Kenworth Dump Truck Install Issue

[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.

[Bill_G]

A wattmeter may show something, but a service monitor would help too. If you didn't make a mistake putting the connector on the cable, I'm betting rf is getting back into the radio collapsing PL, or just messing with the radio in general. Put a mag mnt on the box and run the lead to the radio to see if it works fine.

[Jim202]

I have seen a number of installations where the coax connector at the radio has not had the shield
soldered down to the connector. This produces a very poor connection on the ground side of the
coax and causes just the problem your trying to tell us about.

Seems many radio techs have never been taught how to install coax connectors. Heaven would
fall out of the sky if they ever took the time to read any directions on how the connectors are
installed. Managed to have a number of steak dinners paid for by others when I made a bet with
them on what the problem was even before I climbed into a vehicle with the problem your try
to describe.

Jim

[Quazz209]

Bill,
I will give a mag base antenna a shot but I since I was using a non ground plane antenna I had actually removed the antenna from the mirror and moved it away from the truck. It was actually placed on the hood of the truck next to it. So I am not to confident but at this point will try just about anything. If you feel that RF is getting back to the radio and generally screwing with it what is the best way to remedy that situation.

Jim,
The connectors I use are designed to be a Crimp/Crimp style connector and I have probably used well over 100 of them without incident. However, I am open to soldering if you feel it may help. Should I get a solder style connector or do you recommend soldering the shield to the connector and then Crimping over it? Not sure the crimp sleeve would fit in that manner.

Thanks for the help,
Rick

[jackhackett]

You can do a quick check on the cable with an ohmmeter. Remove the antenna, use the meter to check for a short (low resistance) between the NMO mount center and outer contact, you should see an open circuit (very high resistance). If there's no short, connect a jumper wire across the PL connector and check again, you should now show a short.
You can also check continuity between the connector and mount if you can reach both with your meter leads.

[Bill_G]

Bill,
I will give a mag base antenna a shot but I since I was using a non ground plane antenna I had actually removed the antenna from the mirror and moved it away from the truck. It was actually placed on the hood of the truck next to it. So I am not to confident but at this point will try just about anything. If you feel that RF is getting back to the radio and generally screwing with it what is the best way to remedy that situation.

Jim,
The connectors I use are designed to be a Crimp/Crimp style connector and I have probably used well over 100 of them without incident. However, I am open to soldering if you feel it may help. Should I get a solder style connector or do you recommend soldering the shield to the connector and then Crimping over it? Not sure the crimp sleeve would fit in that manner.

Thanks for the help,
Rick

Rick - If you placed a test antenna on the next truck over, and it didn't work, then you need a tech to tear into this to see what is actually happening when you transmit and receive. Maybe it is as simple as you found the 10 square foot area of no coverage in the yard where the truck is parked. Moving the antenna over a few feet should have changed that, but we don't know the condition of the repeater, or the overall performance of the system. I am a little concerned the batt line drops a volt when keyed, but we don't know the condition of the batteries, the power point you tied to, how good the ground is, how much current you're pulling, or the quality of the cable you used. For s&g's you could bring a power supply from the shop, power up the radio, and see if it works with nothing else changed. I'd get another pair of eyes on the problem who has some test equipment to check it all out.

[Jim202]

Bill,
I will give a mag base antenna a shot but I since I was using a non ground plane antenna I had actually removed the antenna from the mirror and moved it away from the truck. It was actually placed on the hood of the truck next to it. So I am not to confident but at this point will try just about anything. If you feel that RF is getting back to the radio and generally screwing with it what is the best way to remedy that situation.

Jim,
The connectors I use are designed to be a Crimp/Crimp style connector and I have probably used well over 100 of them without incident. However, I am open to soldering if you feel it may help. Should I get a solder style connector or do you recommend soldering the shield to the connector and then Crimping over it? Not sure the crimp sleeve would fit in that manner.

Thanks for the help,
Rick

If your using a good crimp for the shell, you should be OK. I never crimp the center conductor.
Those I always solder regardless of the connector style.

You might take the suggestion of trying a mag mount to see if you can hit the repeater.

Also take Bill's comments about using an ohm meter. There have been a couple of times where
the center conductor at the antenna has had a problem either due to a manufacturing defect,
improper install, or water has done it's thing and rotted off the connection at the mount on the roof.

Jim

[jackhackett]

I must've missed that about the voltage first time through... 11.7V might be pushing it, radios are generally rated for 13.8V, I've seen some get flaky under 12. Have you tried it with the truck running?

[Quazz209]

I have not tried my voltage testing with the truck running but the problem still exists with the truck running. I moved it around the yard and that also did not yield any better reception.

I have already ohmed the Antenna cable. I had center pin to connector center pin resistance of 0 Ohms and a mount to outer connector of 1 ohm. I also did not have any shorts between the center pin and outer connector points. That was a complete open.

One thing I did notice when doing that testing was that if the connector was connected to the radio and I removed the antenna and base I would get a short between the center pin and the mount. I did not have an opportunity to test another unit to see if that behavior was similar.

I will post more once I have my forward, reflected, and SWR readings for this unit. Since its operating in the 458Mhz range I had to order another tool. Most of my stuff is down in the Fire/EMS 150Mhz range.

[jackhackett]

One thing I did notice when doing that testing was that if the connector was connected to the radio and I removed the antenna and base I would get a short between the center pin and the mount.

That should be normal. Radios often have have an RF coil from the antenna jack to ground to bleed off any voltage that builds up on the antenna from static, so they'll show a short on an ohmmeter.

Some antennas show a short as well, especially no ground plane types. If the ones you use do, you can use that to test that it's making good contact with the mount by measuring the radio end of the cable and making sure you get a short when the antenna is screwed on.

[Bill_G]

If you have a wattmeter element for 150, then you can get a relative reading at 450 no problem. A bad antenna will still read bad, and a good ant will still have good vswr. Just don't depend on the numbers as absolutes.

[Quazz209]

I am scheduled to be back on site there early next week. As soon as I have more info I will post it up and see where to go from there. If anyone has ideas in the interim I am all ears.

Thanks again

[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?

Of Course the connectors I was waiting on for my Wattmeter got dropped of by UPS at 8pm yesterday so I plan on heading back tomorrow to fine tune the antenna.

[Jim202]

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?
[/quote]

Here is where a service monitor comes in handy. As a desperation test, program up a portable radio on the
repeater input and listen to it as you do a voice check. It maybe down to a dirty modulation pot if this is
one of the old radios that is not software setting controlled. Other than that, try swapping the radio again
and see if the problem follows the radio. Need to have a clue which way to look.

The reception problem either points to a bad antenna system (connector at radio, coax cable from radio to
antenna, bad connection at base of antenna, bad connection of antenna on roof, or a combination of all of
the above). Need your watt meter to find out what is going on.

Forget now, have you tried a mag mount with the cable going right to the radio?

Jim

[Bill_G]

It's time for a tech to look at this.

[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.