Engineering Radio – Page 175 – One Hundred percent Human Generated Content for your Reading Enjoyment

Blanketing Interference and RFI

Blanketing interference refers to the phenomenon of receiving radio signals on devices not designed to do so. In broadcast radio, this is defined for AM stations in part 73.88 as:

The licensee of each broadcast station is required to satisfy all reasonable complaints of blanketing interference within the 1 V/m contour.

And for FM stations, it is part 73.318:

Areas adjacent to the transmitting antenna that receive a signal with a strength of 115 dBu (562 mV/m) or greater will be assumed to be blanketed.

Any interference to any device with that signal contour is blanketing interference. 73.318 further states that:

permittees or licensees who either (1) commence program tests, or (2) replace their antennas, or (3) request facilities modifications and are issued a new construction permit must satisfy all complaints of blanketing interference which are received by the station during a one year period.

I have always taken a more pragmatic approach to interference complaints. Rather than pass the buck and tell the homeowner or business owner that it is not our (the radio station’s) problem, I’d go and try to help them out. Generally speaking, the interference problems are close to the transmitter site, so on the next trip to that site, I would bring RFI filters and my 25 years of RF experience and solve the problem. I would like to think this helps the station’s and the company’s image in the community.

Most of the problems are pretty easily solved, although once in a while, I have come on some head-scratchers. An AM station playing on the outlets in a guy’s garage, the mic cords on a church PA system, and an off switch on a blender, off all things. The Bare Naked Ladies had a line in the song Light up my Yard: “we can dance to the radio station that plays in our teeth.”

What I have found is to start with the simple stuff first, check the ground on the electrical service entrance panel. One might be surprised to find it disconnected, corroded, or missing completely. On more than one occasion, I fixed all of the RFI problems with a simple turn of the screw holding the ground wire to the grounding electrode. In my experience, this is the most common single failure point. A disconnected ground will cause the entire neutral wiring system to act like a giant AM antenna, with all sorts of bad outcomes.

Most often, telephone answering machines, cordless phones, and other devices powered by wall warts are suspect. Those devices do not have a path to ground. A few turns of all the wires coming and going from said device around a ferrite core such as a snap on TDK RFI EMI filter available from Mouser will take care of it. Mouser has several different versions available.

Occasionally, one needs to put on a detective hat and do some footwork. Mast mount TV antenna preamps can cause untold heartache and problems. One such incident involved the second harmonic of an FM station falling exactly on channel 11’s audio frequency. This was affecting several houses in a one-block area. I finally found the problem at one of the complainant’s houses when I pulled the TV out and found the preamp power supply. Unplugging it made all the problems go away (I hate Radio Shack).

Usually, the process of elimination will discover the problem and thereby reveal a solution. The aforementioned church incident was discovered after I began unplugging microphone cords from the back of the Mackie mixer in the choir loft. It turns out several mic lines were plugged into the back of the mixer, unused and unterminated, creating a large long receiving antenna on the cable shield, which happened to be aligned perfectly to pick up RF from an AM station.

Move AM stations to channel 5 and 6

It might happen, at least according to Commissioner Clyburn, they aren’t saying no right away. According to her prepared statement:

I believe it is time that we consider the fate of Channels 5 and 6 as they relate to current radio service. These channels have proven difficult for television broadcasting, and I have a hard time imagining that they would fare much better as additional spectrum for mobile broadband use. This spectrum is not well suited for digital transmissions. It certainly is possible that this spectrum could be used for LPFM, expanded NCE use, and AM broadcasters.

That would, indeed, be an interesting development, if it were allowed to happen. Of course, there are quite a few hurdles to get over, even if it gets the FCC’s nod, which is a long shot, to say the least. There would likely be some type of congressional “input” into the matter, which could stall things for years if not forever, depending on which way the money flows and which one of our wonderful congressional representatives can be bought and sold.

Getting new radios on the market with the expanded FM band (77 through 87 MHz) will take some time. Thankfully, unlike HD radio, no licensing fees will be required. Manufacturers simply need to increase the frequency range down. It might take several years, but it would happen eventually, as is the case with expanded AM band radios, which are universal now.
Existing AM stations should be given the option to move, those that stay on the AM band will get the option to improve their facilities or go non-directional as the interference contours allow.
Those that choose to abandon AM need to surrender their AM license before commencing broadcasting on FM, none of this expanded band crap where they were supposed to surrender licenses after five years and never did.
Those that choose to abandon the AM band also will not be assured the same theoretical coverage areas they had on the AM band.
AM migrants should not have to compete in an auction.

Indeed, if LPFMs get a boost in the process, all the better. It might actually give radio the shot in the arm it needs, add a good deal of local competition, and satisfy several needs.

Protective gear and worker safety

Safety at work is one of those things that is often overlooked for various reasons. Sometimes we just get into a groove and are not thinking about it. Other times, employers can put workers in a potentially dangerous situation by ignoring regulations or insisting employees do things contrary to common sense.

For a broadcast engineer working in the field, safety can be a matter of life and death. Transmitters, in particular, have a host of potential safety issues; high voltage, thermal burns, RF burns, revolving mechanical parts, and external things like lightning. Transmitter sites themselves can be critter magnets, anything from bees to raccoons, bears, and even the two-legged kind.

