tech stuff – Page 62 – Engineering Radio

Copper theft and how to avoid it

One of the unfortunate signs of the times is the increased theft of valuable materials. Copper, while not as expensive as it once was, still fetches a fair amount at the scrap dealer. One local telephone company has been having a difficult time keeping its aerial cables intact in certain areas. For radio stations, the situation is compounded by remote transmitter sites with lots of copper transmission lines and buried ground radials around AM towers. Reduced staffing levels also mean that the weekly trip to the transmitter site is now every two weeks or perhaps once a month or even less.

Sites that are not visited or monitored very often are prime targets for copper theft. Forget asking the local constabulary to patrol more often, the few times I tried that I was met with a blank stare.

A few common sense type things that I have learned over the years may keep your site intact:

Keep up appearances. A neglected transmitter site is more likely to attract the wrong type of attention from the wrong type of people. Clean up any rubbish, dead equipment, keep the weeds and trees cut down, etc. If a site looks well tended and is often visited, a thief may think twice about lifting valuable metals.
Along with #1, keep things buttoned up. Secure all transmission lines to ice bridges, remove any deadlines, etc. If there are ground radials poking out bury them, same with ground screens, copper straps, etc. Out of sight, out of mind, leaving this stuff exposed is asking for somebody to come along and give a tug.
Fences and locks. Towers are required to be fenced and locked to prevent electric shock hazards. It is also a good idea to fence the building, generator, and fuel tank if possible.
Post all sorts of warning signs, RF warning, high voltage, no trespassing, under video surveillance, pretty much anything to deter trespassing and vandalism.
Add video cameras with a video recording device since most theft occurs during non-working hours. Last year, the company I used to work for traded a video surveillance system for the studio location.
Compensate a neighbor to keep an eye on the place and call you if they see any suspicious activity. It doesn’t even have to be money, I once worked out a deal with a neighbor for some T-shirts and CDs. That was the best alarm system we ever had.

In the long run, keeping all the copper parts where they belong is a great way to avoid those annoying “the station is off the air” phone calls not to mention the expense of replacing damaged transmission and ground systems.

Delta Current Sample Toroid

Another example from my blown-up shit collection, artifacts division:

Delta TCT-1HV current sample toroid destroyed by lightning

This is a Delta TCT-1HV current sample toroid that was pretty well destroyed during a thunderstorm. I mounted it on a piece of plexiglass because I think it looks cool. This unit was installed at the base of the WGY transmitting tower. One June evening, I received a call from the station operator (back when they had live operators) that the air signal sounded kind of “funny.” So I turned on the radio and sure enough, if one thinks a radio station that sounds like a motorboat is funny, then, why yes indeed, it did sound funny.

Since I only lived a few miles away from the site, I jumped in the trusty truck and headed over. Upon arrival, I found the MW50B on the air at full power, with the carrier power swinging wildly from 20-90 KW with modulation. Hmmmm, bad power supply? Turned the transmitter off and tried to place the backup transmitter on the air. Now the old Gates BC5P had never been super reliable in the first place, but it was odd that it would not even run at all.

Then I had a hunch, let’s walk out to the tower I said to my assistant who had shown up to help. When we got to the ATU building it was filled with blue smoke. Ah ha! Somebody let the magic smoke out of one of the components! I was expecting a capacitor blown in half but was surprised to find the copper tubing that connected the ATU to the tower melted in half. Lightning must have caused an arc between the tubing and the toroid and for some reason, the transmitter kept on running while it was arcing. The copper tubing in the picture with the toroid is only missing about six inches, the way the system was mounted at the tower base, fourteen inches of copper tubing was missing, or rather melted into a puddle on the bottom of the ATU.

I quickly found another piece of 1/2-inch copper, cut it to length, flattened out the ends with a hammer, and drilled mounting holes. Luckily I was able to get everything back in order quickly and the station returned to the air about an hour or so after it went off.

Everything has a cause. Investigation showed that the VSWR circuit on the MW50 had been disconnected from the directional coupler. The lead was un-soldered and taped off, so it was quite intentional. I spoke briefly with two of the three prior engineers that had serviced the MW50 over the years, they both blamed the other one. I surmise this; The WGY tower was prone to lightning strikes because of its height. Even if the tower was not directly struck by lightning, oftentimes the guy wires would arc across the insulators, causing the MW50 to momentarily interrupt the PDM signal and drop the carrier for about a second. Some programming people at the station did not like this, it sounded bad on the air, so one of those guys undid the VSWR circuit, and voila! No more momentary outages during a thunderstorm! Brilliant! Except for the 60-90 minute outage one night…

Sometimes it is better to tell the program directors that their idea is not good, then move on.

