It's too bad that you must have such a long coax run for your system. The loss over such a length - even with good cable - would be unacceptable in my book.
Is it necessary to use such a long piece?
Here's a coax calculator for you to figure out your losses due to cable inefficiency:
Coax Calculator
Remember this fact: just 1db/100 ft of loss from your coax reduces the output power of your radio by 20%. For 300 ft, multiply it by 3. That is - if your system is going to throw 1500 watts output; then only 600 watts are getting to the antenna.
Makes you think twice about having a long coax run - eh?
It should.
The crappier the cable - the losses go from bad to worse.
The longer run you need with crappy cable means you are wasting power and efficiency.
SO - use only the length that is necessary and buy the best coax you can!
If you lose 3db/100 ft with crap cable; you have lost HALF of your output.
Check it out...Do the math...