Here is a quote from an article:
Experimentation in the early 20th century determined that the best POWER HANDLING capability could be achieved by using 30 Ohm Coaxial Cable, whereas the lowest signal ATTENUATION (LOSS) could be achieved by using 77 Ohm Coaxial Cable. However, there are few dielectric materials suitable for use in a coaxial cable to support 30 Ohm impedance. Thus, 50 Ohm Coaxial Cable was selected as the ideal compromise; offering high power handling AND low attenuation characteristics.
With 50 Ohm Coaxial Cables being the best compromise solution, practically any application that demands high power handling capacity, i.e. 100 watts or more, will use 50 Ohm Coaxial Cable. A good rule of thumb is that any device that functions as a transmitter or transceiver tends to use 50 Ohm Coaxial Cable. This includes devices such as CB/Ham Radios, Broadcast Radio/TV Transmitters, Wi-Fi and Cellular Phone Repeaters and 2-Way Radios
Back to me : I see that 70 ohm coax is lower loss and you may find that most repeaters using hardline, concidered the best,low loss feedline,are using 70 ohm hardline as it induces LESS LOSS THAN 50 OHM COAX. It really matters little that you use 50 or 70-75 ohm coax and an SWR METER does not indicate loss,only match impedance as most antenna/coax is mismatched anyway as a properly cut quarter wave vertical antenna is 31 ohms (until the owner introduces loss buy cutting it(or radials) to a non quarter wave length) And the typical, 1/4 wave each side,dipole has an impedance of 71 ohms until someone mistakenly trims the length and detunes the antenna,thus introducing LOSS so as to see a lower SWR..
You see ,cutting an antenna to adjust SWR only degrades the antenna performance to achieve a SWR reading that really represents little and costs loss in performance ...I only 'trim' an antenna when SWR in above 1.7 to limit heat buildup in the transmitter. As long as an antenna is within reasonable SWR range to trim it beyond it's measured length for lower than acceptable SWR only fools the meter by introducing loss in the antenna/feedline/ground system
A 70 ohm dipole should not show below 1.5 to one SWR unless loss due to cutting the antenna or otherwise inducing LOSS to the system.
Last thought:
Just do what you want to do. 50 or 70 OHM coax will not cause the losses that cutting an antenna to satisfy an SWR meter will introduce.
Here is a link to read.:
http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/
