Creek overflows roadway, Ulster County, NY. Tropical Storm Irene, August 28, 2011
I read through the FCC’s Hurricane Irene information on which stations were off the air and when, I believe there are a few more to add to the list. Starting from South to North:
- WNYC (820 KHz) (New York Public Radio) See WMCA below.
- WMCA (570 KHz) (Salem Communications) went off when the fetid swamp known as the Meadowlands flooded, which is where their transmitter sites are located.
- WFAF (106.3 MHz) Mount Kisco, (Cumulus Broadcasting) Loss of power, no back up power
- WLNA (1420 KHz) Peekskill (Pamal Broadcasting) another AM located in a fetid swamp, this time in Peekskill. It is likely this station will be off for several days.
- WOSR (91.7 MHz) Middletown (Northeast Public Radio) reasons unknown.
- WKIP (1450 KHz) Poughkeepsie (Clear Channel Broadcasting) transmitter room flooded with about 18 inches of water, water over topped the base insulator and ATU.
- WVKR (91.3 MHz) Poughkeepsie (Vassar College) no backup power, back on at 12:00 pm 8/29
- WKXP (94.3 MHz) Kingston (Clear Channel Broadcasting) Loss of emergency generator when fan belt broke, restored four hours later.
- WAMK (90.9 MHz) Kingston (North East Public Radio) Kingston transmitter site, which looks like it is located in a Revolutionary War relic, is noted for being unreliable. It goes off frequently and was off and on all day.
- WFGB (89.7 MHz) Kingston (Sound of Life Radio) Located in same building as WAMK, is an LP-1 station.
- WKNY (1490 KHz) Kingston (Cumulus Broadcasting) antenna field flooded, back on the air by 9 am 8/29.
- WYJB (95.5 MHz) Albany (Pamal Broadcasting) Generator voltage regulator failed, equipment secured to prevent damage. Is an LP-1 station.
- WZMR (104.9 MHz) Altamont (Pamal Broadcasting) Co-located with WYJB
- WAJZ (96.3 MHz) Voorheesville) (Pamal Broadcasting) Co-located with WYJB
- WROW (590 KHz) Albany (Pamal Broadcasting) STL passed through WYJB transmitter site
- WPTR (96.7 MHz) Clifton Park (Crawford Broadcasting) reasons unknown
- WTMM (104.5 MHz) Mechanicville (Townsquare Media) reasons unknown
- WEQX (102.7 MHz) Manchester, VT (Northshire Broadcasting) loss of power
Most of these stations are now back on the air, however, several suffered much water damage due to flooding and will be off for a while. Lots and lots of roadways washed out, trees down, power still out for tens of thousands of people, it’s a mess.
These stations that went off the are are but a small fraction of the radio stations that serve the Hudson Valley and upstate NY. Most of the large class B stations, and regional AM stations, which are also the LP-1 EAS stations, stayed on the air for the duration of the storm, as did all of the NOAA
Weather Radio All Hazards stations in the area.
Kudos to the DJ’s, meteorologists, news people and field reporters for keeping us informed and safe.
And now, something completely different. It seems there is quite a kerfuffle going on in shortwave (AKA HF) pirate land. It seems there has been some FCC enforcement action of late, leading to at least one HF pirate being closed down, while some others are pointing fingers at another saying he is a rat, or a rabbit. Or something. I dunno, it gets a little hard to follow.
I have written about this in the past; Pirate Shortwave broadcasting. It is a very interesting phenomena that compels a person to gather together all the parts necessary, usually at some expense, and assemble a station. Further, keying the transmitter and broadcasting without benefit of a license is a violation of federal law, which can bring heavy sanctions. While most pirate broadcasters seem to get a slap on the wrist, this lax FCC attitude can change. There have been several steep fines lately for repeat offenders in the FM band. At least on the FM band and somewhat the AM band too, a unlawful broadcaster is assured of some public audience. On the shortwave bands, a pirate broadcaster’s audience is limited to only those that are looking for them, which is a very narrow segment of the population.
