One day, for no good reason, the DA (Directional Antenna) which had previously been rock stable goes haywire. WHAT THE! The phase is way out, or the current ratio is jumping around, nothing makes sense! Time to start cranking on those phasor handles! NOT! AM DA’s are pretty easy to work on, once you get past all the smoke and mirrors hokus pokus stuff.
The first rule of trouble shooting is cause and effect. Every DA has a common point, which is where the input impedance and transmitter power is measured. The common point is the sum of all the tower impedances as seen through the power divider. Any significant changes to the DA will cause the common point current to shift, either up or down.
What does the transmitter say? Most solid state transmitters do not like to run into loads that are significantly departed from 50Ω j0. If the common point has changed, so has the impedance and the transmitter is likely folding back to protect it’s MOSFETS. Tube transmitters are more tolerant, but other signs may exist like increased or decreased modulation levels.
What do the tower base currents say? An AM DA licenses will specify base current ratios as related to the reference tower. That is to say, take the base current of a tower and divide it by the reference tower base current. Those should be within 2% of the licensed value
What do the monitor points say? Every AM DA will have monitor points for each pattern. The maximum values will be specified on the license. In addition to that, a good station practice is to do monitor point readings several times a year and keep a log. Anything out of the ordinary indicates a problem with the DA.
1. Problem: Phase and or current ratios are out on one or more towers. All else is normal. In this case, normal operation of a DA can be confirmed by base current ratios and monitor points. Both those values should be on the license. If base currents and monitor points are normal, then this is most certainly a sample system problem. Most sample systems have either a sample loop on the tower or a toroid current transformer at the base. Working from the tower back to the antenna monitor, use a Simpson 260 VOM to check for continuity. Use a working tower get baseline information, e.g. the sample loop is a DC short, the toroid has 4 ohms DC resistance, etc. Be careful of working in hot ATU’s.
2. Problem: All current/phase rations are out of tolerance and the reflected power on the transmitter has gone up. This would indicate that a change has occurred with one of the tower impedances. Look for changes at the base tower, excessive vegetation, flooding, burned, shorted or open components can cause this. Be sure to check things like sample system and tower light isolation coils.
3. Problem: Common point current has changed one tower current/phase ration is out of tolerance. Many AM arrays have a tower that actually present a negative impedance to the phasor. In this case, the power is “absorbed” from the tower and fed back to the phasor. Check the tower base current of the out of tolerance tower. Often times, something has occurred to take that tower out of the circuit.
4. Problem: All or some of the readings are slightly out of tolerance. Not enough to indicate a real issue, more like a slow drifting over time. This can be caused by seasonal variations. Depending on the wavelength of the tower, tower bases can be very sensitive to small changes in ground conductivity, just as increased or decreased soil moisture.
5. Problem: 2 pattern station, on pattern works, the other does not. This is a phasor control issue. Most DA2 stations rely on contact closures at the tower base to ensure that the pattern switching contactors have moved into the correct position. Often times, a contactor will be hung up or a micro switch will be bad. Another possibility is a hung up contactor in the phasor, or a control circuit problem.
Those are general starting points. Often times, evaluating all of the information on hand will shed some light on the issue. AM DA’s will have the following operating indicating systems:
Transmitter directional coupler and overload circuits
Directional Antenna system monitoring system
Tower base current meters
Monitoring points
Phasor indicator lights
Building all of those indicators into one picture will generally point to the problem. When trouble shooting begin at one end of the circuit and logically work your way to the other.
Sometimes some seasonal effect will cause the readings to be off. Before any phasor handle is turned, make a note of all the readings on the dial cranks. This way, if things get out of whack there is a restore point. If there is a seasonal variation with the antenna array, it should be well documented by previous seasons pattern changes. If not, it could be a slow deterioration of the ground system or ATU components.
If a person were to drive south down I-95 through Bridgeport, CT and look off to the left, they would see a 500 foot smokestack for a coal fired power plant. Side mounted on that smokestack is a 6 bay Shively FM antenna. The antenna is more visible when driving south. That would be the antenna for WEBE 107.9 Mhz. This is right down town, therefore, I would imagine this station has no problems with reception.
