Remember that song on MTV “Video Killed the Radio Star?” Turns out that was wishful thinking. There was a group much worse waiting in the wings to kill the radio star: Accountants.
Radio has been declared dead many times over the last 80 years or so.
First off, the great depression was supposed to kill radio. And some stations died but a good many prospered. Turns out that the entertainment value of radio was just what the nation needed, in many cases, to forget their collective troubles.
Then TV was supposed kill radio, but the problem with TV was it was not very portable. Radio re-invented itself to go in cars and lived on, stronger than ever.
MTV came out in 1978 and you could watch endless music videos, one after another. The first music videos were more or less live shows. It was exciting, at first, to see a some of the videos. Then bands began making videos to tell specific stories with their songs. Sort of a visual interpretation of their music. What we mainly saw was self indulgent whining. Yawn.
Radio lived on. Satellite radio was then the last nail in the coffin. Except that satellite radio audio is barely listenable, especially the music channels. I listened to Sirius’ Jazz channel for about a half an hour in a rental car once, ended up with a splitting headache. XM’s audio was a little better but still full of digital hash. It made AM radio sound good.
Radio still lived on. Then came consolidation. With it, big time non-radio type owners attempting to make stacks of cash. To do this, they got rid of most of the air staffs, installed computers to replace them, did away with any local content and or community service, and launched blended un-entertaining bland formats that are safer than attending a church service. For a while, they made lots of money with very few expenses.
The public: they didn’t buy into it so much. When radio stopped being entertaining, most everyone began buying I-pods. Webcasters sprung up and began offering internet only content, some of which is pretty good. The real telling thing, however, is National Public Radio took off. They used to make fun of NPR on Saturday Night live. No more, NPR is probably one of the last places to get reliable news from.
And what a way to go, death of a thousand cuts as it were. The formula is thus: Take anything that is fun, successful, and entertaining and put a bunch of up tight, stiff white guys in charge of it.
Radio is not dead yet, but the prognosis is not good. The bankers and accountants… They are on there way to the next money making venture.
We have a Harris Z5-CD transmitter for one of our FM stations. Brand H is not my preferred make, however, it was already installed when we bought the station, so I have to live with it.
This particular site gets hit by lightning strikes often. Normally, it does not affect anything until the transmitter gets turned off for maintenance. Then, almost invariably, when turning the transmitter back on one of the modules will fail. Most often this is manifest when one of the two power supplies shut down causing the transmitter to run no more than 20% power.
The way this is trouble shot is to slide each module out and turn the transmitter back on. When the power supply stays on, the bad module has been located. A confirmation test is to check the MOSFET for a short circuit between Drain and Source. This short circuit condition puts a direct short on the power supply causing it to crow bar and turn off.
So, once the bad module has been located, and the spare module is installed in the transmitter, then what? Most engineers call Harris and ship the module back for repair. Most engineers don’t want to mess with unsoldering a surface mount MOSFET and soldering a new one in. I find it moderately entertaining to fix things myself, so I do not do what most engineers do.
NXP BLF177 MOSFETS
The MOSFET in this particular module is the BLF177, made by NXP. Harris will sell you one for quite a bit of money. You can also buy one from Mouser for about half the cost.
Harris FM Z series transmitter PA module with cover removed
Once the parts are obtained, the worst part of the entire job is unsoldering the old MOSFET. This takes some patience and skill. What I found works best is to melt some solder on the foil leads and get them good and hot. Since this MOSFET is already destroyed, we don’t have to worry about heat etc. The one thing you do not want to do it actually break the MOSFET open. That is because it contains beryllium oxide, a known carcinogen. Once all the solder is liquid, carefully pry the foil up with a small screw driver. There are several components that have to be moved to work on this.
Harris Z series PA module with MOSFETS removed
After the old MOSFET is removed, clean up the solder pad with a solder pump and solder wick. I like to use a little liquid flux on the solder wick, it makes things go faster.
