Shortened dipole

[Andrew (grayhat)]

Being bored and having some time in my hands I started fiddling with 4NEC2 to explore an idea I had time ago, but
let me start from the beginning

first of all a fact; if we bend in an U shape the arms of a dipole keeping the "straight portion" (from feedpoint to bend) at 60% of the arm lenght, gain will have a minimal impact [1] and impedance match to 50 Ohm will improve

So, let's go on and do it for a 40m band dipole, fold the arms horizontally "back" then keep the sides around 20cm and fold the wires again toward center

in my model this gave a 40 m dipole with about 8 meters for each arm, so fitting in a somewhat limited space BUT keeping the straight dipole gain (above 7 dBi) and offering a match for about 1.7 MHz, with SWR<2 .... not bad, I believe, and then, no loading coils or matching systems, a regular guanella balun will fit and work well

[1] if you look at the current distribution along the arms of a half wave dipole you'll see that most current (that is most radiation) is in the middle 60% hence the choice of such a shortening value which allows to mantain the gain and avoids the need for complex impedance matching

[Andrew (grayhat)]

Code: Select all

CM shortened dipole
CE

SY freq=7.100  

SY wire=0.0075
SY wave=300/freq

SY hfac=0.60
SY lfac=1.2709
SY frac=0.50

SY tlen=(wave*frac)*lfac
SY arms=tlen/2
SY hght=wave*hfac

SY front=(arms*0.60)
SY other=arms-front

SY sfac=0.082
SY sides=other*sfac
SY back=other-sides


SY segs=3
SY segm=21
SY segl=51

GW  1  segl  0      0 hght      0  -front hght wire
GW  2  segl  0      0 hght      0   front hght wire

GW  3  segs  0 -front hght -sides  -front hght wire
GW  4  segs  0  front hght -sides   front hght wire

GW  5  segm  -sides -front hght -sides -back hght wire
GW  6  segm  -sides  front hght -sides  back hght wire


GE  1
GN  2  0  0  0  13  0.005

EK

EX  6  1  1  0  1  0

FR  0  0  0  0  freq  0
EN

the NEC model to be totally clear just save the code as "short_dipole.nec" and open it with 4nec2

[Andrew (grayhat)]

here's a summary, notice the span and the gain

shortdip.jpg (154.05 KiB) Viewed 41285 times

[Andrew (grayhat)]

OK, changed the model so that the folded portion is below, so that the contruction/installation of the dipole is easier, here's the new model optimized for the 20 metters band (centered on 14.175 MHz)

Code: Select all

CM 20m band shortened dipole
CM feed it using a guanella 1:1
CE


' design frequency
SY freq=14.175  

SY wire=0.0075
SY wave=300/freq

' height from ground 
SY hfac=0.60

' adjust these for best  impedance match
SY lfac=1.2872
SY sfac=0.0815

' wave fraction
SY frac=0.50

' start
SY tlen=(wave*frac)*lfac
SY arms=tlen/2
SY hght=wave*hfac

' fed wires
SY front=(arms*0.60)
SY other=arms-front

 ' sides and finsl stubs
SY sides=other*sfac
SY stub=other-sides
SY hlow=hght-sides

SY segs=3
SY segm=21
SY segl=51

GW  1  segl  0      0 hght      0  -front hght wire
GW  2  segl  0      0 hght      0   front hght wire

GW  3  segs  0 -front hght      0  -front hlow wire
GW  4  segs  0  front hght      0   front hlow wire

GW  5  segm  0 -front hlow 0 -stub hlow wire
GW  6  segm  0  front hlow 0  stub hlow wire

GE  1
GN  2  0  0  0  13  0.005

EK

EX  6  1  1  0  1  0

FR  0  0  0  0  freq  0
EN

loading the model in 4NEC2 and running it results in something like this

shortdip14.jpg (169.33 KiB) Viewed 40119 times

notice that the impedance match is quite good and the bandwidth with SWR<2 is quite wide, the gain curve (not shown) is almost flat, offering more than 7dBi gain when (as in model) the dipole is raised at 0.6 wavelenghts

the overall span is about 8.17m or 26.8ft which may possibly fit some "critical" installations

the idea of bending the arms came from the "inverted U" dipole shown here

https://www.qsl.net/dk7zb/portabel/inverted_u.htm

I just bent the dropping portions to keep them far from ground (and reduce losses), while still having a shortened dipole

willing to recalculate the dipole for other bands, just change the value of "freq" to the desired center frequency, then use the NEC optimizer to recalculate the value of "lfac" for minimal SWR, next do the same for "sfac" and you should be ok, the sizes calculated in NEC will be a good starting point to build the dipole and we may keep it a bit longer to account for environmental conditions and prune it after

[Andrew (grayhat)]

And here's the 3D radiation pattern, as you can see, it's just like the one from a regular size dipole

shortdip14-2.jpg (63.66 KiB) Viewed 39741 times

https://qrper.net/download/file.php?id=194