Antenna under power lines

[Ken9876]

I'm working on a 900 MHz radio system, due to the site location and limitations of the antenna cable length I would need to locate the receiving antennas of two locations under power lines. I would maintain the 10' feet of clearance needed and the antenna would be mounted no higher than the lower cable and phone cables. I'm left with some concerns, one being safety, as long as I maintain the required distance is it safe? The other will the power lines cause any interference with the antenna? There is currently only one set on conductors at the very top of the pole.

 

[Hv&Lv]

As long as clearances are met and the antenna isn't installed in a right of way there shouldn't be a problem. Of course, with that being stated let me tell you true story.

I went on an outage call one afternoon, and immediately rode the line out to find out why the fuse blew. After making sure the line was clear(about 20 minutes), we went and refused the line, restoring power to the neighborhood. Looking further, we found an antenna with burn marks about 4.5 feet apart at the top of the antenna. The lady of the house came outside and wanted to know what happened.(she had came home while we were patrolling the line). We informed her of our concerns and she called around, ultimately finding her husband in the emergency room with holes blown out of the bottom of his feet (small holes). Apparently the husband was installing the antenna too close to the power lines and stuck the antenna into the lines, striking the neutral wire first before hitting the primary line. He lived, actually he came home that day, and he received a visit from our safety director a couple of days later.

The point is, watch what you are doing. Make sure before you start that the antenna will not hit the power line when installing it, or when it falls down in a storm.

As far as RF noise, if there is some, the POCO should be quick to fix this. It is generally caused by arching somewhere on the line, whether it is hardware breaking down, or something as simple as a loose bolt on an insulator.

 

[K8MHZ]

As long as clearances are met and the antenna isn't installed in a right of way there shouldn't be a problem. Of course, with that being stated let me tell you true story.

I went on an outage call one afternoon, and immediately rode the line out to find out why the fuse blew. After making sure the line was clear(about 20 minutes), we went and refused the line, restoring power to the neighborhood. Looking further, we found an antenna with burn marks about 4.5 feet apart at the top of the antenna. The lady of the house came outside and wanted to know what happened.(she had came home while we were patrolling the line). We informed her of our concerns and she called around, ultimately finding her husband in the emergency room with holes blown out of the bottom of his feet (small holes). Apparently the husband was installing the antenna too close to the power lines and stuck the antenna into the lines, striking the neutral wire first before hitting the primary line. He lived, actually he came home that day, and he received a visit from our safety director a couple of days later.

The point is, watch what you are doing. Make sure before you start that the antenna will not hit the power line when installing it, or when it falls down in a storm.

As far as RF noise, if there is some, the POCO should be quick to fix this. It is generally caused by arching somewhere on the line, whether it is hardware breaking down, or something as simple as a loose bolt on an insulator.

Most people think of ham radio as a fairly safe hobby, but the reality is that people are killed just about every year from attempting to install antennas. Some fall from towers, but about the same amount lose control of an antenna and it hits the power lines, often killing them.

The guy is lucky he lived.

 

[T.M.Haja Sahib]

I'm working on a 900 MHz radio system, due to the site location and limitations of the antenna cable length I would need to locate the receiving antennas of two locations under power lines. I would maintain the 10' feet of clearance needed and the antenna would be mounted no higher than the lower cable and phone cables. I'm left with some concerns, one being safety, as long as I maintain the required distance is it safe? The other will the power lines cause any interference with the antenna? There is currently only one set on conductors at the very top of the pole.

Does any code permit your proposal,because voltage with respect to ground may appear on the metal pole supporting the antenna due to induction?

 

[K8MHZ]

Does any code permit your proposal,because voltage with respect to ground may appear on the metal pole supporting the antenna due to induction?

Trust me, voltage to ground will not appear on the metal pole supporting the antenna due to induction.

His first question was about interference. 900 MHz is not very prone to interference from AC lines, if at all.

My concern is safety. Caution should be taken so that the antenna can't fall and make contact with utility wires, and also it must be placed where the utility wires can't make contact with the antenna, should they fall.

He mentions that he will be 10 feet from power lines, but doesn't mention the voltage. POCOs require clearances and those clearances will vary with voltage. IMHO, 10 feet is WAY too close. The POCO may require 30 or more.

Another issue is the FCC. If those antennas are listed with the FCC as being at a particular location and a particular height, the FCC must approve of the change before it's done.

 

[T.M.Haja Sahib]

voltage to ground will not appear on the metal pole supporting the antenna due to induction.

Somewhere in Mikeholt's newsletter the induction issue for metal poles under power lines was mentioned...........

 

[K8MHZ]

Somewhere in Mikeholt's newsletter the induction issue for metal poles under power lines was mentioned...........

I would like to see that. We use metal poles to support power lines, have lights on metal poles with power going to them and I know of hundreds of antennas on metal masts and towers.

If that metal mast is close enough to a power line to get electricity induced into it, it is too close, WAY to close for the other factors to even be considered.

Even if the power lines are insulated triplex, I think that 10 feet is too close.

NEC 810.16 (B) states that outdoor antennas shall be located well away from overhead conductors of electric light and power circuits of over 150 volts to ground.

 

[K8MHZ]

Besides what I mentioned already, will there be a need to change the length of the coax feeding the antenna? 900 MHz is very lossy and requires huge cable with special connectors that use a special tool (and sometimes a solder gun). If you have to add length, it's very likely that you will also have to increase the size of the coax. Coax used for 900 MHz long runs is usually what we call 'hard line'. It has either a solid tube or a corrugated shield and has huge minimum turning radii. You can't just lay it on the roof of building, either. It needs to be supported properly and protected. Any change in the diameter or shape of the outside shield will adversely affect the system, sometimes severely.

 

[Ken9876]

Thanks for the good info. My problem is two antenna locations are in an urban area along a highway. The only room I have to work with is my 10? right of way. After talking with the Mfg. because of the 900 MHz I?m limited to 100? total cable length, I?m using a fairly large cable LMR-400. The antennas are small 18? long Yagis, I plan on mounting them atop a 19? aluminum traffic signal type pole. The primaries at the top of the pole I would put at least 69KV, I would need to put a call in to the POCO to find out exactly what the voltage is. There is no chance of the antenna contacting the primaries, however there is a chance the primaries could fall and contact the antenna. This is a possibility however this is a remote receiving site for a sign and the only damage would be to the equipment. The FCC shouldn?t be a problem as the system is license free. Unfortunately I really don?t have many options here.