Once all the old solder is cleaned off the solder pads, I brush a light coat of liquid flux in the pad. Again, this makes things go faster.
Harris Z series FM transmitter module new MOSFETs waiting to be soldered
The new MOSFETS are very sensitive to static discharge, so I always use a static drain wrist band when handling. I place both MOSFETs on to the circuit board. I then solder them on using as little heat as possible from the soldering iron. Again, the MOSFETs are sensitive to heat and one can easily be destroyed if it gets too hot.
Harris Z FM series PA module repaired
This is the module with the new MOSFETs soldered in. I use defluxing compound to remove all the extra flux. Once it cools off, I test the new module with a DVM:
Harris Z series FM PA circuit board under test, resistance is 3.3 Mohm
If the MOSFETS are good, they will have an internal resistance of around 3.3 MΩ. If the module is bad the MOSFETS will read only a few ohms if shorted:
Harris Z series FM PA module under test, DVM reads 1.6 ohms
That is how you do it. I think Harris charges $775.00 per module to repair. I fixed this one for $240.00, but that is not the reason I did it. I did it for the fun that was in it.
Every now and then something goes wrong. One of the nicer features of a solid state transmitter is a soft failure mode. For example, the loss of a single RF module may bring the transmitter down to 95% power vs 100% power. In a tube transmitter, the failure of the tube would mean 0% power.
This happened recently when a transmitter was turned off for tower maintenance. Upon restart, an RF module failed. Unfortunately, the spare RF module had already been used due to a lightning strike in early July. So we were down a few percent on the output meter until another RF module was ordered and installed. The station was running at 94% power according to the external watt meter. That equates to about half a dB power reduction over normal operations, which is really insignificant.
Naturally, the fact that the transmitter was at low power gave the program director another excuse to pile on. First I received this e-mail:
Getting reports out of both XXX and XXX that it’s nothing but the The (competing station) on 1xx.1 – the tropo is going pretty good right now. I’ll monitor on the ride in but needless to say we can’t fix this soon enough. We’ve been bad in XXX County for the last two weeks and I just assumed tropo and stayed quiet – could this module have been out for a while?
Where are we on a software controller so we can log in and monitor stuff like this?
To which I responded:
The module problem arose after the transmitter was turned back on, so no, this has not been a problem for the last two weeks, it has only been a problem since Sunday Morning at 11:42 am.
As I said below, the new module was ordered and as soon as it arrives, it will be put back in.
I then received this e-mail:
With all the shadowing in our contour and the short-spacing, we just can’t afford to loose 1db without tangible effects. We need every nanowatt possible everyday – especially in the summer.
So, Mr. Smarty paints there thinks that 0.00000001 watt makes a difference. The absurdity of that statement is un-measurable. Why not a femto watt or a yacto watt? Here was my politically correct response:
I understand you want the transmitter fixed.
I have done everything humanly possible to effect repairs including calling Harris on my day off to order parts and have them shipped.
e-mails of this type do not make things go any faster, and are in fact, counter productive.
EAS, or more properly, the Emergency Alert System, is a government mandated system of encoders and decoders designed by the federal government to alert the public in case of war or other emergency. It, and it’s predecessor, EBS (Emergency Broadcast System) have never been activated by the federal government. Both systems, however, are used extensively by local and state governments for things like weather alerts, amber alerts, etc.
Back in the mid-90′s the FCC had a chance to redo the EBS and produce something that was a streamlined and effective tool for public warning. Unfortunately, the EAS system is neither. Rather, it is a cumbersome system of weekly and monthly tests scheduled around pre-conceived notions that how the system is tested every week will be how the system works in an emergency. In practice, this is generally a good theory of system design, but it has failed miserably with EAS. The reasons why are thus:
Most all emergencies are local or at most state wide events. To this day, very few state and or local government emergency managers would be able to activate EAS for their area. The reason is there is minimal if any interface with the LP-1 EAS stations or station personnel. Ignorance and apathy on behalf of both radio station personnel and government officials is the main culprit.
