote from: Co6aka on July 30, 2025, 05:00:28 amcap hats, sure, but are more complex to setup and then, what about impedance and gain?

author: KD4PBS
author_rank: Contributor
timestamp: August 05, 2025, 03:16:27 pm
content: More than 100 years of huge brains and deep pockets doing equivalent research for large corporations, as well as budget-minded people simply tinkering with the hobby, and you think nobody has ever tried this before?There's a reason this is not normally done.There may not be equal and opposite currents in the stub legs, but there is some amount of difference, and as previous posters stated, it turns that section into basically a balanced transmission line.You may as well use an end fed balanced transmission line and cut one of the conductors short the same amount as the gap between the ends of this design. It would be just as efficient given a proper match.

author: A.Z.
author_rank: Super Contributor
timestamp: August 06, 2025, 02:47:10 pm
content: Quote from: KD4PBS on August 05, 2025, 03:16:27 pmnever wrote it, nor I pretend to have invented something, if that's what you're trying to say; all I can say is that it works and anyone can easily test itQuote from: KD4PBS on August 05, 2025, 03:16:27 pmif you want, you may test it, I prefer regular wire; oh and notice that even linear loaded dipoles use a similar approach and they weren't invented yesterday, just FYIhttps://www.m0pzt.com/40m-linear-loaded-dipole/

author: KD4PBS
author_rank: Contributor
timestamp: August 06, 2025, 09:30:12 pm
content: I apologize; I thought you had asserted that you had discovered an antenna that operated so exceptionally that it bears a writeup.Dummy loads are usually shorter than 1/2 wavelength with great VSWR over a huge bandwidth as well. I really don't see any benefit to this design and from my own experience, I see no point to make one. It's certainly shorter than a typical 1/2 wavelength dipole, and even a wet cat's whisker can be made to closely match a 50 ohm source at 160M with a proper matching network, but physics tells us it cannot operate with comparable gain to a dipole for the reasons indicated, unless one defines "comparable" as "at least about half". Generally speaking, built-in design elements which promote source impedance matching while maintaining the maximum amount of "wire in the air" will always obtain higher gain than built-in design elements which promote source impedance matching with less "wire in the air". Folding elements back upon themselves in this manner cannot increase gain because it essentially turns that end into a stub of balanced transmission line which doesn't lend itself to radiation. Traditional "folded dipole" antennas use 1/2 wavelength long elements on both "sides" folded in the middle of them with the far end shunted to do just that - more conductor length = more gain. That's just plain facts.If such a design as you show here worked well, it would have been mass produced and deployed in huge numbers long ago by people with large resources. This appears to be just another fractal antenna type exercise.Don't get me wrong though - it's certainly fun to experiment.Oh - and that link gives me"403 Forbidden Access to this resource on the server is denied!"

author: A.Z.
author_rank: Super Contributor
timestamp: August 07, 2025, 08:47:44 am
content: Quote from: KD4PBS on August 06, 2025, 09:30:12 pmYou do ? Really ? ok, let's move on, MattQuote from: KD4PBS on August 06, 2025, 09:30:12 pmI don't think I wrote that, just take your time and reread ... while I skip the meaningless stuff which should be clear after rereadingQuote from: KD4PBS on August 06, 2025, 09:30:12 pmfunny, just tried it and it seems to work fine

author: KD4PBS
author_rank: Contributor
timestamp: August 07, 2025, 02:58:06 pm
content: Quote from: A.Z. on August 02, 2025, 11:22:17 amI re-re-reread it, and this part still stuck out as why I thought that you had discovered an antenna that operated so exceptionally that it bears a writeup.I think my point was that there's no innovation in reinventing inefficient antennas. Gain is not even close to comparable to a dipole in the real world. Simulations can get us close, but cannot model everything accurately. Theories were given as to why this design doesn't approach the performance of a dipole. Fractal antennas sound like an amazing idea until they are pressed into service where it's been shown that they don't perform to the level the simulation predicts.And yes, I do apologize if this was taken personally; I surely didn't. That's why I wrote how even with all of this design's shortcomings, it's certainly fun to experiment.

author: KD4PBS
author_rank: Contributor
timestamp: August 07, 2025, 03:01:06 pm
content: Quote from: A.Z. on August 07, 2025, 08:47:44 am73!

