Engineering Radio

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.

Something to think about.

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)

Therefore:

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.

WGY transmitting tower, Schenectady, NY

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.

Base insulator, WGY 625 foot square faced transmitting tower

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.

WGY open transmission line between transmitter building and tower base

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.

Harris MW-50B, WGY Schenectady, NY

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…”

WGY transmitter site backup generator

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.

5000 gallon backup generator fuel tank

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.

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 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:

1. 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°.
2. 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.
3. 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.

This was a fun little project I was involved with last year.  Diplexing two AM stations to a single tower.  This particular tower was brand new, replacing an older tower that was rusting from the inside out.  As such, it had slightly different characteristics than the old tower, so it was a whole new project.  Fortunately, the old Antenna Tuning Units (ATU) were made by Kintronics, so they had plenty of head room on both side of the circuit for matching purposes.

The replacement tower is up, the new unipole antenna has been installed and now it is time to match the transmitters to the tower. This involves using some math. At some point in radio history, someone decided that all transmitters should have an output impedance(Z) of 50Ω (ohms). Impedance in an alternating current (AC) circuit (all radio frequency is AC) is like resistance in a direct current (DC) circuit. The only difference is impedance requires the use of the Z axis to calculate. You remember 9th grade algebra and the Cartesian Coordinate graphing system, the X and the Y axis. Looking down on the X and Y axis, the Z axis would be stick straight up, which makes it a three dimensional problem.

Station number one, broadcasts on 980 kHz at 5,000 watts. The tower is 240 feet tall which is close to 1/4 wave length, nearly ideal for an AM station. Using a Delta Operating Impedance Bridge (OIB-1) and a Potomac SD-31 frequency Generator, I measured the impedance at the base of the tower. On 980 khz it is 74Ω with +j160 reactance. Using the ohms law pie chart, anything can be  figured out about electricity:

ohms law pie chart

Therefore, the base current will be I=√(P/R) =√(5000/74) = 8.3 amps. The voltage will be E=√PxR =√(5000×74) = 608 Volts.

A bit about reactance; it is noted by using the letter j, which indicates it is an imaginary number. Basically in an AC circuit, there is inductance and capacitance. They are the reciprocal of each other, sort of (this could get into a long, long post if I have to explain the roles of inductance and capacitance in and AC circuit). Reactance is an undesired inductive or capacitive component that has to do with the lead or lag time between the voltage wave form peak and the current wave form peak. In standard utility company parlance it is know as the “Power Factor”. In RF circuits it causes inefficient power transfer and it needs to be canceled. A +j value indicates that the reactance is inductive, and therefore needs to be canceled out with a capacitor. A -j value indicates the reactance is capacitive and needs to be canceled out with an inductor.

Then there is the difference in impedance, the transmitter and transmission line is 50Ω and the tower is 74Ω. Enter the antenna tuning unit (ATU). The ATU matches the base impedance of the tower through the use of a T network:

To determine the value of each leg of the T network, we need to employ math again. Here is where the details will catch up with you. Remember, there are two stations on this tower, a 980 kHz and a 1430 kHz. We need to make two T networks, one for each station. There are a few characteristics of a T network that can be used to our advantage here. A T network can also function as a low pass or high pass filter depending on the relationship between capacitance and inductance. In an inductive circuit the phase is advanced and in a capacitive circuit the phase is retarded. If we can make the phases of the two stations 180° opposing, this makes an excellent start to a filter network. Therefore, one station should be +90° and the other should be -90°.

So, on 980 kHz we want to match 50Ω to 74Ω with a +90° phase shift. Simple. Each leg of the T network needs to have the following value:

Z_(leg)=√Z_(antenna) x Z_{(transmitter)} or Z=√(50 x 74) = √3700 = 60.8Ω

This is a highly simplified diagram that does not show the pass/reject filters employed between the ATU and the tower to properly combine both stations onto one antenna. That would be an extensive topic that I am not even sure I could adequately describe here:

So each leg needs to have an impedance of 60.8Ω. The input leg is inductive, the ground leg is capacitive and the output leg is inductive. Remember, the output leg is already inductive by +j163. The inductive reactance needs to be canceled out, but some of it can be used in the T network. To make the output network match the rest of the T network, it will need 102.4Ω capacitive reactance (163-60.8=102.4Ω). To calculate these values, we use the L and C formulas which are 980 KHz = .98 mHz):

C = 1/(2π f (mHz)Xc) or 1/(6.28 x .98 x 60.8) or 0.00267 uf (ground leg, 60.8Ω)

C= 1/(2π f (mHz)Xc) or 1/(6.28 x .98 x 102.4) or 0.00159 uf (output leg, 102.4Ω)

and

L= Xl / (2π f(mhz) or 60.8 / (6.28 x .98) or 9.88 uH (input leg, 60.8Ω)

This combination should get us close to the Z 50Ω impedance that the transmitter is looking for.

The next frequency is 1430 kHz with a power of 10,000 watts. This frequency should be retarded by -90 degrees, so the input will be capacitive with in inductive leg to ground and a capacitive output. The tower measures 165Ω with -j105. Perfect!

Again, the current and voltage at the base of the tower on this frequency will be I=√(P/R) = 7.78 amps and E=√PR = 1,285 volts.

Z= √(50×165) = √8250 = 90.82Ω

L = Xl / (2π f(mHz) = 90.82 / (6.28 x 1.43) = 10.11 uH (ground leg, 90.82Ω)

C= 1 / (2π f(mHz) Xc) = 1 / (6.28 x 1.43 x 90.82) = 0.00122 uf (input leg, 90.82Ω)

and

C= 1 / (2π f(mHz) Xc) = 1 / (6.28 x 1.43 x (-j105-90)) = 0.0074 uf (output leg, 75.8Ω)

Since the current and voltage for both stations are additive (with slight variations due to phasing on the two frequencies) the total current at the tower base will be 8.3 amps + 7.8 amps = 16.1 amps and the total voltage will be 608 volts + 1285 volts = 1,893 volts. Now you know why there is a fence around the bottom of the tower!

That is the theoretical part.  Using the OIB-1 and the generator, I tuned leg to ground to give the approximate valued noted above.  The inductive legs are easier to tune than the capacitive legs.  Since the value of each component is stamped on the name plate, I was able to estimate where the tap should go.  The capacitors are fixed, so they required some series/parallel connections to get the values close.

After all that, the transmitters are turned on and the system is measured under power.  Everything was pretty close, but a little bit of final tuning was required.

Once the transmitters were happy with the match, I did an full impedance sweep of both frequencies and recalculated the base currents for each station.  Then all of the harmonics and additive frequencies were checked to make sure that any spurious emissions were below the FCC required maximums.  That involved driving about 1 mile away and using a Potomac Instruments FIM-41.

The Frequencies measured were:

• 530 kHz, lower intermod product
• 1880 kHz, upper intermod product
• 1960 kHz, 980 second harmonic
• 2940 kHz, 980 third harmonic
• 2860 kHz, 1430 second harmonic
• 4290 kHz, 1430 third harmonic

And there you have it, that is how an AM transmitter is coupled to the base of a transmitting tower.