What are they trying to accomplish? Most of the shortwave pirate broadcasts that I have listened to are limited to a couple of songs from one particular genra, send an ID and then are off. Some will send a QSL card via slow scan TV. What compels these operators to go through all the trouble for a few minutes of irregular operation? Some of them have well equipped studios to go along with the transmitting equipment. Then there is the clandestine nature of the undertaking, often with mail drops and spoofed e-mail addresses.
Some seem to exult in sticking it to the man, that man being the FCC, big media corporations or any authority that tells them they are doing wrong. Acts of civil disobedience against authority perceived (rightly or wrongly) as oppressive or evil. Others seem to have some need to perform, no matter how small the audience may be. Some are just fooling around and do it simply because they can. Finally, others like the challenge of building a low power shortwave transmitter from scratch and seeing it to through to it’s end.
If the so said station is broadcasting with any appreciable power, it will get noticed quickly and sooner or later, the FCC will pay a visit. That is a foregone conclusion. The FCC has quite a few new tricks up its sleeve when it comes to direction finding and RF finger printing. That’s right, RF finger printing, it is exactly what it sounds like. Super resolution HFDF eliminates the need for triangulation, multiple vehicles, and wasting a lot of time driving around neighborhoods trying to figure out which residence an illegal broadcaster is using.
While I understand the compulsion to broadcast free radio; the need to inform under served communities, the fact that what we used to rely on for information and news is gone, a once vibrant and exciting art form has been reduced to a hollow shell of its former self, however, we have not yet reached a Magna Carta moment. There are still some legal methods of getting the word out on radio, both conventional and shortwave. International Broadcasting stations WBCQ and WRMI offer time brokered programming and are pretty liberal in the types of programs they accept. Not all US shortwave broadcaster are thus, many allowing only religious programming. Those shortwave stations have large coverage areas and existing audiences. There are also may AM radio stations that will do block programming over the weekend, for a price, of course. Then there is the possibility of setting up an internet station. Eventually, the new Low Power FM (LPFM) rules will go into affect and interested groups will be able to apply for licenses in that service.
The point is, while the deck is stacked against the local or community radio broadcaster, it is still possible to get the word out in a legal way. The cost of buying block programming will likely be the same or less than buying all the equipment to set up a pirate station. Further, if the programming is compelling, you may get noticed and be able to flip the equation and actually get paid to do it.
With Hurricane Irene taking aim at the northeast, now is the time to make preparations for bad weather. This is the five day forcast:
Hurricane Irene five day forecast
Now, five day forecasts are notoriously inaccurate. There are too many variables to make it accurate and even the most seasoned meteorologist will admit, it is an educated guess. However, the large semi-transparent disk is almost always accurate. Therefore, it seems we may be in for a bit of a storm this weekend, with the eastern end of Long Island and the Newport/Providence RI in the landfall area. The Bridgeport, CT stations will likely see the worst of it, if the storm follows the predicted course.
Having a good disaster recovery program in place reduces much of the pre-storm work. This includes backup equipment and personnel allocations to keep the stations on the air and providing valuable information during the event. Wherever and whenever our clients allow us, we make sure that these systems are properly designed, installed and working. When trouble is milling about off shore in the form of a Hurricane, then we make a few final preparations, both personally and for the clients:
- Top off all generator fuel tanks and test them. This includes my home generator.
- Make sure all loose items are secured.
- Make sure other redundant systems; backup transmitters, back up STLs, backup transmitter sites are in order and ready to be deployed.
- Check the personal safety items; first aid kit, rain gear, flashlight batteries, work gloves, eye protection, hard hat, some type of energy food and extra water are in the truck.
- Get out a clean sleeping bag and a set of clean dry clothes and put them in the truck.