Bridgeport Power Plant smokestack, viewed from the west
WEBE is a class B FM with a full 50 KW ERP. Most FM’s around here take advantage of a nearby mountain to gain some altitude and thus reduce the TPO a bit. There are several class B stations that run less than 5 KW into a relatively small antenna, but they are way up in the 900 to 1000 foot HAAT range. In this case, the power plant is located right on the Pequonnock River bay, so the AMSL at the base of the smokestack is only 10 feet. This means lots of watts out and a fairly large antenna.
They are using Broadcast Electronics FM35A for the main and backup transmitters. They were installed in late 1986 and are a little long in the tooth.
Broadcast Electronics FM35A transmitter
They run near 12 KV plate supply, about 3.8 amps making 34 KW TPO. That goes into a six bay Shively 6 bay 6813 antenna centered at 475 feet, which makes the HAAT 117 meters.
One of the problems encountered with at site is the smokestack emissions. It seems that a fair amount of mercury comes out to the top of that thing. In the past, this has caused major problems with the antenna shorting itself out and burning up transmission line. Because of this, the entire antenna system, radomes, and transmission line is supplied with Nitrogen from this liquid nitrogen tank:
Liquid Nitrogen Tank
The antenna then intentionally bleeds N2 into the radomes continuously, overpressurizing them, to keep the smokestack emissions out. This type of tank is needed because a conventional N2 tank would last about a day, whereas the liquid tank lasts about 20 days.
The BE FM35A decided to blow a 200 Amp fuse on Friday afternoon:
Blown 200 Amp fuse
I had a BE FM30A that would randomly trip the 200 amp main breaker every once in a while. I could never find anything wrong with the transmitter, it would just come back on and run normally again after the breaker was reset. I even replaced the breaker thinking breaker fatigue. Still happened. In the end, we replaced that transmitter. In this case, I don’t see that happening anytime soon.
BE FM35A heavy iron:
Broadcast Electronics FM35A plate transformer
I would not want to replace this thing, it must easily weight 1,000 pounds.
And rectifier stacks:
Broadcast Electronics FM35A rectifier stacks
12,000 volts DC. That will light up any dirt, dust, piece of fuzz, etc. in the transmitter.
It is one of the more unique FM transmitter sites I’ve ever been to. Every time I see it, I am reminded of that song, Smokestack Lightning. My favorite version of that song is the live recording by the Yardbirds
I was fooling around with my HTC Android phone yesterday and discovered something that has a definite use for radio remotes. An Application called Hertz will record .wav files, which can then be transfered via e-mail or ftp to the studio and played back on the air. The program is pretty slick, it allows sample rates from 8 to 44.1 khz.
I made a sample recording, the microphone in the HTC phone is okay, a better microphone would sound better. After it was done, I emailed it to myself and listened on the laptop. The email took about 4 minutes for a 20 seconds of a 32 kHz .wav file. One could cut that down by choosing a lower sample rate. I have found that 32 kHz it the minimal acceptable sample rate for analog FM. Anything lower than that sounds choppy.
In another potential use, a news reporter could use this to record audio to save and transfer to a computer using a USB cable. The recording time limit depends on the size of the SIM card and the sample rate. Additionally, my HTC Android phone will detect and use WiFi networks, where available, for data services. Using a WiFi network will avoid those 3G data charges and also increase download/upload speeds.
My Verizon plan has unlimited data transfer, so it really doesn’t matter what sample rate I use, your mileage may vary.
Couple the Hertz app with the VNC app mentioned previously, and a person could do all sorts of things remotely with a radio station. The Hertz app is available for free download from the Android app store.
With the advent of computer file storage and automation came the unmanned operation. Unfortunately, what often happens with unmanned operations is somehow the engineer becomes responsible for station operation and ends up getting all the phone calls when anything goes wrong:
Traffic forgot to transfer the Sunday log and the station is off the air at 12 am Sunday morning. Call the engineer.
Part time DJ didn’t read the directions on merging logs, call the engineer.
Widows has encountered a problem and needs to reboot, call the engineer.
The server has locked up, call the engineer.
Silence sensor, engineer’s phone number
I got sick of driving to the radio station when things got out of whack with the AudioVault, so I installed VNC on all the machines. From that point, I could log on from home and see what the problem was. It was great, when traffic goobered up the log transfer, I called the traffic director at home and had her go in a fix it. Untrained operators, called the program director. Unfortunately, I don’t have Bill Gate’s phone number, so the windows issues are still on me.
All of this was great as long as my laptop was around. Being married, however, I had to occasionally listen to my wife, who insisted that we not take the laptop to diner or the movies with us. There were those occasional times when it would have been nice.
