I tried to model a multiband yagi. I added radiators for 10-15-20M, and reflectors spaced .15 wavelenghts away. This gave a nice result with three sharp dips in SWR in the 10-15-20M center frequencies.
For some limitation of NEC2, I had to make equal wires for all the bands and then extend them with more wires (NEC2 wanted parallel wires to have segments aligned)
I then tried shortening the elements with coils. I shortened these extension wires, and put a series L at the first segment, in the 15 and 20m driven elements and reflectors.
When doing a frequency sweep, the 20 and 15M bands show acceptable dips, but the 10M band does a strange thing: it has a wide SWR dip from 27 to 28mhz, then a spike at 28.1mhz, and then comes back down at 28.2mhz
Is this spike an artifact of simulation, or is it real interaction between the elements altering the frequency?
Here's the NEC file:
CM Back yard dipole, converted with 4nec2 on 26-mar-20 9:20
CE
SY DIEZmdrlen=5.03 '2.66
SY QUINmdrlen=7.07 '7.07
SY VEINmdrlen=10.63 '10.63
SY DIEZmdrpos=0.8 '0
SY QUINmdrpos=1.06 '1.06
SY VEINmdrpos=1.7 '1.7
SY h=8 '8
SY d=.019/2 '.019/2
GW 2121 2 .03 QUINmdrpos h -.03 QUINmdrpos h d
GW 1 20 .03 DIEZmdrpos h DIEZmdrlen/2 DIEZmdrpos h d
GW 2 20 .03 QUINmdrpos h DIEZmdrlen/2 QUINmdrpos h d
GW 3 20 .03 VEINmdrpos h DIEZmdrlen/2 VEINmdrpos h d
GW 111 20 -.03 DIEZmdrpos h -(DIEZmdrlen/2) DIEZmdrpos h d
GW 112 20 -.03 QUINmdrpos h -(DIEZmdrlen/2) QUINmdrpos h d
GW 113 20 -.03 VEINmdrpos h -(DIEZmdrlen/2) VEINmdrpos h d
GW 1111 5 -.03 DIEZmdrpos h -.03 QUINmdrpos h d/2
GW 1112 5 -.03 QUINmdrpos h -.03 VEINmdrpos h d/2
GW 2111 5 .03 DIEZmdrpos h .03 QUINmdrpos h d/2
GW 2112 5 .03 QUINmdrpos h .03 VEINmdrpos h d/2
GW 4 4 DIEZmdrlen/2 QUINmdrpos h DIEZmdrlen/2+.5 QUINmdrpos h d
GW 5 4 -DIEZmdrlen/2 QUINmdrpos h -(DIEZmdrlen/2+.5) QUINmdrpos h d
GW 6 4 DIEZmdrlen/2 VEINmdrpos h (DIEZmdrlen/2+.8) VEINmdrpos h d
GW 7 4 -DIEZmdrlen/2 VEINmdrpos h -(DIEZmdrlen/2+.8) VEINmdrpos h d
GW 31 15 (DIEZmdrlen*1.05/2) -DIEZmdrpos h -((DIEZmdrlen*1.05/2)) -DIEZmdrpos h d
GW 51 15 (DIEZmdrlen*1.05/2) -QUINmdrpos h -((DIEZmdrlen*1.05/2)) -QUINmdrpos h d
GW 61 15 (DIEZmdrlen*1.05/2) -VEINmdrpos h -((DIEZmdrlen*1.05/2)) -VEINmdrpos h d
GW 32 5 (DIEZmdrlen*1.05/2) -QUINmdrpos h (DIEZmdrlen/2+.5) -QUINmdrpos h d
GW 33 5 -(DIEZmdrlen*1.05/2) -QUINmdrpos h -(DIEZmdrlen/2+.5) -QUINmdrpos h d
GW 34 5 (DIEZmdrlen*1.05/2) -VEINmdrpos h (DIEZmdrlen/2+.8) -VEINmdrpos h d
GW 35 5 -(DIEZmdrlen*1.05/2) -VEINmdrpos h -(DIEZmdrlen/2+.8) -VEINmdrpos h d
GE 1
LD 0 6 1 1 0 15.5e-6
LD 0 7 1 1 0 15.5e-6
LD 0 4 1 1 0 3e-6
LD 0 5 1 1 0 3e-6
LD 0 34 1 1 0 15.5e-6
LD 0 35 1 1 0 15.5e-6
LD 0 32 1 1 0 3.5e-6
LD 0 33 1 1 0 3.5e-6
GN 2 0 0 0 13 0.005
EK
EX 0 2121 2 0 0 0 0
FR 0 0 0 0 14.1 0
EN
```