- Gas up the chain saw and put it in the truck with extra gas, bar oil and blade sharpener.
During the event, it is important to recognize when a situation is too dangerous to proceed and wait for the danger to subside. Examples of this are local flooding of roadways, downed power lines, high winds, and or electrical storm while working at transmitter sites.
Radio may have lost much of it’s relevance as an entertainment medium, however, there is still one thing it does very well; broadcast emergency communications and information to the public.
Update: As of 5 am 8/25 it looks as if the hurricane is making a b-line toward Bridgeport, CT. Most of the computer models are now in agreement which means the forecast is getting a better handle on the variables and is becoming more accurate. Strength is still somewhat debatable, but even a category 1 storm could do significant damage. We shall see.
Update 2: As of 5 pm 8/26, Irene is still on course for the greater NYC/Long Island sound area. As much as possible, preparations are complete. There are some things which cannot be helped, like the height above mean sea level of the WICC and WEBE transmitter sites (10 feet) or the lack of a generator at WXPK studios, etc. Estimates are for Hurricane force winds by this time tomorrow, so the only thing left to do now is get a good night’s sleep. Tomorrow may be one of those long days.
Here is an interesting thing; several people have suggested that IBOC signals on both AM and FM NYC stations be turned off so that smaller local stations will be listenable to local residents in NJ and Long Island. A secondary consideration would be the amount of power IBOC uses and possibility of backup generators running out of fuel to run something that has little or no audience. If that isn’t telling….
It should be interesting.
It was a hot day, it was a cold day. The tube transmitter was running, the solid state (HD-1) transmitter was off the air. The books show that the company has deep pockets, but the accountant has short arms. And so it goes. In a sordid, yet familiar tale of leaping three quarters of the way across a river, the builders of this transmitter site seemed to think of everything except the cooling requirements for a 35 KW FM transmitter.
Instead of installing real commercial AC units, someone decided that 34,000 BTU window units were the way to go. At one time, there were eight of those units, all single phase 240 volts sucking down gobs of power and freezing up when the outside temperature dropped below 40°F. This was always a problem, but became more so when we took over the maintenance of this site. When there was a full time engineer, his time, apparently, could be wasted running back and forth turning the window units on or off in the winter as required. Now that a contract company is doing the work, it becomes cost prohibitive to require such things.
Therefore, the time had come to make a change. To that end, six of the 34,000 BTU window units were removed from the building. Two of the existing holes in the wall where used to create an emergency cooling system, consisting of a 4,292 CFM fan and a couple of shutters. Two other holes were used for the new air conditioners and two holes were blocked up. The remaining two window units were left in place in the combiner room, which is a separate cooling zone.
Bard 5 ton wall mount AC unit and cooling fan shutter
The new AC’s are five ton wall mount Bard units. These are three phase and should be more than enough to keep the transmitters cool. Here is how I arrived at that conclusion:
- The entire building load when the main transmitter is running at full power, without the transmitter room air conditioning, is 60 KW.
- All of the building loads except the transmitters go through a single phase panel.
- The load on the single phase panel is 10 KW, thus the transmitter load is 50 KW (this 10 KW is mostly the single phase AC units in the combiner room)
- The TPO is 32 KW, therefore the transmitter is generating 18 KW of waste heat.
- One watt hour = 3.412 BTU of energy, thus
- 18,000 watt hours equals 61,416 BTUs
- One ton = 12,000 BTU, thus
- 61,416 BTU ÷ 12,000 BTU = 5.118 tons
That will take care of the main transmitter waste heat. The HD transmitter generates another 4,000 watts of waste heat or 1.37 tons. The combiner is an another room and doesn’t factor into the calculation. The rest of the equipment is inconsequential, adding up to less than 100 watts.
The solar gain is more difficult to calculate, as it is based on the building structure, the type of construction and the heat gain (loss) through the walls and doors. This building is concrete block, insulated, and has no windows. It is unshaded, however it is painted a light color. All in all, the solar gain should be less than two tons on a hot day. Therefore the total AC load should be 8.25 tons or less.
