Static drain choke

[Radioceo]

I have a site utilizing an Onix Pulsar 5KW. The transmitter suffers damage with each passing electrical storm. The STL receive antenna is mounted on the tower. No isocoupler is used. The feedline is supported by insulators up to the quarter wave point. Here, the shield is bonded to the tower. The shield is bonded to system ground at the base of the tower. Does this negate the need for a static drain choke? And what has been your experience with this type of installation?

 

[RadioFlyerAtl]

I don't see how this arrangement would "negate the need" for a static drain choke. Lightning will follow the path of least impedance, and the tower would likely present a much more favorable impedance to a transient than the shield of a coax line. A static drain choke could help, but make sure your spark gap(s) is(are) spaced correctly. If you're in need of a static drain choke or good spark gap, the folks at Kintronic can help (http://www.kintronic.com/).

It might also be a good time to check grounds around the site. With 5 kW, I would expect to see 4" copper strap in place. Verify low impedance connections from the antenna ground system to the ATU, transmitter, and electrical service. Also, is a surge protector present on the AC mains? Polyphaser (http://www.polyphaser.com/) and LEA International (http://www.leainternational.com/) make great transient protection units.

 

[Radioceo]

Lightning will follow the path of least resistance unless you consider lightning to be AC rather than DC. The purpose of a static drain choke is to drain off static charges before they build up to sufficient potential to cause harm. A static drain choke is not designed to carry the current of a direct lightning strike.

I already have a Kintronics choke and a spark gap on hand, ready for installation. The people at Kintronics are the best at what they do and extremely nice folks to deal with.

A Control Concepts unit is intalled on the incoming AC. I've had great luck with these units. There are issues however with the method of transmitter grounding which was installed by a previous engineer.

 

[ncradioeng]

If you don't have them, install a few ground rods around the tower base and bond them to the strap running up the tower pier to the spark gap. Ground radials do not substitute for for ground rods. I consider a lightning strike a transient event - lots of RF - otherwise they wouldn't be received on radios. If you don't have a turn or two in the line from the ATU to the tower (including the STL line), that may make the problem worse.

 

[Radioceo]

Ground rods are installed. Everything has a loop including the AC feed to the tower lights. All suggestions and opinions are appreciated. Keep them coming.

 

[RadioFlyerAtl]

The typical current waveshape of a lightning strike exhibits a very fast rise time to peak current flow. In fact, it is that characteristic that creates problems for electronics. Hence, even though the strike is DC, we must consider the high frequency effects in our analysis. Since the strike energy is a high frequency pulse, a low inductance path to ground becomes a critical factor. So we must not only be concerned with our DC resistance but the impedance.

If you already have the choke and spark gap on hand, I would say you're headed in the right direction. I have some sites with drain chokes and spark gaps, and some with only spark gaps. Even on those without static drain chokes, the properly spaced spark gaps seem to do the job. We've had little to no problem with lightning at all of our AM sites. As far as Kintronic goes, I second those opinions - Tom, James, and the whole crew are really a great bunch of people to do business with.

I'd also resolve the doubts you might have with the transmitter grounding. Even though copper's at an all-time high, it's still much cheaper than off-air time and transmitter repairs.

I haven't heard much about Control Concepts, but thanks for the information.

 

[littlejohn]

I'd put a static choke in the tuning unit. And I'd take a couple turns of the coax going to the tuning unit around a Mason jar to make a tweo turn choke of the coax. And oput a ferrite slug on the transmitter side of the two turns. And ground the plant inside properly. Lotsa strap and silver rod and a torch.
Remember, since the coax going up the tower is isolated for a distance, a strike on the tower will induce a hell of a pike in the center conductor. Feed it a lightning arrestor.

 

[CW]

Time to maybe look at folded monopole if the lightning is THAT bad.....if you do that, make sure your connections are good, clean and well bonded...otherwise, you may note loss of radiation efficiency in years to come...a folded monopole allows you to bond the coax of the STL to the tower all the way down...better lightning protection for the transmitter as well. I usually dont like the folded monopole but in your case, I would seriously look at it.

