One of the AM station around here that I am familiar with is considering a downgrade, which is to say reduce power and get rid of a directional antenna system in favor of a non-DA antenna. In this particular case, it makes sense, as the station can co-locate with another AM that is closer to the COL by a good distance. The coverage from the new site at reduced power looks to be a good fit. If this can be arraigned, the AM station in question would loose a multi tower AM antenna system that is 50 years old and all the attendant headaches, expenses and labor that goes with it.
Many AM stations that are DA-2 or even DA should consider downgrading to a lower power level and getting rid of their DA system. Directional antenna systems on AM stations are maintenance nightmares. Unfortunately, in the 50’s, 60’s and 70’s, it was often thought that adding power, extra towers to an AM station would give them great swaths of extra coverage. Sometimes it worked out, sometimes it did not. Often what happened was some area was added, but in areas that where nulls toward protected stations, signal strengths went down. What the station ended up with was more towers, more maintenance, monitor points, a sample system, and more expense.
Taking an AM station in the other direction might actually make more sense. Go back to one tower non-directional 1 KW or whatever power can be used in the daytime. Time was when the FCC would only allow certain power levels; .5, 1, 5, 10 and 50 KW. Those were what a new station had to work with. No longer is that the case, any power level can be used so long as it meets interference contours and the city of license contour coverage requirements.
Presunrise authority is normally 500 watts and is available at 6 am, post sunset authority varies but often a PSA extends the on air time to 9 pm in the winter time. For a local radio station, which is what all but the class A AM stations are destined to become, this will be adequate. For a loosing station, it may be that, or turn in the license and sell the land to a developer.
Diplexing on another AM stations tower closer to town is also a good way to get out of maintaining an expensive antenna array with diminishing income.
Here is one of those things that can often be a head scratcher for the uninitiated:
The FCC data base gives antenna height in electrical degrees when what you really want to know is how tall is that tower. Never fear, to figure all this out, requires math. Pretty simple math at that, too. I prefer to do these calculations in metric, it is easier and the final product can be converted to feet, if that is desired.
First of all, radio waves travel at the speed of light, known as “c” in many scientific circles. Therefore, a quick lookup shows the speed of light is 299,792,458 meters per second (m/s). That is in a vacuum, in a steel tower, there is a velocity factor, most often calculated as 95%, so we have to reduce speed of light in a vacuum to the speed of RF in a steel tower.
299,792,458 m/s × .95 = 284,802,835 m/s (speed of a radio wave in a steel tower)
Frequencies for AM radio are often given in KHz, which is 1000 cycles per second. For example, 1,370 KHz × 1000 = 1,370,000 Hz (or c/s)
284,802,835 m/s ÷ 1,370,000 c/s = 207 meters per cycle. Therefore the wavelength is 207 meters.
There are 360 degrees per cycle, therefore:
207 meters ÷ 360° = 0.575 meters per degree
If the height of the tower is 90°, then 90° × 0.575 m/° = 51.57 meters. Add to that the height of the base insulator (if there is one) and the concrete tower base and that is the total tower height.
To convert meters to feet, multiply by 3.2808399.
In the United States, that tower would be 169.78 feet tall.
To any who live in the capital region, the WGY tower near the intersection of I-90 and I-88 in the town of Rotterdam is a familiar site. It is big, tall, and conspicuously marked with a huge “81 WGY” on the southwest face of the tower. At night the call letters used to be lit up by a spot light but that may have been turned off in recent years.
In my time as chief engineer there, I found several file folders of memos and other materials about the building of the tower, which started in 1936. Prior to that, WGY used a T top wire antenna, first from the General Electric plant in Schenectady (1922-25) then from the current tower site in Rotterdam. Located with WGY were GE’s experimental shortwave stations W2XAF and W2XAD.
When the station increased power to 50,000 watts in 1925, may reports of fading were received from locations 20-50 miles away. WGY engineers studied the situation by doing a full proof on the antenna. They found an elliptical shaped pattern with nulls to the north and south. This coincided approximately with the T arms of the T top antenna, likely due to the self resonating effect of the support towers for the ends of the T.
