Antennas

2 antenna's are ok, if you can get them 9' apart. Not going to happen in your case, unless you put one on the front bumper, and one on the back bumper, lol

Folks always think 2 is better than one, but that is not the truth in most cases.

Try using just one, with a single 18' coax. Get the SWR adjusted and try the other antenna. Once you get both set, put them back in cophase (dual) and see what ya get then.

Personally, I like running only 1 antenna, and I like a steel ship type, such as a Predator 10k, Wilson 5000, 2000, and my all time fav is the 102" with a spring. You cannot beat a 102"

 

One 3' K40 antenna in the middle on the headache rack works the best for me, on the dually. It is tall enough to get over the top of the g/n stock trailer.

Ran 2 for a while, on the doors and/or where you have yours. Much better rangewith the single on the rack.

Dual antennas work better for trucks pulling tall and wide trailers, to get the signal around the trailer.

 

I have the exact same radio as you.....In my Freightliner I had to run a separate wire from the antenna to a good ground on the truck the SWR changed dramatically for the better. Also run a heavier wire for the power 8 or 10 ga. stuff it will draw 7.4 amps at its max power. The standard wiring in the freightliner wasn't enough every time I keyed up the display dimmed a bit and with modulation it was noticable till I put a little heavier wire in. If your going to use this type of radio use a good antenna I would recommend Wilson 2000. (just one) and 18' of good coax I got the SWR from 1.8-1 down to 1.2-1 on 1 and 40 after I did the above.

 

Yeah, using a small gauge wire, and not giving the radio what it needs, amperage wise, is starving the transistors (final(s)) for power. In the end, starving them causes them to die.

 

Sounds to me like you have a bad bit of coax. Forget the crap about having to have 18' of coax - cause that really don't matter. I use 75 ohm cable TV coax at home with my ham radio gear tied into a piece of coax that is about 160' long hanging from the trees. Of course I use my MFJ antenna tuner to match the radios to the antenna. In the truck (an Intl 9400i) I use whatever is the least expensive off the shelf at the truck stop and only the 9' length. I get SWR readings on my MFJ meter of about 1.2-1.3:1. Radio is a Uniden Grant XL (about 18 years old) tied into a K-40 antenna using the factory installed, through the mirror bracket coax. I suggest replacing the coax and see what happens. Yes, you might have to adjust antenna, etc. to take care of the SWR.