Most stations are un-maned for large portions of the day. Even if government official could/did call the station, chances are, nobody would be there. If by chance, arrangements were made to contact station employees at home, they would have to interface with the EAS equipment remotely, which ads complexity to an already complex system.
EAS messages are still mainly relayed from radio station to radio station, the so called daisy chain net work that has been shown numerous times to be unreliable.
The system of SAME codes, FIPS identifiers is not necessarily bad, the application in this case leaves something to be desired. The FCC had a chance to update EAS before the HDTV rollout. One would assume that any improvements could have been built into the new TV sets that are now being sold, but again, that opportunity was missed. For example, I suggested that each TV have a set up screen option where the owner could input their zip code. They could also choose what types of alerts they would want to know about and even base the alerts types on the time of day. Live in a flood zone, the FFW (Flash Flood Warning) 24/7. Live in tornado alley, TOR (Tornado Warning), etc. The cable companies then pipe in the local NOAAall hazards radio station. All the sudden there is a real national alert system in place using mostly non-broadcast wireless systems. Add to that the ability to sign up for emergency e-mails and text messages for specific areas (many places are currently doing this) and there is multiple message paths.
The system as is is not reliable and sooner or later that will be shown with a large scale failure. Recently, the FCC held a summit with the Department of Homeland Security. The cliff notes version of this event is: Yes, the system can be made better. Let’s keep throwing the same ideas at the wall and see if anything sticks this time. Excuse me if I don’t do back flips, this is the same information that was discussed during the last “lets revamp EAS” discussion back in 2005 (04-296).
In the mean time, the EAS continues to be a good fund raiser for the FCC enforcement bureau. Which, you know, it is easier to go to a licensed radio station and bust them for not re-transmitting the RMT (Required Monthly Test) than it is to go out and bust some of the numerous pirate radio operators, some of whom are operating in the same city/metropolitan area as a field office.
The shame of it is, it could work without a great deal of cost, very well.
In my early engineering days, I was tasked with all sorts of non-engineering things. Squeaky hinges, broken chairs, changing light bulbs, etc. I drew the line in a few areas, things like unclogging a toilet for example, required a visit from the plumber.
As a part of these building managment janator services, maintaining the septic tank at an acceptable level so the toilets worked fell under my perview. Every so often, I would call the honey dipper to come and haul all that crap away. Then one day the leach field completely failed. It was only a matter of time, the thing had been in service since 1947. So, the decision was made to connect up to the city sewer line that had been installed a few years previous. Why they didn’t just connect to it when the town was installing it is beyond me, however, that all happened before my employment.
It took a while to line up the permits, get a contractor, get a plumber, hire an excavator etc. Of course, the toilets were still being used, because people need to poop after all, at work. It is there right. Under the constitution. Therefore, I hired the honey dipper to come every few days and pump the tank, there were no other options. Until the general manager had an idea, we could just pump the raw sewage out onto the field by the antenna towers.
No, I said, that is illegal. Besides, there is a swamp right there and the sewage would get into the swamp.
His reply was “Deer shit in the woods, I’m telling you to pump out the tank. I don’t want to pay for the shit pumper anymore.”
No.
Raising his voice another few dB “PUMP OUT THAT TANK OR ELSE.”
I very calmly said “Or else what?”
He stomped off to his office to sulk. I never did pump out that septic tank into the field and it took several more weeks to make the sewer connection. Every three to four days, that septic service truck would be out there, sucking all the shit out of the tank, in full view of his office. I would snicker, I wish I could have rigged up a camera to tape his reaction.
What if I had caved and did as he asked. Then one day, the health department stops by because the neighbors complained about the smell. I would have been held responsible, even though I was doing what my manager asked me to. That is the moral of the story, even if you are ordered to do something that you know is illegal, it is ultimately you who will be held responsible. It is better to go job hunting that to face potential legal consequences, not to mention the loss of reputation, this is a very small business after all.
