This video shows some of the maintenance required for a HF (AKA Shortwave) broadcast station. It starts with transmission line tensioning, some shots of a curtain array then goes on to show the inside of a transmitter building. Transmitters shown are Harris HF-100 (a 1980’s model tube type PDM design) and Continental 418, but I didn’t see the letter number. They are likely tube modulated units.
These are from the international service of Australia Broadcasting Corporation (ABC), or Radio Australia International.
The contracting company I work for takes care of 40-50 radio stations at any given time spanning from NYC and southern Connecticut up to the Canadian boarder. Stations or groups sign contracts and some go by the wayside on a fairly regular basis. All of this makes for the “Giant Box of Keys,” to be carried around in the work truck or whatever vehicle one is driving while on call.
The problem with the Giant Box of Keys is it never seems to be up to date. New stations or groups join and didn’t get added. Other stations fire employees and feel compelled to change the locks after doing so. Therefore, I have found the Giant Box of Keys to be a hit or miss proposition. In my former life as Director of Engineering for Pamal Broadcasting and Dame Media before that, I sought to simplify things as much as possible.
I found that replacing all keyed pad locks with self set combo locks made life much easier. All of the tower fences, transmitter site fences and road gates now had the same combination and could be changed to another combination easily if needed. That eliminated digging trough drawers looking for tower fences keys and or long walks back to the transmitter building because I forgot the key or the key didn’t work. Taking that a step further, I began to replace the door locks with these:
Unfortunately, those door locks are a good deal more expensive than the pad locks, somewhere between $300-400 per copy. I only got a few done before the end came. An alternative to this is a key lock box:
Of course, these are not fool proof either. Occasionally, some previous user will forget to put the key back in the box. I have also had one such box frozen under a lump of ice and once somebody swiped the whole box off of the wall with the aid of a sledge hammer or something.
There are some places where this approach will not work, such as high security sites or leased sites where the land lord dictates the lock policy.
Still, whenever possible, using combo locks saves time and money in the long run. Rather than using precious engineering hours to make and distribute copies of keys, we can be more focused on doing real engineering work.
This is at WEBE and this particular section of transmission line is running 34 KW into the analog/digital combiner in the next room. The clamps are tight, but you can see a little scorch mark on the stainless steel clamp right over the slot in the field flange. That is where there is a gap between the outer conductors, which possibly means the inner conductor was cut slightly too long during installation. I suspect this and or a problem with the bullet is causing the heating issue. I was never (and still am not) a fan of those field flange type elbows, I’d much prefer the flanged type with a field flange on the straight line section.
34 KW is getting into the semi-serious power level for FM broadcasting. At those levels, even small impedance mismatches can lead to big problems. We have a new elbow, field flanges, bullets on order. Unfortunately, we will have to take the station off the air to replace this.
This is all a part of an air conditioning project. There was a plywood partition wall between the front and the back of the transmitters which was impeding air flow. All of the HVAC contractors who bid on the AC job identified it as an problem which needed to be addressed before the big 5 ton wall mount AC units were installed.
Update: Replaced elbow last night (8/4). Went off at 10pm and back on at 10:25. Found the inner conductor had been pushed out of place and was off center on the outside (toward the wall side) of the elbow. This was an older elbow that did not have the nylon inner spacers on the center conductor. The inner conductor was dark purple. Before replacement, the elbow was 138°F (59°C) under full power (34 KW). After replacement, it was 97°F (36°C) as was the rest of the transmission line. At these power levels and frequencies, even small, minor imperfections cause impedance shifts and become issues.
I cleaned up and reused the bullets and the outer conductor with scotch brite. I also used more support wires because I believe the elbow was starting to pull apart, which can sort of be seen in the first picture.
Most (if not all) radio engineers cringe when they here a clap of thunder. Then the waiting begins. What are we waiting for? The cellphone to start ringing, of course. Over the twenty or so years I have been doing this, I have learned a few things. One of them is you cannot over ground something.
That being said, you can, of course, ground something improperly.
The worst areas we have for lightening damage is the Gainesville/Pensacola markets. Those places are in the lightning capital of the US. Time was our class C FM station was getting knocked off a couple of times a month.
There is hope. When we upgraded the stations and installed new transmitters in 2004 I insisted that the tower and building be properly grounded. I even got into an argument with the CFO about the “mission creep” as he put it. Never mind that I put $20K in the initial work specification for grounding.
There are a couple of strategies to use when dealing with lightning at transmitter sites:
Grounding: First, foremost and always. Grounding should consist of multiple ground rods driven as deeply into the earth as possible. At the Trenton Florida transmitter site we used 20 foot long ground rods driven in 20 feet apart all the way around the building and in five 60 foot spokes around the tower. All of these ground rods and tower base were bonded with #2 solid copper wire CAD (exothermically) welded to the ground rods. All turns were kept to a large diameter radius to keep inductance down. When lightning strikes the tower, this creates a large electron sink to dissipate the strike energy into.
Bonding: All equipment cabinets, racks, and everything metal is bonded together and to the same ground point presented by the grounding system. When lightning strikes, often the ground cannot dissipate the energy fast enough. When this happens, the entire ground area around the tower gets charged up. Current will only flow down a less resistive path. If everything is bonded together, the potential between any piece of equipment or component is the same, even if that potential is +10,000 volts. No flow of current means no damage.
The transmitter building is located away from the tower. Almost every FM and TV transmitter site I have visited, the building is right smack at the tower base. By moving the building away about 100 feet or so, the EMP from the tower strike has dissipated (log function) significantly before it passes through the transmitter building. It is a little more expensive to install due to the added transmission line lengths and losses, however it works.
I have been at the Trenton Florida transmitter site when lightning struck the tower. The result, not even a transmitter overload. Nothing noticed on the air, no damage sustained by any equipment. For the last five years, there has been no off air time due to lightning damage at this site.
The studio site has a similar story. We built a new studio building in 2005, there is a 100 foot monopole that holds the STL antennas. You know that it gets hit during a storm. I remember the manager and IT guy from Pensacola commenting about how nice the new SAS Rubicon consoles were. Both of them also said that they wouldn’t last through the first summer because of lightning damage. Four years later, not a single incident of damage to the consoles, computers or anything else in the building because we grounded everything as I described above.