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Basic troubleshooting procedures require a process of elimination in order to get to the culprit.

Remember easy & cheap 1st, hardest and expensive last.

With the antenna system the possibilities are the coax cable, a stud mount, antenna or antenna’s, the antenna ground, and in some cases the mounting location of the antenna.

Check them out in this order – ground, antenna, stud mount, coax and antenna placement.

Try moving the coax back and fourth, then grab the antenna mounting bracket.

Take caution here; stay away from the antenna and don’t touch it or the top of the stud mount.  If the SWR fluctuates you have a ground problem.

The SWR should be below a 2.5 and not fluctuate more than .1 up or down when moving the coax or grabbing the bracket. In other words 1.9-2.1 of fluctuation is ok. Remember, paint and primer are your enemy.

Pinched or melted coax jacket is not good- replace it. Use an 18 foot long RG58 coax cable for a single antenna system and two 18 foot legs of RG59 for a dual antenna system. Read about coax cable.

Spin the antenna in a circular direction and check the SWR. Look for serious fluctuation or a spike. If it exhibits either replace it and same for the stud mount is screws into. The SWR will fluctuate when the antenna is spinning.
For example: The SWR is a 3.0, you spin the antenna and the SWR fluctuates up to a 4.3. This is normal especially if their is anything close to the antenna such as an exhaust stack, pickup cab, headache rack and so on.
If you experience an off scale spike or fluctuation close to off scale, replace the antenna and the stud mount it screws in to.

Ground is a consideration and tends to be the problem much of the time. For example, the antenna mount must be bolted to clean metal with no paint or primer. The structure the mount is bolted to, must also connect to the chassis and make an electrical connection.

A good example of this is a tool box mount in a pickup bed. Most people mount the tool box on top of the bed rail only after sandwiching rubber between the bed rails and tool box. Now you have paint, primer and rubber preventing the electrical connection. I advise drilling two holes, one at each end of the tool box located so that a self tapping screw can be inserted to connect the tool box to the bed rail top. Clean the contact surface (the area the self tapper’s head will contact) to bare metal. Insert one on each side of the tool box. This will ground the tool box to the truck body. (Don’t use nuts & bolts)

Most people make the same mistake at the antenna mount itself. Four bolts holding the mount in place will not ground the bracket if the tool box is painted. Either clean the contact surface (the area the bolt’s head will contact) or drill a fifth small hole and insert a self tapper in place. The self tapper screw will bite into the tool box and ground the bracket.

The point is that the entire path must ground not just the antenna mount for example, and don’t even consider ground wires. At 27 MHz, ground runs on top of the wire not through it. It tapers off quickly normally within a foot or two. If using a ground wire, use a basket weave type material with much surface area and make it as short as possible. We use base coax, strip the shield out, flatten it, and apply heat shrink to clean it up.

Trunk lip mounts require special attention for proper ground. This type of mount normally uses two allen screws to tight the mount to the trunk (From the bottom side). The contact area on the trunk must be scrapped to bare metal removing ALL paint and primer. A short ground strap should also be installed at each supporting trunk arm close to the hinge.
Use self tapping screws and attach the ground strap from the vehicle body to the supporting arm.
The goal is to bypass the hinge. Many hinges have teflon or some other type of non conducting material built into its construction.

The final possibilities are antenna placement, height, and the way the antenna is reacting on the vehicle.

If using a center load type of antenna verify the bottom of the load is equal to or above the top of the vehicle. The majority of the signal radiates from this part of the antenna. The SWR will normally drop and your overall performance will improve if you follow this rule.

If the antenna is mounted to a Big Rigs mirror arm, try leaning the antenna forward 5-7 degrees. Signal from the antenna can strike some other object such as an exhaust stack, and reflect or (bounce back) striking the antenna and raising the SWR and degrading overall performance.

The antenna could be reacting badly to your auto body size & shape.
If all other recommendations have been followed and the SWR problem has not been resolved try relocating the antenna.
The antenna can quickly be relocated in order to rule out placement or to simply change the way it is reacting.
Antennas can react badly under some circumstances causing one to believe a problem exists when their is no fault with any part of the system.
Don’t be concerned with the look, you are not looking for a permanent mounting location, just another good grounded location away from anything that could interfere with the antenna system.
Once the antenna has been relocated check the SWR.
If the problem has not been resolved at this point try changing the antenna to a different type of antenna.

Vehicle resonance – ALL vehicles; cars, trucks, vans, motorcycles, etc., resonate at a particular frequency. This is called vehicle resonance.
The vehicle resonance reacts with your antenna system 100% of the time. Sometimes this reaction causes antenna systems to have poor performance and or high SWR.
Under this circumstance you won’t be able to pinpoint a problem.
Each piece of the system can be changed with no improvement.
The ground is good, all pieces of the system have been verified to be good, but the SWR is still high.

What do we do now?
Change the antenna to a different type. If the antenna is a base loaded antenna try a center load or a top loaded fiberglass antenna and recheck the SWR. If the antenna is a fiberglass or top loaded antenna try a center loaded or base loaded antenna.

If none of the above resolves your problem and you can’t find a local professional that can resolve your issue, bring it to us. We will resolve your issue.

Please call for an appointment before showing up as we may be booked for the day.
We can service your equipment – read more

 

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Clay Thompson

Owner, Lead Tech & Head of R&D at Clays Radio Shop.


(Clays Radio Shop