As radio engineers, we have a whole host of rules that must be adhered to, FCC rules, building codes, fire codes, FAA rules, OSHA rules, etc. It is up to you to know them and ensure that they are followed within your department.
A few pictures from my last trip to one of our FM transmitter sites. This is a mountain top site, in as much as a medium sized hill is a mountain around here. This site has a 2.3 mile road through the woods that is almost impassable 3-4 months out of the year.
Previous engineers have walked up the hill with a tool box. I can say this with all honesty; not me. In the past they have also rented a helicopter, used a snow cat, snowmobiles or an ATV with snow tracks. I’d do those things provided they are safe and insured. As I get older (and wiser), I realize that the only person who going to look after my well being is me.
Anyway, the trip starts here, at the gate:
Gate to transmitter road
Then it goes up the hill:
Transmitter site access road
Some sections are worse than others:
washed out road
Along the way there are some nice views:
City reservior near transmitter site
Finally, the gate to the tower farm:
Access gate to transmitter site
There are two digital TV stations, several cell phone carriers, some government two way gear, some FM translators, Media Flow, and us at this site. There are also some Ham radio repeaters off to the side in another building. All in all, a pretty RF intense site.
The view from the top:
view looking north
The reason why we came:
Transmitter room
That is a 24 year old BE FM5B transmitter. The back up is a Gates FM5G, which aren’t we glad we have a solid reliable transmitter selection for such a remote site. Actually, we were supposed to put in a new Nautel V-10 here last year, but the money was spent on computers instead. Oh well, good thing there will be no computer crashes when we go off the air.
A standard maintenance trip consists of meter readings, comparing the reading to the last set of readings, changing the air filters, checking the remote control and calibrating it to the transmitter, checking the tower light sensor, etc.
Normally, the backup transmitter would be run into the dummy load, but the backup transmitter no longer works. Parts are not available to fix it, so we operate without a net. One of the previous general managers asked if that keeps me awake at night, to which my answer was no, not at all.
If you work at a radio station that still has a local program director instead of one at the corporate programming lair (I know, sooooo old school), then you might be interested in this. I compiled a list of things that radio station program directors like:
Good ratings. A good rating book means that they are great program directors and they really know their stuff. Bad ratings means that engineering dropped the ball (again) when the station went off the air for 30 seconds during afternoon drive.
Taking credit for anything good. Sort of goes with the good ratings above, but this extends out to all other aspects of a radio station, promotions, sales, news, and even engineering.
New Processing. Any new gizmo or gadget that changes the sound of the microphone or entire station, for better or worse, is good. The more flashing lights the better. The more knobs to adjust the better. Things that can be plugged into computers and remotely controlled are the ultimate.
More. More of anything is better, more compression, more expansion, more highs, more mid-range, more lows, more gain, more de-essing, more loudness, more power, more punch, more reverb, more crack, more more more. If they could just have a little more, the station would be number one.
Any other new piece of equipment. Watching a program director look at a new studio is like watching a two year old open presents on Christmas morning. I know, I have a two year old. Unfortunately, the studios don’t stay new looking for long.
Taping notes up in the studio. I have one studio where every stationary piece of equipment has a note taped to it. Mind you, the notes have nothing to do with the equipment they are covering up, they are more like general directions, phone numbers, and other miscellaneous pieces of information.
Free stuff. Used to be called payola or plugola, now it is a free lap top, or a trip to Disney paid for by the record rep. I’ve even seen some mysterious mike processors show up (see number 3).
Rigging up lights to alert operators. This is a great one, the studio operator does not know if the Marti (or Matrix or ISDN) is active, so they want a light to indicate there is someone there. Or a light on the phone hotline, or a light for the EAS machine, the back door, the coffee machine, the silence sensor (never mind they are in the studio, they still need a silence sensor light)
Blaming other people when things go wrong. The program director is infallible. If something goes wrong, it is somebody else’s fault. Always. And forever. Amen.
