"it is the only way in the town i live in and i like having things yesterday - the local radio shack has NO COAX except for 75 ohm rg59, RG6 etc. what happened to radio shack!"
For *most* items, give up on local sources. They're mostly dead. The sooner you embrace the fact you have to buy everything online and start planning ahead the more fun you'll have actually *working* on your projects instead of trying to buy stuff. When I have to order coax, order a bunch extra. When I order some PowerPoles, I buy the 50 pack. Do I need one transistor, four capacitors, and a couple values of resistor for a project? Five transistors, twenty capacitors and a ten pack of each of the values plus a few I'm running low on to go in the drawer.
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The other comment is that RG-6 is actually decent HF transmitting coax. It's roughly the same loss as RG-8 or RG-213, even including a 1.5:1 mismatch, actually, so it's quite a bit better than RG-58 if you can deal with the mismatch issues, which will sometimes be quite minor (more on that in a minute)
VK1OD has some info:http://www.vk1od.net/RG6/index.htm
The copper-clad steel center conductor stuff **could** show extra loss on low frequencies if the copper cladding is thin, but my research into the particular brand of RG-6/QS stuff I bought at Home Depot shows that published loss, measured loss, and loss calculated from VK1OD's calculator for a different sort of RG-6 type cable all agree pretty well.
The mismatch is only an issue compared to using 50 ohm coax when you are trying to feed antennas that are overall a good match to 50 ohms but on the low side. So, say, a 35 ohm antenna.
That's a 1.4:1 SWR on 50 ohm line, and since SWR remains constant along the line, you'll see 1.4:1 at the other end.
However, *impedance* does not remain constant along a transmission line. It goes around a circle of constant SWR as you change the line length, varying from the original resistance up to 1.4 times the line impedance. If you use an electrical half wave or multiple, you'll see 35 ohms again. If you use an odd multiple of a quarter wave, you'll see maybe 71 ohms. Both a low enough SWR as far as your transmitter is concerned. Since your rig is 50 ohms, it's seeing a 1.4:1 SWR. Rig's happy, you're happy.
But now, hook your 35 ohm antenna to 75 ohm line. That's a 2.1:1 SWR as far as the 75 ohm line is concerned... and just like the 50 ohm line, it varies between the original 35 ohms (which your transmitter is happy with) and 2.1x the 75 ohm line impedance... 158 ohms.
If your transmitter is like mine, it will be distinctly unhappy to work into a mismatch of more than 3:1. So the 75 ohm coax acts as a *transformer* that takes your 35 ohm antenna up to 158 ohms. ( This sort of thing is nice in reverse, by the way, taking a 100 ohm quad loop down to 56 ohms by sticking a quarter wavelength of 75 ohm coax at the feedpoint... )
So this is the one way in which running 75 ohm coax can really mess up your SWR. If it's a single band antenna, just changing the line length a quarter wavelength can fix this. If it's a multiple band antenna, the best thing to do is to, at least, make the *antenna* close to 75 ohms on each band by design or with a transformer. Then the line sees a 1:1 SWR referenced to 75 ohms and anywhere you cut it your transmitter will see a 1.5:1 SWR and be happy, or you can use another transformer if you like your SWR meter needle to never move while you're transmitting ;-)
But, with the caution that you need to take this particular impedance transformation issue into account, RG-6 is good stuff for the ham on a budget (especially who needs the coax *yesterday*. Loss of RG-8 or RG-213, most of it handles a fair bit of power, and it's what, twelve cents a foot? Connectors are cheap too.
It's feeding a 15/12/10m antenna and my 20m/17m Moxon here. I use the twelfth-wave transformers referenced in VK1OD's page to match the Moxon, and for the 15/12/10m antenna, I have a different (similar) series section transformer on the antenna side and a broadband ferrite cored 1.5:1 UNUN inside.