Dont waste your time using coax, using coax achieves nothing that can be measured in the real world.
Use plain old copper wire. If you need a broadband antenna either use a fan dipole or cage dipole. There are broadband feed schemes around that allows coverage of the whole 80 meter band.
I dont think its worth all the trouble. If it was me I would just antenna antenna tuner as a line SWR flattener for going up or down the band.
Modeling is not going to tell you much, coax dipoles dont have gain over wire dipoles! (unless you have curtain array of dipoles on 300ft towers)
If the coax is old or gets a break in the jacket it will soak up moisture and you will have a lousy antenna very fast.
What matters more is height above ground. You will gain much more by doubling the height of your antenna above ground than playing around with crazy antenna schemes like coax
bazooka antennas. Since coax is so heavy you will be losing a lot of height because of the droop. Just stick up a plain old wire dipole thats good enough. If you need more gain at the 35ft height get
a amplifier. 6 or 10db of gain from the amp is far more than you will ever get by playing around with bits of wire or coax.
Another thing that can be useful is using 2 dipoles as a turnstile antenna. If you get creative you can switch
polarization and sense of the turnstile. Its amazing how somethings you can pick up 6 to 10db on receiver just selecting the right sense or polarization even from 2 low dipoles.
After the basic half wave dipole the next best antenna you can use is the double extended zepp antenna. Feed the zepp with open wire line and you can use it on all bands.
If you just talking locally you dont want an antenna that reduces your beam-width. Although at 35ft high you wont have much of a directional pattern.
Another option that you might want to try is vertical or an inverted L antenna. If you fed your low copper wire dipole with open wire line you could short out the feedline and feed it against some radials and use the dipole
as a T vertical antenna. Dont believe people when they say vertical antennas are useless for talking down the road. You can open up a band locally on 75 meters by just switching from a low dipole to a vertical. Not everything is high angle and not everything is low angle. You cant have too many guns in your armory!
You have lots of options for other antenna designs that are proven. Unfortunately coax bazookas antennas are just voodoo antennas that offer no real advantage in the real world.
I'm trying to make a semi-stealthy dipole for a rooftop, and after knowing that you can use 2 runs of coax together as semi-ladder line, it got me to thinking.
What characteristics would an antenna have, if made from coax, and the shield was separated in the middle, and tied to a run of "coax ladder line" back to a tuner?
The coax would be on top of the roof pitch, and would blend in, with the pitch being about 35ft off ground.
I don't have any antenna modeling software, and I though I'd run this by you guys first before I started waiting good coax.
Thanks in advance!