Bard 5 ton wall mount AC unit
All that is left now was to install the things. Just pull up the truck and use a crane to lift them in place, except, no; that plan won’t work. This is the transmitter site at the power plant and the 138 KV lines overhead precluded any lifting with a crane. We instead had to build ramps and move the things around on large hand trucks. One unit is installed on the rear of the building, the other on the front. It required several days to make the ramps and four people to muscle the things into place.
The bottom air intake holes needed to be cut out for the new units. Cutting into the concrete block while the BE FM 35A was running proved to be another challenge. We used several sheets of plastic, shop vacs and extra air filters on the transmitters to keep the concrete dust out of the PA cavities and motor bearings.
Plan B cooling consists of a 4,292 CFM Venturi fan mounted on the rear wall of the building. The fan is controlled by a ceiling mounted thermostat set to 95 degrees. If the AC’s fail, the ceiling temperature will rise and the fan will turn on.
Transmitter site emergency cooling fan
The room volume is about 3600 cubic feet, therefore this fan will change the room air about once every 60 seconds or so. It is not the best plan to move humid, potentially dirty outside air through a building, but it it keeps the station on the air while the main AC units are being repaired, then so be it.
The entire system went on line last week and is working well.
This is cute. A small (VS allegedly stands for “Very Small”) integrated 2,500 watt FM transmitter. This one we just finished installing as a backup transmitter for WSPK, on Mount Beacon, New York.
Nautel VS-2.5 FM Transmitter
This site has a Nautel V-7.5 as the main transmitter. That unit is very reliable, however, this transmitter site is non-accessible 4-5 months out of the year due to ice and snow. The last time we had an off air emergency due to a crippling ice storm, it took an entire week to clear away all the downed trees so we could gain access to the site via snowmobile. As such, every system needs dual or even triple redundancy. Lack of said redundancy has lead to several prolonged outages in the past.
WSPK signal flow diagram
Last year, we were finally able to install a backup antenna after 63 years without one. This year, it is time to upgrade the rest of the backup equipment. The new auxiliary transmitter is connected directly to the auxiliary antenna via a five port coax switch. This allows for use of the dummy load for testing when we are present, but removes a potential failure point in the coax switch. There have been at least two incidences of the disk jockey accidentally transferring the transmitter into the dummy load when taking transmitter readings. Hopefully this configuration will be fairly idiot proof. I am making an interlock panel that will prevent both transmitters from being on the air at the same time.
Nautel VS 2.5 connections
This site is a work in progress.
The backup processor is at the transmitter site, the main processor is in the rack room at the studio. This works well because the main processor occasionally looses its mind and needs to be rebooted. It would be a significant pain to drive all the way up to the transmitter site just to reboot the processor. It might not happen at all during the winter. The back up processor has no mind so it is not an issue.
The VS transmitter is attractive because it has a built in exciter that accepts composite, AES or IP audio. The exciter also has a built in Orban processor as an option. Thus, if it really hit the fan, we could use the LAN extender to get the audio to the site. Further, it could be addressed by any studio in the company WAN. Which is cool, when you think about it.
Nautel continues to crank out innovative, dependable products and there is nothing wrong with that.
I give you joy, the unmitigated joy and shear pleasure of the Harris SX 5 AM transmitter. This particular unit dates from 1984 and is installed at WUPE in Pittsfield, MA. It has a few issues of late.
Harris SX 5 medium frequency AM transmitter
The first of which is the unbalanced or out of ratio condition of the PA current and voltage. When changing power levels, the PA current and voltage are supposed to track together. When they do not, it is an almost sure sign that one or several of the MOSFETS in the PA are shorted. Shorted IRF-350 MOSFETS are indicated by blown fuses on the PA boards and should be replaced in pairs. The reason for the damaged devices also needs to be investigated. It is entirely possible that the site receives a lot of lightning, which can cause this damage. It could also be heat related, as the site is not currently air conditioned. The other possibility is under drive conditions.
