Cardioid loop antenna for SW reception

[A.Z.]

Hi there, folks, just stumbled upon an interesting antenna design described here

http://www.crosscountrywireless.net/cardioid_loop_antenna.htm

now, the idea isn't new, since it's basically the same "loop and whip" idea used for radio direction finding, what's interesting is that in this case the loop itself acts as both the loop and the whip; being curious, while I didn't try it yet (will do, probably next week if time permits), I decided to put together a NEC model and run some simulations to see what to expect from the antenna and if I could somewhat improve it, the resulting (optimized) model is this one

Code: [Select]


CM File: loopant.nec
CM optimized vertical cardioid loop antenna
CM impedance arount 450 Ohm from 0.1 to 30 MHz
CM feed with a 9:1 BalUn
CE

' symbols
SY frq=7.100      ' test frequency
SY rad=0.5        ' loop radius
SY hgh=1.20       ' height from ground
SY ray=0.00125    ' wire radius
SY res=510        ' resistor
SY seg=3          ' segmentation
SY se2=11         ' ground wire segmentation

' loop geometry
GW   1 seg 0 rad*sin(0)   hgh+rad*cos(0)   0 rad*sin(10)  hgh+rad*cos(10)  ray
GW   2 seg 0 rad*sin(10)  hgh+rad*cos(10)  0 rad*sin(20)  hgh+rad*cos(20)  ray
GW   3 seg 0 rad*sin(20)  hgh+rad*cos(20)  0 rad*sin(30)  hgh+rad*cos(30)  ray
GW   4 seg 0 rad*sin(30)  hgh+rad*cos(30)  0 rad*sin(40)  hgh+rad*cos(40)  ray
GW   5 seg 0 rad*sin(40)  hgh+rad*cos(40)  0 rad*sin(50)  hgh+rad*cos(50)  ray
GW   6 seg 0 rad*sin(50)  hgh+rad*cos(50)  0 rad*sin(60)  hgh+rad*cos(60)  ray
GW   7 seg 0 rad*sin(60)  hgh+rad*cos(60)  0 rad*sin(70)  hgh+rad*cos(70)  ray
GW   8 seg 0 rad*sin(70)  hgh+rad*cos(70)  0 rad*sin(80)  hgh+rad*cos(80)  ray
GW   9 seg 0 rad*sin(80)  hgh+rad*cos(80)  0 rad*sin(90)  hgh+rad*cos(90)  ray
GW  10 seg 0 rad*sin(90)  hgh+rad*cos(90)  0 rad*sin(100) hgh+rad*cos(100) ray
GW  11 seg 0 rad*sin(100) hgh+rad*cos(100) 0 rad*sin(110) hgh+rad*cos(110) ray
GW  12 seg 0 rad*sin(110) hgh+rad*cos(110) 0 rad*sin(120) hgh+rad*cos(120) ray
GW  13 seg 0 rad*sin(120) hgh+rad*cos(120) 0 rad*sin(130) hgh+rad*cos(130) ray
GW  14 seg 0 rad*sin(130) hgh+rad*cos(130) 0 rad*sin(140) hgh+rad*cos(140) ray
GW  15 seg 0 rad*sin(140) hgh+rad*cos(140) 0 rad*sin(150) hgh+rad*cos(150) ray
GW  16 seg 0 rad*sin(150) hgh+rad*cos(150) 0 rad*sin(160) hgh+rad*cos(160) ray
GW  17 seg 0 rad*sin(160) hgh+rad*cos(160) 0 rad*sin(170) hgh+rad*cos(170) ray
GW  18 seg 0 rad*sin(170) hgh+rad*cos(170) 0 rad*sin(180) hgh+rad*cos(180) ray
GW  19 seg 0 rad*sin(180) hgh+rad*cos(180) 0 rad*sin(190) hgh+rad*cos(190) ray
GW  20 seg 0 rad*sin(190) hgh+rad*cos(190) 0 rad*sin(200) hgh+rad*cos(200) ray
GW  21 seg 0 rad*sin(200) hgh+rad*cos(200) 0 rad*sin(210) hgh+rad*cos(210) ray
GW  22 seg 0 rad*sin(210) hgh+rad*cos(210) 0 rad*sin(220) hgh+rad*cos(220) ray
GW  23 seg 0 rad*sin(220) hgh+rad*cos(220) 0 rad*sin(230) hgh+rad*cos(230) ray
GW  24 seg 0 rad*sin(230) hgh+rad*cos(230) 0 rad*sin(240) hgh+rad*cos(240) ray
GW  25 seg 0 rad*sin(240) hgh+rad*cos(240) 0 rad*sin(250) hgh+rad*cos(250) ray
GW  26 seg 0 rad*sin(250) hgh+rad*cos(250) 0 rad*sin(260) hgh+rad*cos(260) ray
GW  27 seg 0 rad*sin(260) hgh+rad*cos(260) 0 rad*sin(270) hgh+rad*cos(270) ray
GW  28 seg 0 rad*sin(270) hgh+rad*cos(270) 0 rad*sin(280) hgh+rad*cos(280) ray
GW  29 seg 0 rad*sin(280) hgh+rad*cos(280) 0 rad*sin(290) hgh+rad*cos(290) ray
GW  30 seg 0 rad*sin(290) hgh+rad*cos(290) 0 rad*sin(300) hgh+rad*cos(300) ray
GW  31 seg 0 rad*sin(300) hgh+rad*cos(300) 0 rad*sin(310) hgh+rad*cos(310) ray
GW  32 seg 0 rad*sin(310) hgh+rad*cos(310) 0 rad*sin(320) hgh+rad*cos(320) ray
GW  33 seg 0 rad*sin(320) hgh+rad*cos(320) 0 rad*sin(330) hgh+rad*cos(330) ray
GW  34 seg 0 rad*sin(330) hgh+rad*cos(330) 0 rad*sin(340) hgh+rad*cos(340) ray
GW  35 seg 0 rad*sin(340) hgh+rad*cos(340) 0 rad*sin(350) hgh+rad*cos(350) ray
GW  36 seg 0 rad*sin(350) hgh+rad*cos(350) 0 rad*sin(360) hgh+rad*cos(360) ray