With the purchase of the Android phone, however, I no longer have to worry about that. Android VNC is a free app that allows an Android phone to connect to any VNC server application. The user can save all the VNC connection information in the phone. It has several mouse options including touch pad, touch pad mouse, mouse track ball, etc. It connects to most VNC servers: incl TightVNC, RealVNC on Win and Linux, x11vnc, and Apple Remote Desktop on OS/X. 0.4.3. Special commands such as ctl-alt-del are available through the menu. It is also fully zoomable. All in all, I can do almost anything with the Android phone that I can do on the laptop. My wife is thrilled.
If you have ever wondered about those ubiquitous NOAA all hazards radio (formerly National Weather Service radio) station, wonder no more. These stations transmit on one of five frequencies in the 162 mhz band with power ranges between 250 and 1,000 watts. There are over 1,000 transmitters scattered throughout the country including outlaying territories like American Samoa, Guam, Northern Marianas, Virgin Islands and Puerto Rico.
The original 1958 plan was for these stations to transmit Aviation and Marine weather forecasts. The system was expanded for use by the general public in the early sixties. Since that time, it has been slowly expanding until, with the most recent survey concluding that NOAA weather radio station can be received by 95% of the US population.
One of those stations in my neck of the woods is due for a transmitter upgrade. WXL-37 uses two Scientific Radio Systems SR-416P transmitters, as a main and a standby. The programming audio comes from the NWS office in Albany, New York, via TELCO line. The old transmitters are tube type made by in 1976. They are reliable transmitters, however, after 34 years of continous use, they are getting a little tired. They are also big and bulky and since Scientific Radio Systems went out of business, not supported.
This year, NOAA is replacing these transmitters with a Nautel NG1000. I have worked with Nautel’s military grade transmitters before and found them to be extremely rugged. Those transmitters are what the original AMPFET design was based on. Nautel is not the only vendor that NOAA is using however, others include Armstrong Transmitters and Crown Broadcast.
NOAA Scientific Radio Systems SR416P VHF transmitters
The Nautel NG1000 is a little thing, taking up about half an equipment rack with an outboard cavity filter and dummy load. There are two drawers, a controller an antenna switch and a remote control. Each drawer is it’s own 1 KW transmitter. The GUI is a on a laptop, which is what I prefer. If there must be some sort of computer driven GUI, then make it removable, so that when lightning strikes the 1,000 foot steel lightning rod 25 feet away, it doesn’t get blown up. Each transmitter is connected to a 30 AMP 240 Volt breaker via a 4 prong twist lock plug.
Nautel NG1000 NOAA transmitter
The antenna for this station near the middle of this 1,000 foot tower, thus the station gets excellent coverage with a TPO of 1,000 watts.
American Tower site, Highland, NY
On a related side note, the computer synthesizedvoices normally heard on NOAA stations took several years to evolve. Remembering when this began back in the mid 1990′s with “Paul.” Several years later, “Craig” and “Donna” were introduced. Finally, “Tom,” which is able to change voice inflections for emphasis. When I was in the Coast Guard, we did high seas synopsis and forecast on HF without aide of computers. At times, especially during typhoon season, it got a little busy in the weather broadcast position. There are advantages and disadvantages to both methods. Personally, I’d rather hear a human voice, especially in a crisis.
As broadcasters, we don’t really hear that much about ceramic power vacuum tubes these days, as more and more broadcast transmitters migrate to solid state devices. Once upon a time, however, power tubes where the engine that drove the entire operation. Tubes had to be budgeted for, stocked, rotated and replaced on a regular schedule. Some of those dern things were expensive too.
Take the 4CX35,000A which was used in the Harris MW50 transmitters. The transmitter used two of these tubes, one in the RF section and one it the modulator. As I recall, new tubes cost somewhere north of $8,000.00 each from EIMAC. Plus, in the A models there were two 4CX1500A driver tubes. All of which could add up to an expensive maintenance cost every two years or so.
The next best option was to buy rebuilt tubes. Rebuilt tubes were about half the cost of brand new ones. Some people complain that rebuilds don’t last as long, or only last half as long as the new tubes. I never found that to be the case. I often found other factors effected tube life far greater, such as filament voltage management, cooling and by extension, cleanliness.