The MOSFETS turn on and off at a rate of 1/(carrier frequency (hertz)) times per second. If they are under driven, they will go full on and short circuit. Minimum drive is 27.5 volts peak to peak, anything less than that is marginal and can lead to destruction of the PA devices. Under drive indicates an issue with the oscillator, which has it’s own set of peculiar failure modes.
Since this is an older unit, all of the large electrolytic capacitors are also suspect and need to be replaced. There are three power supply capacitors in the bottom of the transmitter, two 76,000 μF 40 VDC for low voltage and one 7500 μF, 350 VDC for high voltage. The modulator section also has six 5100 μF 350 VDC capacitors, collectively known as “dynamite sticks” due to there explosive potential if installed incorrectly.
Harris SX 5 modulator section
While replacing the dynamite sticks, I noticed this PDM pickup board has a whole burned through it. This is a part of the modulator section and if it burned completely open, would likely cause all sorts of problems with this transmitter, likely spurs all around the dial, distorted modulation or perhaps overload and fail altogether.
Harris SX 5 transmitter damaged PDM pull up board
I managed to fix it with a jumper between what is left of the circuit board trace and the capacitor mounting bracket. I soldered the jumper to the board face and soldered the wire lug. After scraping all the oxidized metal off of the capacitor mounting bracket, I attached with a screw. The board itself needs to be replaced, if it is still available from Harris, which it may not be as support for this transmitter was dropped in 2008.
Harris SX5 PDM pull up board temporarily repaired with wire jumper
The worst, and I mean worst possible situation with these transmitters is some type of control malfunction. The control boards and oscillator are in that large vertical pull out drawer. God protect and preserve the digital control and S and M boards, as they are a major headache to troubleshoot. They have 7300 TLL (5 volt logic) that controls all functions and only little problem will cause the entire transmitter to shut down.
Other SX series transmitter tips can be found here.
I didn’t get to replacing the blown devices because of a looming electrical storm, which precludes working inside of transmitters. I’ll get back there next week and finish the job.
Update: I finished the repair job today 8/24. There were 16 blown MOSFETS on the PA boards. I checked the drive levels on the input side of the RF torrid load resistors and it is with normal range. I also found this snake in the bottom of the transmitter across the HV shorting bar.
Small grey rat snake, electrocuted by Harris SX 5 transmitter
Could have been chasing diner. Overall, the site needs help. The air conditioner is coming next week.
Harris SX 5 transmitter fully operational
After repairs, the transmitter is back at full power and modulating +125% again.
Or rather, what is the significance of this:
Wall graffiti at a transmitter site
A few hints; it was found (written on a wall) of an old, mountain top transmitter site. We are renovating this site and it was underneath an old old alarm panel from the 1970′s that I removed. It reads 468 ÷ 45 = 10. and the 468 is significant.
Once upon a time, a person could go to the TELCO demark and get all of the inside numbers for the CO and any number of CO’s in the area. They would be scribbled on the wall next to the equipment along with many other numbers. This was especially helpful when doing emergency trouble shooting on a circuit that was down. Try to do that these days and the most likely result is an unanswered phone. Most of the smaller CO’s are not normally manned unless there is a trouble ticket in process.
Six weeks to go and time is moving fast. Many stations of a particular group have not done the required CAP upgrades to their EAS equipment. I’d imagine that we will be running around installing and programming these things at the last moment.
A list of certified EAS equipment vendors is available from the FCC website. The new FCC website, which seems to be dedicated to bringing broadband to America, somewhat at the expense of everything else. More about that later.
