Cardioid loop antenna for SW reception - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: June 15, 2022, 03:32:18 pm
content: Hi there, folks, just stumbled upon an interesting antenna design described herehttp://www.crosscountrywireless.net/cardioid_loop_antenna.htmnow, 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 oneCode:CM File: loopant.necCM optimized vertical cardioid loop antennaCM impedance arount 450 Ohm from 0.1 to 30 MHzCM feed with a 9:1 BalUnCE' symbolsSY frq=7.100 ' test frequencySY rad=0.5 ' loop radiusSY hgh=1.20 ' height from groundSY ray=0.00125 ' wire radiusSY res=510 ' resistorSY seg=3 ' segmentationSY se2=11 ' ground wire segmentation' loop geometryGW 1 seg 0 rad*sin(0) hgh+rad*cos(0) 0 rad*sin(10) hgh+rad*cos(10) rayGW 2 seg 0 rad*sin(10) hgh+rad*cos(10) 0 rad*sin(20) hgh+rad*cos(20) rayGW 3 seg 0 rad*sin(20) hgh+rad*cos(20) 0 rad*sin(30) hgh+rad*cos(30) rayGW 4 seg 0 rad*sin(30) hgh+rad*cos(30) 0 rad*sin(40) hgh+rad*cos(40) rayGW 5 seg 0 rad*sin(40) hgh+rad*cos(40) 0 rad*sin(50) hgh+rad*cos(50) rayGW 6 seg 0 rad*sin(50) hgh+rad*cos(50) 0 rad*sin(60) hgh+rad*cos(60) rayGW 7 seg 0 rad*sin(60) hgh+rad*cos(60) 0 rad*sin(70) hgh+rad*cos(70) rayGW 8 seg 0 rad*sin(70) hgh+rad*cos(70) 0 rad*sin(80) hgh+rad*cos(80) rayGW 9 seg 0 rad*sin(80) hgh+rad*cos(80) 0 rad*sin(90) hgh+rad*cos(90) rayGW 10 seg 0 rad*sin(90) hgh+rad*cos(90) 0 rad*sin(100) hgh+rad*cos(100) rayGW 11 seg 0 rad*sin(100) hgh+rad*cos(100) 0 rad*sin(110) hgh+rad*cos(110) rayGW 12 seg 0 rad*sin(110) hgh+rad*cos(110) 0 rad*sin(120) hgh+rad*cos(120) rayGW 13 seg 0 rad*sin(120) hgh+rad*cos(120) 0 rad*sin(130) hgh+rad*cos(130) rayGW 14 seg 0 rad*sin(130) hgh+rad*cos(130) 0 rad*sin(140) hgh+rad*cos(140) rayGW 15 seg 0 rad*sin(140) hgh+rad*cos(140) 0 rad*sin(150) hgh+rad*cos(150) rayGW 16 seg 0 rad*sin(150) hgh+rad*cos(150) 0 rad*sin(160) hgh+rad*cos(160) rayGW 17 seg 0 rad*sin(160) hgh+rad*cos(160) 0 rad*sin(170) hgh+rad*cos(170) rayGW 18 seg 0 rad*sin(170) hgh+rad*cos(170) 0 rad*sin(180) hgh+rad*cos(180) rayGW 19 seg 0 rad*sin(180) hgh+rad*cos(180) 0 rad*sin(190) hgh+rad*cos(190) rayGW 20 seg 0 rad*sin(190) hgh+rad*cos(190) 0 rad*sin(200) hgh+rad*cos(200) rayGW 21 seg 0 rad*sin(200) hgh+rad*cos(200) 0 rad*sin(210) hgh+rad*cos(210) rayGW 22 seg 0 rad*sin(210) hgh+rad*cos(210) 0 rad*sin(220) hgh+rad*cos(220) rayGW 23 seg 0 rad*sin(220) hgh+rad*cos(220) 0 rad*sin(230) hgh+rad*cos(230) rayGW 24 seg 0 rad*sin(230) hgh+rad*cos(230) 0 rad*sin(240) hgh+rad*cos(240) rayGW 25 seg 0 rad*sin(240) hgh+rad*cos(240) 0 rad*sin(250) hgh+rad*cos(250) rayGW 26 seg 0 rad*sin(250) hgh+rad*cos(250) 0 rad*sin(260) hgh+rad*cos(260) rayGW 27 seg 0 rad*sin(260) hgh+rad*cos(260) 0 rad*sin(270) hgh+rad*cos(270) rayGW 28 seg 0 rad*sin(270) hgh+rad*cos(270) 0 rad*sin(280) hgh+rad*cos(280) rayGW 29 seg 0 rad*sin(280) hgh+rad*cos(280) 0 rad*sin(290) hgh+rad*cos(290) rayGW 30 seg 0 rad*sin(290) hgh+rad*cos(290) 0 rad*sin(300) hgh+rad*cos(300) rayGW 31 seg 0 rad*sin(300) hgh+rad*cos(300) 0 rad*sin(310) hgh+rad*cos(310) rayGW 32 seg 0 rad*sin(310) hgh+rad*cos(310) 0 rad*sin(320) hgh+rad*cos(320) rayGW 33 seg 0 rad*sin(320) hgh+rad*cos(320) 0 rad*sin(330) hgh+rad*cos(330) rayGW 34 seg 0 rad*sin(330) hgh+rad*cos(330) 0 rad*sin(340) hgh+rad*cos(340) rayGW 35 seg 0 rad*sin(340) hgh+rad*cos(340) 0 rad*sin(350) hgh+rad*cos(350) rayGW 36 seg 0 rad*sin(350) hgh+rad*cos(350) 0 rad*sin(360) hgh+rad*cos(360) ray' ground connection wireGW 99 se2 0 rad*sin(170) hgh+rad*cos(170) 0 rad*sin(170) 0 ray' end of geometryGE 1' ground parametersGN 2 0 0 0 13 0.005LD 7 0 0 0 2.1 ray ' insulationLD 5 0 0 0 58000000 ' copperLD 0 1 1 1 res 0 0 ' resistor' feedingEKEX 0 18 seg 0 1.0 0.0' frequencyFR 0 1 0 0 frq 1' end of modelENthe 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 followsbasically 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 oneas 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 curveas 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