Some one suggested that I put up the video “More, more, more” by Andrea True Connection to go along #4. Well, okay, I guess. It is not a terrible song but the video kinda suxor. From what I can tell, Andrea True is a former p0r n star that turned signer for just this one hit. Looks like it was filmed on a p0r n set too.
Feel free to add anything else that I may have forgotten. Of course, this is all in good fun. I’ll to a “stuff radio engineers like” post as soon as I figure out what that is.
Technically speaking, no. Here is how radio is defined in the dictionary:
ra⋅di⋅o
[rey-dee-oh]
-noun
1. wireless telegraphy or telephony: speeches broadcast by radio.
2. an apparatus for receiving or transmitting radio broadcasts.
3. a message transmitted by radio.
Therefore, the internet, something relying on wired connections for the transmission of data for the most part, is not radio. A radio station that is streaming audio, is a different matter.
Aside from that technicality, there is something else that is important to note. Internet broadcasters (AKA webcaster) lack some other key components that make a radio station a radio station; A specific set of rules that govern their behavior. Things like profanity, copyright infringement, slander, payola, plugola, syndication rights, advertising rules (things tobacco, alcohol) emergency information, public issues and so on.
A radio station license is granted in the public interest. Time was that radio station were required to do a certain amount of public service broadcasting, things like the news, religious programs, community interest programs. Many station still do this. An internet broadcaster is under so such constraints. Some would say that is better and it just might be. However, when Tim Westergren says “don’t call it internet radio, just call it radio,” sir, you are wrong.
Once upon a time, in the not too distant past, all long distance communication in the US was handled by one company, AT&T. There was no other company that could transmit data over medium to long distances. The breath and scope of their communications network is not understood by most people these days. Most people know that AT&T handled long distance telephone calls for the Bell Telephone System until the Bell breakup in 1984. However, AT&T did a lot more than long distance phone.
For example, if you watched the network news or network TV show anytime before 1980, it was likely brought to you via AT&T microwave system, known as AT&T long lines. Listen to the news on the radio, same deal. Before the wide spread use of communication satellites and fiber optics, the AT&T microwave relay network was the only way to get various types of electronic media signals from one place to another.
Beginning in the late 1980′s, competing local and long distance telephone companies began installing fiber optic cables between company offices. That coupled with the increased use of satellite systems for mass media video and audio delivery services made the huge AT&T microwave network obsolete. Some of the old microwave sites that are located in down town areas have been reused by local phone companies and cell phone providers. Many of the rural sites now sit empty.
ATT long lines microwave site with towers
This is the former AT&T microwave relay site located near Kingston, NY. It is now owned by American Tower, Inc. There are two towers behind the building, only the tower on the right has a few active communications antennas on it. The taller tower is 190 feet tall and was built in 1957. The shorter tower is 120 feet tall and was built in 1961. Both towers and everything on them was made by Western Electric, the same company that manufactured the telephone sets. Chances are, Western Electric contracted the actual manufacture of equipment out to others, then billed AT&T, their parent company a markup. Something that would make all MBAs proud.
Western electric 190 foot tower, built in 1957
This tower was built in 1957. The structure and galvanizing are still in excellent condition.
The large antennas you see on the towers are microwave horn antennas. They are no longer in use. Several transmitters and receivers would have been connected to each one of these antennas by use of RF multiplexers. Each microwave transmitter/receiver would have had several data channels. Generally, this was C Band microwave equipment, so it was in the 4, 6, and 8 gHz frequency range.
Western Electric KS-15676 microwave antenna
All of this telephone traffic was transmitted on digital data channels un-encrypted. Many have argued that this allowed the government (most notably the NSA or National Security Agency) to intercept and listen to most domestic long distance telephone calls within the US. There is a book called Puzzle palaceby James Bamford if you are interested in NSA history. It was written more than 20 years ago, so it doesn’t really apply today, but it is an interesting look at what the government was up to.