About CAP and EAS; I hope the effort and expense is worth it. Those CAP converters and CAP compliant EAS machines are not cheap. I would think that after all that mandated spending, the new, new EAS works better than the old new EAS which left quite a bit to be desired. I am still skeptical of the internet based distribution system, as the internet has proven to be less than 100% reliable in all locations. The method seems to violate the KISS principle, of which I am a firm believer. In any case, I will reserve judgement until after the nation wide test on November 9, 2011.
Regarding that test, I have already read some “expectations management” statements from FEMA and the FCC. I have to say, if, given a years notice to install the equipment and do local testing before the big test, the new system does not show great improvements over the old EAS, it is time to scrap EAS altogether. It would be far better to tell the public they are on their own and recommend buying NOAA weather radios or rather NOAA All Hazards radios to receive their emergency information on. Supplement that with an e-mail notification system like NY Alert, which I use for urgent traffic and weather information and be done with it. This way, at least, nobody will be expecting the nearest automated station from the borg collective to supply meaningful, up to date information.
I thought it would be interesting to do a comparison between the two types of transmitters, both AM and FM. I have been doing this thing for 25 years and have quite a bit of experience working on all types of transmitters. Some of the broadcast transmitters I have personally worked on over the years include:
- Harris: FM-20H, FM-5G, HT-35, HT-10, HT-3.5, FM-25K, FM-5K, Z5-CD, MW-50A, MW-50B, MW-1A, MW-5A, BC-5H, SX-5, SX-1A, Gates 1
- Broadcast Electronics: FM-5B, FM 3.5A, FM5A, FM30A, FM35A, FM30T, FM20T, FM10S, FM5C, FM1C, AM10A, AM6A, AM5E, AM1A
- Continental Electronics: 816R-2, 814R-1
- Collins Rockwell: 831F-1, 838E-1
- Nautel: ND-1, ND-5, XL-60, V-40, V-10, V-7.5, NV-40
- Gates: BC5P, BC1T, FM5B
- General Electric: BTA-25
- RCA: FM20ES1, BTA5J, BTA1-AR
- CSI/CCA, Visual, Energy Onix, Bauer, McMartin, QEI, some Italian something or other, etc. Various makes and models.
I think I have a fair amount of transmitter experience under my belt. What I have found is that certain brands of transmitters are better than others, regardless of whether they are tube or solid state. There are several differences in each type, obviously. As to some blanket statement about which is better, solid state or tube, I don’t have one. My statement would be “It depends.”
Tube transmitters are more rugged and will take more abuse than a solid state unit. Things like heat, lightning, EMP, mismatched antenna won’t phase a well designed, well manufactured tube transmitter. On the other hand, they are less efficient AC to RF, have higher B+ voltages, have hard failure modes, and are more difficult to linearize, if that is required for some reason.
Solid state transmitters are more broadbanded, easier to change frequency, they have soft failure mode due to redundant amplifiers and power supplies. The voltages are lower, thus they are safer to work on.
Here is a complete list of advantages and disadvantages of each type:
||Very rugged, able to take heat, EMP, lightning, mistuned antenna, poor operating environment, etc
||Not heat tolerant, lightning and EMP can damage MOSFETS, switching power supplies sensitive to AC mains issues
||Advantage: Solid State, however efficiency gain can be wiped out due to larger air conditioning requirement
||Hard, most often
||Soft, most often
||Advantage: Solid State, failure of a single module or power supply generally will not take unit off the air
||Advantage: FM Solid state transmitters can easily be moved. AM transmitters still require extensive retuning.
