...mounted to the aluminum catwalk on my 09 Freightliner Cascadia...
I'm using a Wilson Teardrop surface mount and have run a 12 gauge ground wire from it directly to my frame.
Several potential problems here.
First, how much of the antenna is up in the clear above the rest of the metal structure of the vehicle?
Or, to put it another way, how much of the antenna is running close to the metal structure of the vehicle?
That has a huge effect on antenna tuning and performance. And, to the extent that the antenna
sways in the wind, the tuning can shift with speed. The ideal location is with the whip mounted above
any other metal, but that isn't likely with a tall antenna on a truck that is already pushing the height
How large is the catwalk? How long is the #12 wire connecting the base of the antenna to the
A #12 wire is rather small in this application. Not that you need to carry a lot of current, but you need
a lot of surface area to reduce the inductance and make a low-impedance path. 1" wide flat copper
strip is a better choice, and using several of them to different points on the frame will improve things
75m mobile whips can be very difficult to tune, and should have very narrow bandwidth if it is at
all efficient. Tuning across the band in 100kHz steps you might not find any point that is better
than 5 : 1 if the resonance happens to be midway between the points you measure. As Steve
said, even finding the band in the first place can be difficult, and the SWR at resonance for such
a short antenna (again, if it is at all efficient) may be 2 : 1 or higher.
One of the best tools for the job is an SWR analyzer that reads the sign of the reactance: put
it RIGHT AT the feedpoint and a negative reactance means you need to lengthen the antenna.
But if there is any significant length of coax (more than a couple feet) this becomes less accurate
(though it might help you find the band at least.)
One thing you can do is to tune though the band from ,say, 3 to 5 MHz and listen for a rise in the
background noise. A significant increase over a range of perhaps 30 to 60 kHz may mean that you've
found the resonant frequency. I've also tied a rope to the whip and bent it towards or away from
the vehicle - if you hear a peak in the noise with the antenna pulled away it means that the
antenna is too long. (Note that you have to stand out of the field of the antenna to take these
measurements - your body standing next to the antenna while adjusting the stinger is enough to
throw the tuning off. I used to lay under my Volkswagen when checking the antenna so I was
out of the pattern, though that still changed the capacitance between the chassis and ground.
And I was skinnier in those days, too.)
Meanwhile a quick resistance check with a VOM should confirm whether there is a short in the
base or coax. You can also check the performance on a different band with a different antenna
to see if there is a problem - if a quarter wave CB whip doesn't show a low SWR on 10m or 12m
then you need to start by fixing the mount. Make sure that is working before worrying about
trying to tune the 75m antenna.