UHF antenna shorted out.......

[Cuttsy87]

Hey guys,

I have a uniden 7700nb UHF installed. Tonight when I got home I had to unload a washing machine. It was dark so I flicked on my rear work lights. Now this is where it gets interesting. I walked out after installing the washing machine to find my car full of smoke and my antenna bent over. Upon onspection. The UHF aerial cable has got extremely hot and melted the coaxial cable. Just wondered if anyone has has this happen to them..... I have picked the power up from the power wire for my work lights (led 27w). It hasn't popped any fuses or anything. So i am leaning towards the antenna being chaffed somewhere. Just odd because the unit wasn't even switched on.

Help me understand please????

 

[camo.b]

Wow very lucky!!! Is the worklight earthing souly on the uhf antenna mount at all?

 

[Navmania]

That is strange. The antenna doesn't carry any power or load unless transmitting and then normally the radio will blow if something is wrong before the antenna would melt. It looks like you have put a power source to the coax somehow. Because an antenna like the one you are using connects to ground / earth, I would say you have had a dead short in the coax and its melted. You wouldn't have blown a fuse due to the diodes in the back of the radio stoping the power from feeding back to the radios internals. The only way the radio will blow is if you have the centre of the coax touching the shield in the plug or if the coax has damage, and then it will only damage the radio if you are transmitting for a period of time and it's not very long at all. This blows what's called the finals diodes in the radio. How you got power of that amperage to the coax to melt it is a mystery that's for sure.

 

[Cuttsy87]

Mmmmm I've got no idea at all. Maybe the unit is faulty. Or maybe the coax is rubbing somewhere. Is the outershield positive?? Think I might just take it to a specialist.

 

[camo.b]

Navmania.i wonder if it was a long small load....that finally got the cable up to temp

 

[Cuttsy87]

Just don't know when the unit was not even on.....

 

[ozstickman]

Outta shield is earth, inside core is kinda the positive but a CB band uhf wouldn't have a enough power running through it on transmission to melt it.

 

[camo.b]

Just don't know when the unit was not even on.....

Where did you earth the lights?

 

[Cuttsy87]

Just a bolt under the dash.

 

[Cuttsy87]

Im leaning towards a faulty unit for some reason

 

[Navmania]

I don't think so cambo. To build up enough heat to melt it I think it would have Been pulling some serious amps of direct power from somewhere. Transmitting power is different to battery power. The coax doesn't have a positive or negative so to speak, the inner core is for the transmission and the out shield is to stop interference. And if your antenna is a ground independent it could only do that if its rubbed to a positive supply wire and its grounded at the same time. I have feed 1.2kw up an antenna and it won't even get warm let alone melt it. That's 1200 watts. The only place that the coax should touch is the plug in the back of the radio and the antenna base. That's it.

 

[Navmania]

Does the radio still turn on?

 

[camo.b]

Just wondering if his earth point is actually earthing onto the uhf body..from there its going thru the coax outer sheild and finally to earth

 

[camo.b]

Thats the led light earth point...

 

[camo.b]

Does the radio still turn on?

That could be interesting..

 

[nuckin futs]

where the coax runs through the firewall is there any other wires running next to the coax

 

[Navmania]

Just wondering if his earth point is actually earthing onto the uhf body..from there its going thru the coax outer sheild and finally to earth

That's possible, but they are all earths. If a positive wire has rubbed and is touching the steel case of the radio that could do it. But you think the fuse would blow unless the fuse was bigger than the wire in the antenna and it melted it instead. The centre wire in the antenna is only small.

 

[nuckin futs]

unless the fuse is behind the dash and not near the battery

 

[camo.b]

Im also remembering my rg6 cable i used for my antenna stuff..large single copper core and multi strand outer...is uhf coax smaller

 

[Cuttsy87]

Yeh got me stumped. Trip to the auto sparky I think. I'm no specialist in the area. So ill get the pros to have a look. It's in the overhead console. So it could be somewhere in the headlining as well I guess. I'll keep you all posted anyways. Cheers for the input guys.