Somewhere in Utah, a phone company is missing it’s microwave site…

I followed this link to this site called “SurvivalRealty.com” and saw this article about what looks to be a former ATT microwave relay site in Utah turned into a residence. The site is much smaller than the former ATT site in Kingston that I profiled in this post. Still, that is a Western Electric tower and those are KS-15676 antennas.

Former ATT microwave site turned into a residence

If I were that guy, I’d take those antennas down a scrap them. Looks like the waveguides are already gone. I might have tried to put some windows in while I was renovating it. It would drive me crazy to live in a house without any windows. I guess if one were waiting for the big one, windows might not be a desired feature of a survival bunker.

I wouldn’t really call it a “communications bunker” though. I’ve been in communications bunkers, they are mostly underground and are much more robust than that building. Still, it is built better than an ordinary commercial building or a regular house. It would take a special person to live out in the middle of nowhere like that.

Broadbanded AM antennas

Partly for my own edification, partly just because, here is some information about AM antenna systems and their bandwidth. An AM tower is a radiator that, simply by the physical constraints of the tower structure itself, is pretty narrow-banded, even under the best conditions. Add to that, antenna tuning units, transmission line phasing, antenna phasing units, diplexing units, and things can get very squished outside of the immediate carrier frequency. This seems to be a particular problem with directional antennas, which most AM stations employ.

WGY 810 kHz, Schenectady, NY transmitting tower w/open feed line — WGY 810 kHz, Schenectady, NY transmitting tower with open transmission line

As an engineer, you can get some idea of how narrow an antenna system’s bandwidth is by looking at the base impedance measurement. Every AM station is required to keep the latest impedance measurement on file. When looking at these measurements, there will be one curve that indicates base resistance (R) and another curve that indicates reactance ( X, although often noted as + or -j). If the resistance and or reactance curve is slopped steeply at the carrier frequency and out to 20-30 kHz, it is a narrow tower. Add to that the different phase shifts of an ATU and or Phasor and things will be compounded. That is why it takes a professional to design and tune up these things, a poor design will never sound right.

Another way to get some idea of bandwidth requires a field strength meter. Modulate the transmitter with a 10 kHz tone at 50% modulation. Then, away from the near field, measure the carrier and 10 kHz +/- the carrier frequency on the log scale. The sidebands should be symmetrical and about 1/4 the carrier level.

Generally speaking, antenna systems need to be designed for low VSWR across the entire side band range (+/- 10 kHz from the carrier) as well as symmetrical distribution of radiated energy across the lower and upper sidebands. Several factors influence these conditions:

Electrical tower height is perhaps the hardest thing to change once a tower is constructed. Short towers (less than 80 electrical degrees), or very tall towers, (taller than 200 electrical degrees) present problems. If one were constructing an AM station and could choose any tower height, something between 120 to 190 electrical degrees would be ideal. Existing towers can be top-loaded to add electrical height for an additional 30 degrees or so. Beyond 30 degrees it becomes difficult to physically attain and therefore impractical in most situations. Top loading and bottom loading of a tower can reduce bandwidth if done improperly. Bottom loading an AM tower is almost never done due to the very high voltage and current as the electrical length approaches 180°.
Antenna matching networks can greatly improve or degrade bandwidth, depending on how they are designed. A T-matching network has more parts and is more expensive, however, it allows for optimum control over the R and jX phasing. This becomes much more difficult with directional antenna where phase considerations are a part of the station’s antenna field pattern development.
Phasors present the biggest challenge, particularly in the power divider sections. A tank circuit power divider is the worst choice, and a shunt circuit power divider is the best bandwidth choice, however, it is the hardest to conceptualize.

Obviously, the more complicated the antenna system, the harder it will be to keep the bandwidth open over 20 kHz of spectrum. This is especially true on lower-frequency AM signals, where the bandwidth is a much larger percentage of the frequency. Multiple patterns, multiple tower DAs are a nightmare. Single-tower non-directional stations are the easiest to modify.

As far as the circuit itself, higher Q circuits have smaller bandwidths. Simply stated, in an alternating current circuit, Q=X/R. The better the reduction of X, which also has a lot to do with the relationship of the current and voltage phasing, the better the Q will be. This is why a T network is the best design for an ATU. With a 90° or 180° tower, this is relatively straightforward. In towers that are shorter or taller than that, it becomes more difficult as the value of R becomes less friendly.

In most cases, some sort of L/C network can be deployed to decrease the Q of an antenna system at the base of the tower. Directional stations also need to have the phasing equipment looked at, because, as noted above, certain designs can create bandwidth bottlenecks. All in all, it is usually an expensive proposition for a multi-tower directional station to broadband its antenna system. This is another reason why IBOC on AM is destined to fail, many AM towers cannot pass the extended sidebands adequately.