You have to put a shortening factor there, so for erko 300/145.15MHz=2.066m/4=0.516m*0.95=0.49m.
It goes well with the counterweight, but worth a try to do
you're Slim Jim's, see
http://www.m0ukd.com/calculators/slim-jim-and-j-pole-calculator .
Or do it from the TV dual line according to http://ok1ike.c-a-v.com/soubory/2m_slim_jim.htm. There may not be a tube for a portable.