I can say I never had a warranty issue with ECONCO tubes. I cannot say that about EIMAC, as during the late 90′s and early 00′s (or whatever you call that decade) I had several brand new 4CX3500 tubes that were bad right out of the box. These days, ECONCO and EIMAC are both owned by CPI.
I spoke with John Canevari from ECONCO who had a lot of information. For example, as the tube ages, the filament gets more flexible, not less. Most ceramic power tubes use a carbonized tungsten filament containing some small amount of thorium. As the tube ages, the filament can no longer boil off enough electrons and the emission begins to drop off. That is the normal end of life for a power tube. Occasionally, some catastrophic failure will occur.
There are many steps in the rebuilding process:
Dud is received from the field, the serial number is recorded and the tube is tested in.
The tube is prepped by sand blasting the sealing rings
It is opened
Filament is replaced. In 60-70% of the cases, the grid is replaced. In those tubes that have a screen assembly, 20-60% of those will be replaced.
Interior of the tube is cleaned
Tube is resealed and tested for leaks with a gas spectrometer
Tube is placed on the vacuum machine. Tubes are evacuated hot, smaller tubes take 12 to 24 hours, very large tubes can take up to one week.
The tube is nipped off of the vacuum while still hot. When the tube is fully cooled the vacuum scale is normally around 10-12
Exterior of tube is cleaned and replated. Silver for tubes that are socketed and Nickel for tubes that have leads.
Tube is retested to manufacture’s original specification or greater.
After that, the tube is sent back to its owner or returned to stock. John mentioned that they are very proud of there vacuum tube processing machines, so I asked if he could send along a picture. They certainly look impressive to me, too:
Vacuum tube processing machine, photo courtesy of ECONCO
Not exactly sure which tube type these are, but they sure to look like 4CX15,000:
Vacuum pump working on rebuilt ceramic power tubes, photo courtesy ECONCO
Econco has been in business since 1968 and rebuilds about 600-1,000 tubes per month. In the past, broadcasters used most of the larger tube types. However, with the majority of broadcast transmitters shifting to solid state, other markets have opened up such as industrial heating, military, research and medical equipment.
Does any one need any parts for one? Long time reader and commenter John has one that looks to be in good shape that he is willing to part with or part out. I had three of these units in Harrisburg and my recollection is they were pretty solid units. When tuned properly, they were low noise and sounded good on the air.
The one issue I had was with the small 100 pf pass through/by pass capacitors in the IPA. Several went bad and were no longer functioning as bypass caps. The result was the transmitter would self oscillate. I think there were seven or eight of them and I replaced them all at once. The exact model of that particular transmitter was a BTF-20ES1, which was one of the last FM transmitters off of the factory floor before the broadcast division went under.
Late model RCA BTF-20E FM transmitter
John says:
If anyone needs parts out there, I will probably cannibalize this unit as I have just too many. The separate power supply is inside the garage along with the latest version of the harmonic filter entirely made of copper.
I don’t know if the entire unit actually runs or if it is parts only. It certainly looks like a clean unit. As I recall, Comark bought out all the RCA broadcast parts and service. Comark was then sold to Thomcast, which was sold to Thales which I think spun off it’s transmitter division to Grass Valley Group. Grass Valley started by making TV master control switchers, routers and other video equipment.
Anyway, if you are looking for RCA parts for FM transmitters, contact me, I’ll put you in touch with John.
Old blue, I like to call them, the Harris 1980′s model transmitters with black faces, white cabinets and blue trim. I have yet to find one that I really like, the FM 25-K is, well okay. Sort of like that 200,000 mile jeep that works, most of the time, and it’s paid for.
This particular FM-25K transmitter is located at WIZN in Charlotte, VT.
This transmitter was new in 1987. It had a bad day yesterday, deciding to throw a temper tantrum and trip the HV power supply breaker. Fortunately, the station has a back up transmitter. When we arrived, we found the HV power supply feed through insulator at E1 arced over and broken. Again, fortunately this station’s management believes in stocking spare parts and a replacement was on hand.
This is part of the RF filter for the HV power supply. This happened once before, about two months ago. The replacement insulator then was used, so that might be a factor. Two months ago, both capacitors in the Pi filter and the HV power supply cable (RG-8 coax) was replaced all the way back to the rectifier stacks.