Good general practices can go a great way in reducing injuries and downtime. Take this young fellow here:

improperly attired worker attempting unsafe operation

First of all, it appears he has gone to work in his pajamas, which is a no-no. Secondly, he has the right idea, wearing safety goggles while undertaking the risky operation of cutting low-density polyethylene with a pair of hand shears, however, those look more like swim goggles. They appear to be improperly donned. He is using a right-handed shearing device with his left hand and the work area looks cluttered and unkempt.

Seriously, we are all responsible for ourselves. While at work, it is important to use common sense. I may be a wimp, but if I have a question on whether the breaker is on or off, I go check. High-voltage power supplies offer no second chances. Here is a list of things to be cognizant of while working at transmitter sites:

Weather. If the transmitter got knocked off the air by lightning, wait till the storm is over to fix it. It is still coupled to the tower, even if the backup transmitter is on the air.
Fall hazards. OHSA requires fall protection for any worker working at an elevation higher than 4 feet. Fall protection can vary CFR 29 subpart E 1926 has all the details.
Falling object hazards. Tower works have been known to drop a wrench from time to time. A hard hat should be required whenever climbers are on the tower. Also, I watched ice shedding from a 1000-foot TV tower practically destroy a fuel delivery truck in a matter of minutes.
RF safety. I require all tower climbers to wear personal RF alarms when climbing on any tower that has RF radiators active. Do not work in hot ATUs or Phasors. ATUs and Phasors should have provisions to make all necessary measurements with protective covers in place and minimally exposed RF parts.
Electrical safety. Never work alone at a transmitter site. Turn off breakers before opening transmitter doors, do not defeat interlocks, and always discharge high voltage with a ground stick. Hang ground stick on HV power supply output. Tag out breakers if in a separate room from the transmitter.
Critters. Use bee spray on ATUs and other outdoor structures. Be careful around wild animals, even mice, and mice droppings can spread disease, use hand cleaner after cleaning up mice nests. Snakes love tuning houses, generator sheds or just about any building that is not inhabited.
Generators and backup power. Generators pose several hazards; fuel and batteries can be explosion risks, revolving parts, thermal burns, and high voltage.
Access to the site. In some areas where mountaintop transmitter sites are common, access during winter months can be tricky.

Much of this is common sense, remember, a radio station is a radio station, and there is only one you.

Al Fansome, Call your office

Shortwave Pirate broadcasting has been going on for years. While it is illegal to transmit radio signals without a license, it is not illegal to receive those signals. There is something sort of sneaky like you are hearing something you shouldn’t, part of some underground thing, listening to these guys. I am almost remiss to write something about it because I don’t want anyone to get into trouble.

Anyway, on any given day or night, pirate broadcasters roam the shortwave airwaves. Much of what they do is typical sophomoric humor, such as playing a song where the only lyrics are “god damn you” over and over again. Some of it can be somewhat entertaining. A lot of what they tell as jokes are inside baseball, you have to listen and do a little research to get it, the Al Fansome reference is one. There are no set times or frequencies. It is quite common to hear one guy tuning up and getting ready to go on the air when the current frequency occupant signs off (happens often on 6925 KHz).

Most of these guys build their own transmitters based on designs found on various websites. Power levels vary, but 10 to 50 watts is common. Because of this, a good low-noise receiving antenna is required to pull them out of the noise floor. I have used, with good success, a K9AY terminated loop antenna. We are in a rural area, so it is pretty low noise to begin with. Even so, the coverage with a 50-watt transmitter is remarkable when propagation conditions are good. Sound quality can be quite good for a homemade AM transmitter.

There is a lot of focus on FM pirates these days, that particular setup is likely the easiest to attain for most non-technical types. There are a few AM pirates floating about, those are likely the most difficult to construct and conceal once they are on the air. Both of those broadcast bands have the advantage that there are many kits and or instructions on how to build a pretty good-sounding transmitter. Shortwave seems to be a small cadre of dedicated hobbyists that simply like to fool around on the radio.

In any case, with the FCC stretched thin, it is unlikely that a SW pirate will be busted but not unheard of. One SW pirate station was busted in Florida about a month ago. Even so, that was the first one in almost five years.

For the most part, the activity seems to center between 6850 to 6970 KHz or so. Some others operate around 15055 to 15070 KHz. Here is a brief selection of what one might find on the SW pirate frequencies:

WHYP on 6925 USB “Who Wants To Be A Pirate Radio Operator” at 0156z.
WMPR on 6925 AM “This is WMPR Dancy Party” ID at 0040z.
Captain Morgan Shortwave Radio on 6925AM “Positive Captain Morgan Shortwave ID, email, and twilight zone theme at 2209z.
Radio Ronin Shortwave on 6950AM oriental-sounding interval signal, id at 01:04, Outer Limits intro, “She Blinded Me with Science”, strange version of “SOS”, anti-BP comedy skits, id at 01:19
Indira Calling on 6925AM pop music that I can’t identify, “Rock-It”, Indian music, id at about 00:37, “Beach Party 2000” show, Calcutta mail drop, Beach Boys medley
WEAK Radio on 6925 Shout out to Voice Of Honor. 0048z “Godzilla” 0055z Sabbath.
Channel Z radio on 15067AM 2218z ID and contact info, must be an old show (Blue Ridge Summit maildrop)

And so on. Those call signs are usually spoofs on something. Occasionally, contact information is given out, usually in the form of a mail drop. If so inclined, one might write a letter and receive a QSL card.

There is plenty of information floating around out there about shortwave pirate radio if one cares to look for it. Two of the more popular discussion forums are HF underground and Free Radio Network Grapevines.