Twin coax is _not_ always a good, or even a fair, substitute for
open-wire line. Here's an illustration of how bad it could be.
The driving-point impedance of my quasi-G5RV doublet for f = 10.1 MHz
is equal to ( 2020 - j 3126 ) ohms, according to a NEC-4 simulation
using all the exact dimensions, realistic soil parameters, etc.
Divide this value by two to get what one of the two coaxial cables
would see: ( 1010 - j 1563) ohms. Put this halved value into W9CF's
transmission-line calculator <http://fermi.la.asu.edu/w9cf/tran/>
with just 15 meters of RG-8A or RG-213 and you find that the loss in
this relatively short length of coax would be 5.6 dB.
In other words, nearly three-quarters of my transmitted power would
be dissipated in the coax. Ouch!
OTOH, with 600-ohm ladder line the loss is 0.16 dB -- trivial.
I use 600-ohm ladder line. Needless to say, I'm not about to switch
to twin RG-8A. Yes, I know that 30 m is the worst band for this
antenna, but this is my antenna; I do operate on 30 m (as well as 80,
40, 20, 17, 15, 12, and 10 m) with it; and it works fine.
73 de Chuck, W1HIS