The FM25-K can produce spontaneous high frequency oscillations if not tuned properly. We looked at transmitter output with a Rode Schwartz spectrum analyzer and found it to be clean. Exactly why it blew out another feed through insulator is a bit of a mystery. Since the first replacement was a used part, we surmise that it may have been cracked. If this replacement insulator arcs, there needs to be a full investigation.
As I said in the beginning, I have found these transmitters to be okay, not the best, not the worst. Most of the problems I have encountered with the K series FM transmitters had to do with the controller cards. There are two, one analog and one digital. That’s what Harris calls them anyway. Like the SX transmitter, and the MW transmitter to a certain extent, the control circuits are way over complicated and full of +/- 5 volt CMOS logic. Having that type of control logic connected to a radio tower (e.g. lightning rod) is asking for trouble.
Ahh, since I posted about my android, a few readers have emailed me and would like to know. If you have tried to stream audio using a smartphone web browser, you have found out that it simply doesn’t work. The web browser is unable to decode the radio station stream because most of them are in AAC, AAC+, HeAACv1 or some other codec. At this point, most people give up on the idea and move on. I, on the other hand, determined that it should be doable.
First, I attempted to down load a few apps, but they either crashed or didn’t do what I wanted or weren’t in the right language, or something.
Clear Channel has something called iHeartRadio, which is a clearing house for mobile users that want to listen to Clear Channel radio streams on their iPhones. I don’t know, once you have heard one Kiss-FM station, you’ve heard them all as far as I am concerned. Most other Clear Channel programming is boring and uninspired.
What I finally ended up doing was going to Moodio and reading up on a few things. Here is a good step by step way to use Moodio to listen to radio station web streams on any mobile device.
Be aware that not all data plans are the same. ATT, Sprint, and others now cap data transfer and charge extra if a subscriber goes over. Know your plan.
Select from there list, the stations you want to listen to. They have many US stations as well as many from Europe. If the station you are looking for is not there, you can request that it be added.
Select the default data rate. Since I have unlimited data, I chose the highest rate for the best sounding audio. Others may want lower data rates so as not to exceed data caps.
The stations on your listen list will be displayed.
That is a lot of steps to take. Somebody has to be very into radio or a radio station to do something like that. A forward thinking radio station or group will be writing or paying somebody to write mobile streaming apps for their stream(s). A forward thinking radio station or group would then feature links to these apps prominently on their web pages. Very prominently if they are in a PPM market. Ahem, very prominently if they are in a PPM market.
That is what a forward thinking radio station would be doing…
More bee related RF stories. This happens often this time of year, the paper wasps have worked hard all spring to build their nests up in size and during July, they become large enough to block the aperture of the antenna mounted on a satellite dish. As the nest fills up with eggs and larva, it becomes denser and blocks more RF from the antenna. Soon, the signal on the satellite receiver drops and audio dropouts occur.
I have noticed that the newer generation satellite receivers are not as good as the older Starguide III and II units. The Starguide receivers were pretty light duty when compared to the Scientific Atlanta 7300 or 2300 series units. Those things were build like tanks, took up a lot of rack space, and so long as one replaced the power supply capacitors every so often, never failed. The newer satellite receivers are very intolerant of phase shifts or any carrier disruptions. Many times, the signal strength might look to be above the drop out threshold (usually 4.5 to 5 dB), but the audio still occasionally cuts out. That symptom is almost always bees in the feed horn.
3.2 meter COMTECH satellite dish
This dish is mounted up high above the roof of the building on 6 inch well casing. In order to service the feed horn, one has to either rent a cherry picker or loosen the azimuth bolts and spin the entire dish around so the feed horn is over the roof area. Then an eight foot step ladder is need to get to the feed horn. Luckily, it is a flat roof. Needless to say, I made sure the feed horn had the proper cover over it so that no bees could get in.
Satellite dish feed horn with insect cover installed
Bee fade is best cured with a can of Raid hornet and wasp spray. The culprits are almost always paper wasps, which, I can tell you from experience, have a nasty sting. Once the nest is cleaned out of the antenna aperture, a proper cover must be installed. If one finds that they don’t have a proper cover, I have found that a top from a spray paint can will work as a temporary cover until a proper one can be installed. I would not call a spray paint can cap a permanent solution because the sun will eventually degrade the plastic and it will fall apart.
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
~1st amendment to the United States Constitution
Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
~Benjamin Franklin
...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner
Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~Universal Declaration Of Human Rights, Article 19
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