thanks for the replies - poor SWR will certainly kill signal in a hurry on this antenna and power is likely not an issue (VHF propagation is better vs. UHF as pointed out above).
i really dont want to add an antenna just to pick up 1 channel (sorry ModernFamily on ABC) -- reading around on digitalhome.ca and other forums, there are a few CM4221HD hacks mentioned (
http://www.digitalhome.ca/forum/showthread.php?t=104837) and I will try these to see if it brings in Ch7/KGO. The size of the reflector screen seems to matter (current 24" may not be sufficient to reflect VHF frequencies, quarter-wave @ 180MHz = 40cm). UHF signals appear strong enough that losing some gain at UHF with a larger reflector may still be ok.
From
http://www.hdtvprimer.com/ANTENNAS/comparing.html it appears CM4228 may have much better VHF reception at the cost of increased size/cost.