First Pass at a Backyard Loaded Dipole

40 meters is a great amateur band because it's open all night. That is, it's not like the 20 meter band that stops skipping radio signals around the globe as soon as the sun goes down. Unfortunately a 40 meter dipole is 20 meters long end-to end and my backyard can only fit about 15 meters if it's run diagonally. The trick is to add load coils about 1/3 of the way from the end to lower the resonant frequency of the shortened elements.


It looks something like this in the XNEC2 simulator:

The yellow boxes represent the loads. The red box is the location of the balun that connects the antenna to the feed line. In simulation, the trick is to alternately shorten the elements then increase the inductance of the loads to prevent the resonant frequency of the antenna from rising. Resonance is identified by the frequency at which VSWR is at a minimum.

Here's the NEC2 card stack I finally arrived at:


CM --- NEC2 Input File created or edited by xnec2c 3.6.1-beta ---
CM 40 meter loaded dipole
CM (null)
CE --- End Comments ---
GW     1    15  -6.00000E+00  0.00000E+00  7.62000E+00  6.00000E+00  0.00000E+00  7.62000E+00  1.50000E-03
GE     0     0   0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
EX     0     1     8      0  1.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
FR     0   201     0      0  6.00000E+00  1.00000E-02  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
NH     0     0     0      0  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00
NE     0    10     1     10 -1.35000E+00  0.00000E+00 -1.35000E+00  3.00000E-01  0.00000E+00  3.00000E-01
LD     0     1     3      3  1.00000E+00  2.45000E-05  1.00000E-06  0.00000E+00  0.00000E+00  0.00000E+00
LD     0     1    13     13  1.00000E+00  2.45000E-05  1.00000E-06  0.00000E+00  0.00000E+00  0.00000E+00
RP     0    19    37   1000  0.00000E+00  0.00000E+00  1.00000E+01  1.00000E+01  0.00000E+00  0.00000E+00
EN     0     0     0      0  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00  0.00000E+00

After fiddling with the GW and LD cards I came up with a loaded dipole optimized for 14.070 MHz (the PSK frequency).

Antenna Length: 12 Meters
Number of Segments: 15
Load Segment Locations: 3, 13
Load Inductance: 24.5 uH

Next step is to see if a 24.5 uH inductor, 12/15 meters long is even practical. The next iteration will probably be to move the inductor closer to the feed point, and increase the number of segments.

Next I'll compare my design to published designs as a sanity check on the simulation. If that check out, I'll build it and measure the performance with an antenna analyzer.

Anyway, here's what the VSWR and radiation pattern look like.