||Advantage: Solid State, as tube changes are required every two to three years
||Requires skilled engineers to service and trouble shoot
||Modules and power supplies are often hot swappable and returned to manufacture for repair
||Advantage: Solid State, however either type requires occasional measurements with specialized test equipment
||High voltages, contact will be fatal
||Lower voltages, but can still be fatal
||Advantage: Solid State
||Advantage: Solid State, multiple power amps and power supplies give solid state units more redundancy
||Advantage: Dependant on TPO, Higher powered solid state transmitters are much more expensive than there tube type counterparts
||Good used market, some new FM transmitters still being built
||Good new and used
|| Dependent on brand
|| Dependent on brand
|| Advantage: neither
For some reason, the latest Broadcast Electronics tube type transmitters seem to have very long tube life. I installed an FM20T at WYJB in Albany, New York, in early 2001 and it is still on the original tube, some ten years later. Same can be said for the 2005 FM20T and FM30T installation at WHHZ/WKZY, Gainesville, Florida. Those tubes shows no sign of giving up anytime soon. I don’t know if that is an unusual trait of the transmitter or that particular tube.
WKZY, Gainesville, Florida
The above comparison seems to heavily favor a solid state transmitter. As a general rule, brand new solid state transmitters both AM and FM have advantages in almost every category except high power FM transmitters, where tube types still make sense. From a used transmitter standpoint, there is nothing wrong with a tube type transmitter, provided it has a solid state IPA. I have noticed the 4CX250B driver tubes most often used in FM IPA stages have markedly reduced reliability of late. I would also tend away from transmitter makes and models where the manufacture is no longer in business or no longer supports the product.
Audio Engineers will know this subject well. Grounding has many purposes, including electrical safety, lightning protection, RF shielding and audio noise mitigation. Although all types of grounds are related in that they are designed to conduct stray electrons to a safe place to be dissipated, the designs of each type are somewhat different. What might be an excellent audio ground may not be the best lightning ground and vice versa. Sometimes good audio grounds can lead to stray RF pickup.
The basic ground loop looks something like this:
Ground Loop schematic
Where RG should equal zero, in this representation it is some other resistance. This causes a different potential on the circuit (V1), which in turn causes current to flow (I1). It is that unexpected flow of current that creates the problems, causing voltage (V2) to be induced on another part of the circuit. In cabling applications, this will result in a loud, usually 60 cycle hum impressed on the audio or video being transmitted through the cable.
The resistance can come from something as mundane as the length of the conductor going to ground. This can often happen when using shielded audio wire in installations when the connected equipment is already grounded through the electrical plug.
There are two proven methods for eliminating ground loops, both of which are best implemented in the design phase of construction (aren’t most things).
Radio Station Common Point Grounding
The first is a single ground point topology, also known as a common point or star grounding system. A common ground system consists of one grounding point or buss bonded together so that it has the same potential. All grounded equipment is then connected to that point creating a single path to ground. All modern electrical equipment has a path to ground via the third prong of its electrical cord. Problems can or will occur when audio equipment is plugged into separate AC circuits, grounded via the electrical plug and then tied together via an audio ground. The longer the separate grounding paths, the more severe 60 cycle (or some harmonic thereof) hum can result.
To eliminate this problem, the shields should be broken at one end of the audio cable. Never cut the third prong off of an electric cord, which can create another problem called electrocution. Given the choice between a ground loop and electrocution, I’d stay away from electrocution, mine or somebody else’s.
For installations in high RF fields, the open shield or ground drain can act like an antenna. In those situations, the open end can be bypassed to ground using a 0.01 uf ceramic disk capacitor. Electrically, this will look like an open at DC or 60 cycles, but allow stray RF a path to ground. This problem can be a common occurrence when studios are co-located with transmitters.
The second is by using balanced audio or differential signals as much as possible. This poses a problem for those stations that use consumer grade components, especially in high RF fields. For shorter cable lengths, two or three feet, it is usually not a problem. Anything beyond that however, and trouble awaits.
It is relatively easy and inexpensive to convert audio from unbalanced to balanced. As much as possible, equipment and sound cards that have balanced audio inputs and outputs should be used. In the end, it will simply sound better to use higher quality equipment. Also, longer cable runs need to be properly terminated at both ends.
Installing equipment using good engineering principles and techniques will eliminate these problems before they start.