just keep the power below 200 watts and expect to see a 1.5 to 1 swr when tuned to lowest point.
Well, not necessarily. If the antenna is 50 ohms then the SWR on the 75 ohm coax would
indeed be 1.5 : 1, but the SWR relative to 50 ohms at the transmitter end could be as high
as 2.25 : 1, or it might be 1 : 1, or anywhere in between, depending on the length of the
coax (in wavelengths.)
For example, if the RG-6 is 1/4 wave long, then 50 ohms at the far end would get transformed
to 112 ohms at the shack. But if the coax is 1/2 wave it would be 50 ohms. Of course this
would vary with frequency, since a quarter wave at 40m is a half wave on 20m.
It gets even more complicated because the antenna might not be exactly 50 ohms. Let's say
that the SWR at the antenna relative to 50 ohms is 1.33 : 1. If this is because the antenna is
37.5 ohms, then a quarter wave of 75 ohm coax would transform it to 150 ohms, and the
SWR in the shack would be 3 : 1. But if the antenna were 66.7 ohms (which would have
the same SWR) then a quarter wave section would transform it to 85 ohms, for an SWR of
1.7 : 1, which is probably still usable without a tuner.
If the antenna happens to be 75 ohms, then we'll have a 1.5 : 1 SWR in the shack (measured
relative to 50 ohms, as will be the case with most equipment) regardless of the coax length.
But with 50 ohms, or other impedances that would give a usable SWR on 50 ohm coax, it will
vary more widely due to the transformation effect of the 75 ohm coax.
If you cut the feedline to a half wave on 40m (which would be about 55 feet) then it will
be close to a multiple of 1/2 wavelength on 40, 20, 15 and 10m, and should give a usable
SWR without too much trouble. On some other bands you might need a tuner.
