I've been bouncing around the internet and have found that vertical dipoles seem to be a great idea to use on a big rig due to lack of metal in big rigs today. however I wonder wouldnt what little bit of metal that is on a big rig effects swrs of said dipole? I'm thinking of using a 108 whip as the main element and the Radio Shack with the fiberglass base load and 50 something inch stinger as the counter poise. Thanks in advance for answers and comments.
With the dipole, one side is the HOT side and the other is the RF ground side. Obviously you would use the HOT side as the top vertical section of the dipole. You can use MFJ -347 dipole mount or make your own as long as the HOT antenna uses the insulator bushing and the RF ground does not.
As long as you weren't mounting the antenna on the exhaust stack grab handle on a Pete, I wouldn't worry about what little metal is left around the antenna.
You SHOULD use equal lengths antennas to do this correctly. However 2 full size 1/4 wave whips will be too long at being over 18 ft with mounts. Even using just one 1/4 wave whip mounted to the lower mirror bracket arm on a Freightliner or Volvo or any truck with upper and lower mirror posts will exceed 14 ft.
You would be better off using loaded antennas of the same brand and lengths, either 4 or 5 ft. fiberglass or steel ones. Most fiberglass antennas these days are 5/8 wave but that doesn't really matter. I believe the Francis antennas are still 1/4 wave so I would use two 4 1/2 ft antennas for this project.
There is one point to consider, Since a dipole is a balanced antenna, it is not 50 ohms. Even using 2 full size 1/4/ wave whips in this manner will have an feed-point impedance around 75 ohms, not 50 ohms, so the SWR will not be flat if that's what you were hoping for.
With 2 loaded antennas matched for 50 ohms and even using 50 ohms coax, I would expect your feed-point impedance to be even higher around 100 ohms. This can be verified easily if you have an antenna analyzer one or know a real CB shop that has one.
This is a easy fix using a 2:1 balun. A balun is a transformer device used to balance an unbalanced feed-line for feeding a balanced antenna, Hence the name BAL-UN. At a 2:1 ratio, this means it will cut the feed-point impedance down at 100 ohms in half to 50 ohms which should give you a nice flat SWR match. 50 ohm coax cable is an unbalanced feed-line.
You could also use a mobile antenna tuner which will be easier. But antennas tuners don't really tune an antenna, they just match your radio to a mismatched antenna.Your antenna will still have a higher SWR from the impedance mismatch, but at least your radio will see a near perfect match.
Good luck with your project!