[TowerTalk] 80 meter dipole on 160

[Steve]

I have 80 and 40 meter dipoles on a common center insulator. They are 
fed with about 60 ft of open wire transmission line whose characteristic 
impedance is about 500 ohms. I'd like to use this antenna on 160 meters 
and am looking to do so with an antenna tuner that has only 50 ohm 
unbalanced output.

What's a good balun to use at the rear of the tuner to convert from the 
unbalanced tuner output to the open wire line? Buy or build? I need to 
accommodate legal-limit power.

Is there any point in trying to do some impedance matching with the balun?

Thanks.

Steve K8JQ