' ground connection wire
GW  99 se2 0 rad*sin(170) hgh+rad*cos(170) 0 rad*sin(170)               0 ray

' end of geometry
GE  1

' ground parameters
GN  2  0  0  0  13  0.005

LD  7  0  0  0  2.1  ray        ' insulation
LD  5  0  0  0  58000000        ' copper
LD  0  1  1  1  res  0 0        ' resistor

' feeding
EK
EX 0 18  seg  0  1.0 0.0

' frequency
FR 0 1 0 0 frq 1

' end of model
EN


the model describes a circular loop antenna with a diameter of 1 meter, placed at 1.20 meters from ground and with a 510 Ohm resistor at the top; it's important to notice that raising the antenna (to e.g. 2 meters) would cause the directional pattern to disappear at frequencies above 8 MHz (the pattern would become omnidirectional); also, the choice of the 510 Ohm resistor gives an impedance around 450 Ohm at the feedpoint which is easily matched using a 9:1 BalUn so the antenna could then be connected to whatever preamplifier or other circuit; the structure of the antenna is as follows




basically there's a resistor at the top which splits the loop in two halves, one of those is connected to ground through a run of wire, the resulting antenna offers a radiation pattern like the following one





as you see, from 0.5MHz up to 30MHz the antenna pattern is directional with a pretty low takeoff angle, also, as I wrote, the 510 Ohm resistor allows to find a good match offering an impedance of around 450 Ohm at the antenna feedpoint, the result is the following SWR curve




as you see, the mismatch is minimal, so feeding the antenna through a 9:1 BalUn will allow to directly connect it to whatever standard preamplifier; again, didn't (yet) find the time to build it, but will do as soon as possible, I was also considering the idea of recovering my MLA-30 from the basement and trying to transform it into a "cardioid loop", that one should be an interesting experiment too, although in such a case I'll need to find a value for the resistor which will offer a decent match to the MLA preamp unit