Same Setup, Different Vehicle, Reduced Range

[BrandonS]

Firstly, I want to say I have not checked SWR on the system currently or on my last vehicle.

So that out of the way, I have a Cobra 19 DX III, 18' coax, and a 4' Firestick (no tunable tip). I previously had it on my Discovery II. On the Discovery, I drilled holes through the spare tire carrier and bolted on a generic chrome antenna bracket. The antenna did not stick up over my roof and roofrack very far at all; maybe a foot. I only did one test and my buddy could hear me at 2 miles, we never tried any farther.

I moved all the equipment, except for the bracket, over to my FJ. I made a new mount on the FJ and there is no bare metal or continuity between vehicle ground and antenna ground. I actually have electrical tape between the painted mount and the door hinge to keep the paint on the vehicle from wearing through. The FJ receives fine and has suprisingly low static compared to the Disco, but my buddy can't hear me past 50 yards or so. The antenna on the FJ clears the roof and rack by a considerable amount, so I don't think it's that.

What would cause such a drastic change? Would it be a lack of ground between the antenna, bracket and the vehicle?

Mount:

Antenna Clearance:

 

[Arclight]

I know it's unpleasant, but you really do want to scrape a section of paint down to bare metal where your antenna connects to the vehicle chassis. You can apply some electrical grease if you're worried about corrosion.

Basically, the antenna screws into a 3/8" mount that is bonded to the center portion of the coax cable, and the "body" of the antenna mount is soldered/screwed to the braid portion of the coax. Your 4' Firestick is considered a "ground plane" antenna, meaning that it depends on the metal of your truck to be attached to that braid portion at the antenna, to act as the "counterpoise" to the firestick.

One other thing: You might want to get a multimeter and check that the coax isn't shorting between the ground and center conductor. This will severely affect your range.


Arclight

 

[BrandonS]

I know it's unpleasant, but you really do want to scrape a section of paint down to bare metal where your antenna connects to the vehicle chassis. You can apply some electrical grease if you're worried about corrosion.

Basically, the antenna screws into a 3/8" mount that is bonded to the center portion of the coax cable, and the "body" of the antenna mount is soldered/screwed to the braid portion of the coax. Your 4' Firestick is considered a "ground plane" antenna, meaning that it depends on the metal of your truck to be attached to that braid portion at the antenna, to act as the "counterpoise" to the firestick.

One other thing: You might want to get a multimeter and check that the coax isn't shorting between the ground and center conductor. This will severely affect your range.


Arclight

Thanks for the suggestions... Sounds like I'll need to get my multimeter out.

 

[BigJimCruising]

Above x2. Also remember to put a spring on that antenna since that sticks up pretty far above the roof of your rig it's certain to catch some tree limbs and other stuff!

 

[BrandonS]

Would a ground strap between the bottom portion of the antenna mount and vehicle mount going to a ground point on the vehicle suitable for this? Basically providing ground to the bottom part where the cable attaches.

 

[4x4junkie]

If the wire is real short (<6 inches) it should help, though it would still be far better to have the mount itself grounded directly to the vehicle body right where it's attached.
I would suspect your SWR is probably off the scale, which wires a foot long or longer will do little-to-nothing for.

Is this vehicle a lease? I'll bet no one will ever notice you scraped a small bit of paint off the door hinge for your bracket if you touch it back up good with some touch-up paint.

 

[BrandonS]

No it's "mine" as much as a vehicle with a loan can be. Just was trying to avoid removing paint if I could.

 

[KA5IVR]

My guess would be that the FJ has less of a Ground Plane with it's body compared to the Discovery. You need to check the SWR and I would think it would be high and impossible to tune without a ground.

Ground your mount (coax shield side) to the frame or even run some hidden ground wires.

 

[Tennmogger]

It's easy to run a good ground along with the coaxial cable. See photo of how i did it (wish it were a little more detail). The ground is of utmost importance.

Bob WB4ETT

 

[BrandonS]

I ended up not needing a ground strap. The end on the coax was shorting the center conductor to the outer shielding.