Kenworth Dump Truck Install Problems

[Quazz209]

I am working on a number of radio installs for a customer and of the 6 vehicles that I have finished 1 is proving to be very difficult to work with. Now the install is using a UHF no ground plane antenna(cut to length according to the manufacturers chart), and a NMO style mirror mount with RG58 cable. The frequency of the project is at 458Mhz and is transmitting to a repeater. The radios are Kenwood TK-880H and were customer provided. The install went fine in 5 vehicles, all of which transmit and receive perfectly fine. The last vehicle is 1 of 3 Dump trucks that are identical. All of them are 2000 Kenworth Dumps with the same CBs, Ligthing, Bodies, Satellite Radio, etc. Nothing differs. However this one truck will receive (slight static though)but will not transmit consistently. It seemed to intermittently hit the receiver and latch on when I first installed the new antenna but now will not transmit at all. I have swapped radios with other vehicles, at which point this radio will transmit in another vehicle. I have swapped antennas which will also function on the other trucks. I have replaced the mount and cable. I have rewired power directly to the battery and have a solid 12.6V with the vehicle off and 11.7V while attempting to transmit. I have removed power from all other electronic components while performing the tests. However I still cannot get this truck to hit the repeater. Has anyone else had an issue like this or have any other recommendations? It seems as if their is an Antenna issue but I have been unable isolate it to a component. I have a new wattmeter on order so that I can check reflected power on this frequency but am completely stumped. Any help you may be able to offer would be appreciated. Sorry for the long post but I wanted to be as thorough as possible.

 

[davidgcet]

sounds like a bad ground to me. you should not be dropping nearly a volt on a big truck. also could be the antenna, have you swapped it between trucks to verify as well?

 

[Quazz209]

To address the voltage drop - I did this without the vehicle running so that may account for some of the drop. In regards to the antenna - I have swapped the whip and base with another truck, it functions fine on any other truck. I have also replaced the NMO mount and cable twice now. I have re-terminated the cable end 3 times to ensure I had no errors there. The Power, Ground, antenna cable core, and antenna braid all show no shorts and less than 1ohm of resistance to their respective sources. Also I ran jumpers directly to the batteries and bypassed the in cab wiring with the same result. Makes me rule out a ground issue at this point.

At this point I am almost hoping to see a ton of reflective power and be able to better tune the antenna. Perhaps the cut chart was WAY off for this application.

 

[davidgcet]

is the antenna mounted near the dump body over hang? it could be reflecting it back and making it directional. maybe move it further out on the mirror arm if possible?

also, try sticking just a regular UHF stinger up there. a NGP is best for mirror mounts, but at UHF a stinger will work just fine.

 

[Quazz209]

Yes, the antenna is mounted near the Box overhang. There is probably 8" between the antenna and the box at the closest point. However I actually removed the antenna from the Mirror and moved it away from the box with no change in ability to transmit. I was going to try a mag base on top of the box just for comparison purposes.

 

[roadranger]

Did you mean...

Kenworth T2000, or model year 2000?

 

[Quazz209]

Model Year 2000. I am not sure of the model. They are Tri-axle dump trucks.

 

[iamhere300]

11.7 volts, and 12.6 volts signify a problem with the electrical. It should be 13.6 or close in recieve, and
not much less on TX.

If I read you right, the other trucks have the antenna mounted in the same location, so the reflected should
not be an issue off the dump body if the others work OK.

If the cut of the rod is the same as all three, and the location the same, you should not see a significant
difference in the VSWR.

Look at that electrical. That is certainly an issue. Many radios will NOT TX well at such a low voltage.

 

[Quazz209]

Well, the voltage issue seems to be resolved. I bypassed the chassis wiring for the radio and ran 12AWG directly to the fuse panel in the truck. I know drop from 12.9 to 12.3 with the truck off and from 14.26 to 13.7 with the truck running while transmitting. I am also able to consistently hit the repeater when transmitting.
However, now that I can actually transmit another problem has presented itself. Reception on this radio has more static than any of the other trucks and when transmitting only static and a very very quiet voice transmission can be received on any of the other trucks or the base station. I swapped mics but did not have to do further testing. Anyone seen these Kenwoods do this before or have any idea where I should go for this new symptom?

 

[W0JJK]

Could be the radio has an issue, take it to a Radio shop to have it looked at.

 

[code3cowboy]

Have you checked the feed line and antenna? Have you checked the tuning on the radio?

 

[Quazz209]

Well finally got this issue sorted out late last week. After finally getting the power issue figured out I found out that I had a ton of reflected power. Enough that it was actually off the scale. A little experimentation and I found that my antenna needed to be 2.5" longer to get the reflected power down to about .4 watts. All of the other trucks run the shorter antenna with a similar reflected power(.4 watts). This one truck is just an oddball I guess.

To further complicate things though it turned out that the radio was also defective. It will only transmit at extremely low volumes and receive fine. Not sure if the high reflective power damaged it during preliminary testing or it was defective beforehand but its out to get repaired and the installation of all radios is now complete. Thanks for all the help guys.