 

[ncradioeng]

I'm assuming that your STL is surviving the strikes and the transmitter is not. There should be a copper strap running from the ATU to the transmitter that is also well bonded to the incoming AC power ground/neutral. If that strap is not there (I've seen stations that don't have it), then the coax outer conductor is channeling some of the strike current to the transmitter. If the strap is there, check the bonding at the power panel for arcing (bad connection). If you haven't dug down and inspected the connections around the ATU and radial system at the tower base in a while, it may be worth a look.

 

[phatdaddy]

Has this tranny suffered lightning damage from the time it was installed, or is this something that started sometime afterward? As mentioned by previous posters, all equipment in a shelter should be bonded to a common ground point. In my comm sites I used to use 2" copper strap and make a baseboard halo around the inside perimeter of the building with at least one 10 foot rod at each corner. This halo was then tied to the tower ground system by another copper strap. Each rack or equipment cabinet was then bonded to the halo with it's own separate 1" ground strap. The idea here was to prevent strike energy from flowing from one cabinet through another cabinet to reach ground. All feedlines entering the shack were fed through a Polyphaser entrance panel with appropriate protectors in each feedline. This panel was grounded to the halo with 2" copper strap. The AC service panel was protected with a pellet protector and each cabinet and rack had its own Polyphaser AC protector as well. All phone lines were coupled through protectors and grounded to the halo.

Once all of this was done I no longer had to drive two hours each time it stormed to replace fuses and a handful of parts. It's a costly pain-in-the-butt to retrofit a site this way but the time and expense proved to be well worth the effort.

 

[K4WRF]

All of these are very good points. You should know the mentioned transmitter has well known lighting timidity. There is some information missing that is important. How far is the transmitter building from the tower? And are you saying the lightning is coming in on the STL line? I would recommend you go to the Nautel website, look under Resource Center and lightning protection for radio stations and see if you can implement that solution as a starter. The transmitter house location next to the tower or outside the ground system could require different solution.

W/

 

[Radioceo]

I had intended to pay a visit to this site tonight, but an approaching storm system cancelled those plans. No, I'm not saying the lightning is coming in on the STL line. I was asking for opinions as to whether the STL coax being grounded to the tower at the quarter wave point and again grounded at the tower base would represent the equivalent of a drain choke.

The transmitter building is about 40 feet from the base of the tower. From what I have observed, good engineering practices have been utilized at this site relative to the ground system. The Onix installation does not match up with previous practices. The original transmitter and the Onix sit on top of a wiring trough and
this is where the main ground strap runs. The original transmitter is well bonded. The Onix has been bonded to the cabinet of the other transmitter instead of directly to the ground strap. I had intended to remedy this situation and install the choke and spark gap tonight, but Mother Nature had other plans.

One thing is for sure, this thread could be retitled 'Everything you ever wanted to know about grounding' Good suggestions and advice here.

 

[littlejohn]

I recommend against standing in the transmitter shack during a thunderstorm. It cn be a little more exciting than you like. I'd out a drain choke... do you really want the STL coax to griound thwe thing for DC?
Also, sketch the ground from the tower to the strap in the bottom of your trough... what you >don't< want is for the Onyux box itself to ber a part of the part for a strike going down the tower. That is, if the coax to the tower only grounds at the top of thye radio, and the radio is grounded at the bottom or side, then it's part of the ground path. This will do Bad Things to it. Be sure the coax jacket makes a good ground before the transmitter! (We found an FM we bought in this condition. two pieces of strap and a coax grounding kit saved thousands in blown pieces)

 

[radionekkid]

i once heard oggie prestholt say if at all possible, best locate transmiter bldg off of the ground system and far from tower.

 

[ncradioeng]

Yes, the tower would be at DC ground due to the STL line being connected to the tower at the quarter-wave point and to ground at the tower base - substituting for a static drain choke. There may also be a DC path to ground through an inductor in the ATU depending on its design. Installing the choke may require a remeasurement of your tower impedance as it may change it - if it does, you will have to file with the FCC for a change of antenna impedance. This would show itself as a change in the antenna current and/or transmitter loading after the SDC is connected.

 

[boiseengineer]

Haven't seen any difference in the effect of lightning between a unipole and a series fed tower with a good ground system, other than the added maintenance needed for the unipole. The unipole I'm stuck with is highly sensitive to minor icing. Even when the wind blows hard and warps the skirts it VSWR's. (Yes, the tension is good.)

No consulting engineer I know think the cost/benefit is worth it.