The CS CCR repeater I think has been having link issues, the Denver 145.310/-0.600/123.0Hz machine hears people talking into it but I'm not sure the CS 145.130/-0.600/123.0Hz machine re-transmits the linked audio. The CS machine seems to work locally in the Springs, so it seems to just be the link. The Denver machine seems to work fine, both locally and state-wide. I might be wrong, the truck is down, so I've been driving our car and it doesn't have a radio in it (I forget without exception to bring an HT).
That said, with the standard issue Larsen 2/70 open coil on my truck I can get S6ish into the Springs repeater from south metro Denver. At home I can do either the Denver or Springs CCR repeater with a Diamond X30 on the roof (about 20' AGL). I would expect a Slim Jim to work fine, but would depend on elevation. With VHF and UHF it's all about height and line of sight. A 1/4 w.l. over a decent ground can work long distances as long as there's no obstacles between stations. If there is a house, a tree, a mountain, then even a yagi isn't gonna do miracles. Put the Slim Jim on the roof and I bet it works better. This assumes you have a clear view of Mt Thorodin, which is west of Boulder.
'91 Pickup - Imelda
'08 Tacoma TRD
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
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