I'm not familiar with the article that you are referencing but I use a trap
in my 160/80m Inv L. The vert leg is 60ft and the trap is placed at the top
of the vert section.
I wound my trap on a 4" piece of PVC and used RG213 coax. I used the
attached freeware program to calculate the dimensions of the trap and it
came out right on the money. Most of the antenna analyzers on the market can
be used to check the resonant frequency of the trap.
Let me know if I can be of any further assistance.
1200 Manor Crest Ct.
Marietta GA. 30068
h 770 321-9159
c 678 642-3603