A few months ago I built a coil loaded vertical, and put it on my rooftop. My roof is made of corrugated metal, so I connected GND to it, to act as a ground plane (or is it a counterpoise?). With some shunt matching I got a decent antenna.
Out of curiosity, I tried to simulate it on 4NEC2. I started with one of the example files included wih it, the GndScreen.NEC file. This file has 16 radials spreading out. I assume "a ridiculous amount of radials" would be the same as a solid ground plane, at least for HF. So I sized the radials to the shorter side of my roof, changed the radiator to match my radiator length, copied the radials and shortened, to emulate a capacitive hat, and added a load 3/4 of the way up, like I have with my vertical, and elevated it 6M off the ground, like my roof is. The end result looks like this:
The frequency plot matched my empirical tests very closely. But when I tried running the Far field analysis for 3.5MHz, I got this:
This antenna is showing only 30% radiation efficiency. What causes it to be so low? The radiator is about 8M long and it has a 30uH coil 3/4 of the way up. The radiator for a full-sized quarter wave monopole would need to be 20M tall. Mine is 8M. Is a radiation efficiency of only 30% the expected result for a radiator slightly less than half its ideal size?
Here's my NEC file:
CM
CE
SY len=8.8 'Total wire length
SY hgh=9 'Tower heigth
SY segV=int(hgh)*5 'Vertical nr of segments
SY segH=int(len-hgh) 'Horizontal nr of segments
SY ra=360/16, radl=10 'Nr radials, radial length
SY radh =6 'radial height sabove ground
SY rseg = int(radl) '(integer) number of segments
SY cu=5.8e7, fe=1.39e6 'Wire loading for Copper, Steel
SY wrad =.0015 '1.5 mm2 house-wiring
SY RL=0.5
SY lenf=7.5
GW 1 10 0 0 radh 0 0 lenf+radh .015
GW 11 rseg 0 0 radh radl*cos(1*ra) radl*sin(1*ra) radh-0.6 wrad 'Ground screen
GW 12 rseg 0 0 radh radl*cos(2*ra) radl*sin(2*ra) radh-0.6 wrad
GW 13 rseg 0 0 radh radl*cos(3*ra) radl*sin(3*ra) radh-0.6 wrad
GW 14 rseg 0 0 radh radl*cos(4*ra) radl*sin(4*ra) radh-0.6 wrad
GW 15 rseg 0 0 radh radl*cos(5*ra) radl*sin(5*ra) radh-0.6 wrad
GW 16 rseg 0 0 radh radl*cos(6*ra) radl*sin(6*ra) radh-0.6 wrad
GW 17 rseg 0 0 radh radl*cos(7*ra) radl*sin(7*ra) radh-0.6 wrad
GW 18 rseg 0 0 radh radl*cos(8*ra) radl*sin(8*ra) radh-0.6 wrad
GW 19 rseg 0 0 radh radl*cos(9*ra) radl*sin(9*ra) radh-0.6 wrad
GW 20 rseg 0 0 radh radl*cos(10*ra) radl*sin(10*ra) radh-0.6 wrad
GW 21 rseg 0 0 radh radl*cos(11*ra) radl*sin(11*ra) radh-0.6 wrad
GW 22 rseg 0 0 radh radl*cos(12*ra) radl*sin(12*ra) radh-0.6 wrad
GW 23 rseg 0 0 radh radl*cos(13*ra) radl*sin(13*ra) radh-0.6 wrad
GW 24 rseg 0 0 radh radl*cos(14*ra) radl*sin(14*ra) radh-0.6 wrad
GW 25 rseg 0 0 radh radl*cos(15*ra) radl*sin(15*ra) radh-0.6 wrad
GW 26 rseg 0 0 radh radl*cos(16*ra) radl*sin(16*ra) radh-0.6 wrad
GW 11 3 0 0 lenf+radh RL*cos(1*ra) RL*sin(1*ra) lenf+radh wrad 'Ground screen
GW 12 3 0 0 lenf+radh RL*cos(2*ra) RL*sin(2*ra) lenf+radh wrad
GW 13 3 0 0 lenf+radh RL*cos(3*ra) RL*sin(3*ra) lenf+radh wrad
GW 14 3 0 0 lenf+radh RL*cos(4*ra) RL*sin(4*ra) lenf+radh wrad
GW 15 3 0 0 lenf+radh RL*cos(5*ra) RL*sin(5*ra) lenf+radh wrad
GW 16 3 0 0 lenf+radh RL*cos(6*ra) RL*sin(6*ra) lenf+radh wrad
GW 17 3 0 0 lenf+radh RL*cos(7*ra) RL*sin(7*ra) lenf+radh wrad
GW 18 3 0 0 lenf+radh RL*cos(8*ra) RL*sin(8*ra) lenf+radh wrad
GW 19 3 0 0 lenf+radh RL*cos(9*ra) RL*sin(9*ra) lenf+radh wrad
GW 20 3 0 0 lenf+radh RL*cos(10*ra) RL*sin(10*ra) lenf+radh wrad
GW 21 3 0 0 lenf+radh RL*cos(11*ra) RL*sin(11*ra) lenf+radh wrad
GW 22 3 0 0 lenf+radh RL*cos(12*ra) RL*sin(12*ra) lenf+radh wrad
GW 23 3 0 0 lenf+radh RL*cos(13*ra) RL*sin(13*ra) lenf+radh wrad
GW 24 3 0 0 lenf+radh RL*cos(14*ra) RL*sin(14*ra) lenf+radh wrad
GW 25 3 0 0 lenf+radh RL*cos(15*ra) RL*sin(15*ra) lenf+radh wrad
GW 26 3 0 0 lenf+radh RL*cos(16*ra) RL*sin(16*ra) lenf+radh wrad
GE -1
LD 5 1 0 40 37700000 'Left wire
LD 0 1 6 6 0 30e-6 0
GN 2 0 0 0 13 0.005
EK
EX 0 1 1 0 1 0 0
FR 0 0 0 0 3.5 0
EN