author: A.Z.
author_rank: Super Contributor
timestamp: September 02, 2025, 09:47:07 am
content: . bump .

author: iMo
author_rank: Super Contributor
timestamp: September 03, 2025, 07:56:37 am
content: The biggest challenge for many new hams—especially those 90% living in urban areas—is figuring out how to get on the air from their home QTH with very limited space for antennas. It is a common and frequent topic in our local ham club as well. With little room to work with many operators feel compelled to build short antennas, even though they (perhaps) know the efficiency will suffer a lot. That is why the discussions about “miracle antennas” will always continue to draw interest, imho..

author: radiolistener
author_rank: Super Contributor
timestamp: September 04, 2025, 08:20:59 am
content: Quote from: iMo on September 03, 2025, 07:56:37 amYou are absolutely right that space constraints drive many hams to experiment with shortened antennas. However, a dipole with the ends bent back - forming what is essentially a symmetric section doesn’t really solve the space problem. The bent-back portion carries currents in opposite directions, so it doesn’t radiate, electrically, it’s just a matching section. Yet it still occupies significant physical space.For operators for whom footprint is critical, it’s more effective to remove the non-radiating section entirely and implement a matching network at the feed point. This way, the antenna’s overall size is reduced, but its radiating efficiency remains roughly the same, making much better use of limited space.It’s important to keep in mind that making an antenna physically smaller does not automatically mean it will occupy less effective space for operation. A shortened antenna still has an effective electrical length determined by the wavelength, not its physical size. So if you have limited space for a full-size antenna, the same limitations apply to a shortened one. The difference is that with a physically shorter antenna, you may not notice it as obviously, but it will tend to pick up more noise from nearby apartments, electronics, and wiring. Conversely, when transmitting, a significant portion of power can couple into those same surroundings, potentially interfering with neighbors’ electronics.It should also be noted that a shortened antenna requires a much higher electric/magnetic field strength to radiate the same power into free space. As a result, being close to such an antenna can be much more hazardous than close to a large antenna, since the local field intensity may reach dangerously high levels.It’s analogous to standing on a beach under the sun versus focusing sunlight through a magnifying lens onto a single point. Similarly, being near a small, shortened antenna can be hazardous, whereas being near a full-size antenna at the same transmitted power is generally much safer. This is because the larger antenna distributes the radiated energy over a greater volume, reducing the intensity concentrated at any single point.

author: iMo
author_rank: Super Contributor
timestamp: September 04, 2025, 08:45:57 am
content: New hams usually want to work at 40M/20M and perhaps 15m from their home QTH when talking lower bands, where the lengths are still pretty large. Many are happy with EFHW antennas for 40-20-10m as its short version is only around 11m long.So sometimes it has more sense to have an antenna with 30-40% efficiency of a dipole (not counting the losses in baluns), as none.Moreover, one or two S-es deficit plays no role if you want to go on air in order to collect your first QSOs..

author: A.Z.
author_rank: Super Contributor
timestamp: September 04, 2025, 04:25:40 pm
content: the bare radiation pattern is the *same* of a regular dipole, the gain us just a bit less, nothing one would notice on a S meter and the span is 60% of the one used by a straight dipole which isn't bad for (say) a 40m dipole also since the proposed antenna is cheap and easy to build

author: profdc9
author_rank: Frequent Contributor
timestamp: September 05, 2025, 02:38:11 am
content: I wrote an article about a 20 foot long dual band 20 m and 40 m shortened dipole I made, including the NEC model, tests of its performance, and estimates of its radiation efficiency.I will say that in practice, such antennas are quite easy to unbalance, and making a very high quality balun is necessary if you want the actual antenna to radiate and not just the feedline. I show how to construct one in the article.

author: A.Z.
author_rank: Super Contributor
timestamp: October 13, 2025, 04:00:51 pm
content: Quote from: KD4PBS on August 07, 2025, 02:58:06 pmthere's no "fractal" here; and then I built the antenna for 2, 6 and 10 meters and compared it to a normal horizontal dipole placed ad the same height and.. if there was a difference. I didn't notice it, both the regular and the shortened dipoles offered the same performanceOk, maybe the test wasn't so "scientific" but it served to demonstrate that the 4NEC2 model wasn't all that wrongAnd again, no "miracle" or whatever, just a bit of applied reasoning, nothing else.