NBC, then owners of WJZ (now WABC) in NYC had studied this problem for years and came up with a new antenna design for Standard Broadcast, the uniform cross section guyed tower. Starting in 1935, WGY began to investigate installing such a tower in South Schenectady, as the transmitter site was then known. One report showed an efficiency gain of 430% over the T top antenna that was in use. The General Electric construction and engineering department raised several objects to the standard triangular tower then and now commonly used for AM radiators.
Much mechanical planning and effort went into the design of the tower, which is a square tower, 9 foot face, 625 feet tall. During the planning phase, KDKA was installing a simuliar tower, which collapsed when it was being erected in 1936. An analyisis of the failure showed that one of the guy anchor cable sockets pulled out of the concrete (which was improperly poured). This may also be the reason why the KDKA tower collapsed in 2003, although I never read the engineering report on that failure. Nevertheless, GE engineering felt that forging the members of a triangular tower weakens them and was too risky, thus, a square tower was the solution.
Further, every component of the tower was tested individually. Often, two of a type where build, with one being tested to destruction. Two base insulators were made for this specific tower. The first was tested to destruction at the National Standards and Institutes laboratory in Washington DC. It was found that the insulator withstood slightly more than 1,200,000 pounds of pressure. The working load (tower dead weight) of the base insulator is calculated to be about 430,000 pounds, thus almost a 3:1 safety margin.
The wire rope used for the guy wires was also tested to destruction. The working load on the upper guy is about 24,000 pounds, the wire rope broke at nearly 120,000 pounds. The concrete, guy anchor sockets, T bars, and all other parts were likewise tested.
Electrically, the tower is 186 degrees (it was 180 degrees on 790 kHz, the former WGY frequency). It had a 40 X 40 foot ground mat with 120 buried ground radials. The ground radials were #4 hard drawn stranded copper. When we investigated the system in 1999, it was complete and unbroken. The radials, ground screen, strap and all other metal component showed no signs of deterioration. It helps that the soil surrounding the tower is a sandy loam and well drained.
The tower was fed with 600 ohm open transmission line, 180 degrees long. Initially, the system had been designed for high power operation up to 500 KW. However, when the transmitter was replaced in 1980, a new Harris ATU was installed, which can only handle 50 KW. I recall the base resistance to be 192 ohms with -j85 reactance.
A concrete wall surrounds the base insulator. This was installed in early 1942 to prevent the base insulator from being shot out by sabbators during WWII.
When I worked there, the station had a Harris MW-50B transmitter. This unit was in slightly better shape than its counterpart at WPTR across town. I did find some of the same quirky things with it, however. Our consulting engineer had a good line, “Harris, where no economy is spared…”
The site had a FEMA owned backup generator installed in the 60’s. This was an Onan 225 KW diesel powered unit. 225KW is likely a conservative estimate as those units were way overbuilt. The original fuel tank was buried out behind the building. FEMA contracted for it’s removal in 1995 because of concerns of leaks and soil contamination. When they dug it up, the primer was still on the tank. After getting the tank out of the ground, the contractor cut a large hole in it and lowered a person into the tank to clean it out. Something that should be profiled on the Dirties Jobs TV show.
The new tank was installed in the old outdoor transformer vault. It is a 5000 gallon double walled above ground tank with monitoring system.
It has been several years since I have been to this site. I know they installed a Harris DX-50 sometime in 2001 or so. They also may have replaced the open transmission line. WGY now transmits in HD radio, which they are able to do because the tower was well designed and installed.
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 which, 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.
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 on curve which 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 differing 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 side bands 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 carrier) as well as symmetrical distribution of radiated energy across the lower and upper sidebands. Several factors influence these conditions:
Electrical tower height, 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 where 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 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 stations antenna field pattern development.
Phasors present the biggest challenge, particularly in the power divider sections. A tank circuit power divider is the worst choice, 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 pattern, 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 straight forward. 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 created bandwidth bottlenecks. All in all, it is usually an expensive proposition for a multi tower directional station to broadband it’s antenna system. This is another reason why IBOC on AM is destined to fail, many AM towers cannot pass the extended sidebands adequately.