The building itself is huge, the first floor is 16,000+ square feet and the second floor is 10,000+ square feet. Only about 1000 square feet of this space is actively being used.
I believe this building was built in the late 1940′s or early 1950′s, just as Kingston was growing into a major IBM manufacturing site. The IBM buildings are located a few miles to the south east of this location, they are another cold war relic for discussion later. The IBM buildings were a major computer research and development site in the 1950′s until it closed in 1992. It was assumed that the Soviets had several spy satellites trying to steal secrets from the area, and the IBM facility was a primary nuclear target.
Blast baffle for generator cooling air intake
The microwave relay site has 12 inch re-enforced concrete walls. The ventilation air intakes have blast baffles to prevent a pressure wave (from a nuclear explosion) from blowing the ventilation equipment off of its mounts.
pnumatic actuator panel, seals all outside openings with steel blast doors
All of the outside openings were able to be sealed with steal blast deflectors using a pneumatic control panel located in the control room. There was a five minute timer, presumably to allow the HVAC units to be secured before the doors where closed. They where heavy gauge steel shutter designed to deflect the pressure wave of a nuclear explosion. Since this is an earlier building, it is likely that it is built to a 2 PSI pressure wave spec. Newer buildings were built to 20 or even 50 PSI. This microwave relay site would not have withstood a direct hit from a nuclear warhead, especially the higher yield warheads that came later on.
Water chillers for HVAC system
There where three large water chillers to provide cooling to the HVAC units. Since this was the 1950′s all of the electronic equipment would have had tubes, which would have generated a lot of heat while operating. There were two loops in the HVAC system. The refrigerant loop, which ran between these units and the huge condensers on the second floor roof, and the chilled water loop which ran between these units and the air handlers located in various parts of the building.
There is a bomb shelter in the basement. I found a couple of olive drab cans of civil defense water laying around. The lights were not working at the bottom of the stairs, so I chose not to go into the bomb shelter itself.
Stairs going down to the bomb shelter
“Okay everybody, the missiles are on there way, so lets head down these stairs and pray”
There where two diesel generators, one was 325 KW which could run the entire building. The other was a 200 KW which could run the critical building functions. The fuel storage consisted of two 10,000 gallon tanks buried in the ground outside. Each steel fuel tank had a cathodic protection circuit. Basically a small negative electrical current was passed to the steel tank to keep it from rusting. Apparently it worked because when the tanks were removed in 2000 after 45 years in the ground, the primer was still on the outside of the tank.
Electrical switch gear, part of power company sub-station
The building has it’s own power substation. The electric from the utility company comes off the pole at 13,800 volts and goes to a large step down transformer on a pad outside. From there 480 volts is fed to this switch panel, where it is routed to motors loads or other step down transformers within the building.
Frame room floor, equipment removed
On the main floor, there were rows and rows of wire terminal equipment, microwave transmitters, receivers and data and RF multiplexers in racks. The room in the above picture is about 10,000 square feet, there is another 6,000 square feet beyond the plastic heat barrier. This microwave gear received and transmitted data from Albany and Germantown to the north; Poughkeepsie, Putnam Valley, Ellenville, and Spring Valley to the south. All of that equipment is gone now, replaced by empty space.
Now the whole place is a little creepy.
There are about 500 copper wire pairs of telephone cable that came into various parts of the building to carry the DS-1 and DS-3 circuits that interfaced with the TELCO office in Kingston.
All in all, this was a serious building, no expense was spared in the construction and equipment outfitting. The entire building is shielded with copper mesh screen embedded in the concrete walls. There where redundant systems on top of redundant systems, something that you do not see these days, even in government buildings such as emergency operation centers (EOCs) and 911 call centers.
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.
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