AM – Page 13 – Engineering Radio

The AM Receiver problem

The technical problems with AM broadcasting can be broken down into three broad categories:

Interference from other AM stations
Interference from unintentional radiators (AKA electrical noise)
Poor receivers

Much of the poor fidelity issues with AM broadcast audio come from the narrow IF bandwidth of the typical AM receiver. Older AM receivers had much wider IF bandwidths, sometimes as much as 15 KHz +/- carrier. As the AM band was overfilled with stations starting in the late 1940s, this became a big problem. The tube-type front ends with great sensitivity but not very much selectivity was unable to cope with adjacent channel interference, leading to what was known as “monkey chatter.” This type of interference can be technically described as the higher audio frequency peaks from adjacent channel stations being demodulated. Those hearing this type of interference found it very annoying and rightly so. Thus, receiver manufacturers were deluged with complaints about the poor quality of their units. The solution was simple; narrow the bandwidth until the “monkey chatter” disappeared. This new de facto standard IF bandwidth turned out to be +/- 3 KHz carrier.

It does not take a rocket scientist to see that 3 KHz audio is slightly better than telephone quality. This was the beginning of the perceived AM low fidelity problem. In the meantime, FM broadcasting, after years of lagging behind in spite of its superior audio, made great strides into mainstream acceptance.

NRSC-1 was supposed to reduce this type of interference by limiting AM broadcasting stations’ audio bandwidth to 10 KHz. The idea was to attempt to keep the modulation index somewhat within the allotted channel. This standard was mandated by the FCC in 1989, after which receiver manufacturers were to change their design to allow for broader IF bandwidths, thus improving AM fidelity. There was even an AMAX standard adopted by some receiver manufacturers. Unfortunately, by this time, the majority of AM stations were transitioning from music to talk radio. The new standards were too little much too late.

A quick scan with a quality AM receiver shows that many stations are transmitting high-quality audio, which, with a properly adjusted IF bandwidth can sound remarkably good:

Screen shot - WEOK true oldies channel — Screen shot – WEOK True Oldies Channel

This is a screenshot from an SDR (Software Defined Radio) showing WEOK, Poughkeepsie, NY broadcasting the True Oldies Channel. The signal strength is slightly low, but this is a rural area and the noise floor is also low. I limited the bandwidth to +/- 7.5 KHz carrier because of the pre-emphasis used on most AM stations makes the high-end sound strident. Looking at the spectral display, there is more audio available beyond what I am listening to. This brings me to this; AM fidelity is not inherently inferior, it can sound quite good. There is no reason why AM receiver manufacturers cannot improve their products to include some advanced features;

Variable IF bandwidth based on signal strength
Variable user selected IF bandwidth
Sharp selectivity – adjacent channel rejection
Selectable sideband demodulation (carrier plus upper or carrier plus lower sideband)

While this will never sound as good as FM stereo, it still can sound pretty good, especially with older music recorded before say 1975 or so.

Manufacturers would have to have some impetus to include these features in their chipsets, such as multiple requests by listeners who are looking for better AM quality, which leads us back to programming…

The other issues with AM electrical noise reception and interference from other radio stations are surmountable, so long as there is a reason to. This, leads us back to… programming.

AM revitalization comments

I have been reading the comments regarding the FCC’s NPRM (13-249). Clearly, many people are interested in keeping the AM broadcasting band both active and relevant. Some of these suggestions have merit but are unlikely to be adopted by the FCC. Others are viable and could alleviate at least a few of the technical shortcomings of the AM band. The rest fall along expected positions. Here is a brief rundown:

Clear Channel, iBiquity: Allow stations to transmit in all digital mode. Likelihood: Possible. The hybrid version of AM HD Radio has been a failure on several fronts; added interference to adjacent channels, self-interference, poor adoption, wonky CODECs, etc. However, letting stations choose to broadcast in all digital AM HD Radio may decide the issue once and for all. As long as the all-digital carriers fall within the current analog channels, this would be fine. Actually, I would add that stations transmitting in all digital be allowed to choose DRM as well as HD Radio
REC Networks, MMTC: Move AM stations to former TV channels 5 and 6. Likelihood: Unlikely. It would be a neat solution, however, there are currently many full and low-power TV stations still using those frequencies.
Clear Channel, SBE, MMTC, Crawford, et al: Allow AM stations a special translator filing window. Likelihood: Almost assured. This has been broached by the FCC itself. I would add that Class D and Class C stations be given priority.
SBE, du trial, Lundin and Rackely, MMTC et. al: Remove the “ratchet rule,” reduce antenna efficiency requirements and city of license contour requirements. Likelihood: probable. Over the years, the FCC’s rules and regulations designed to help AM broadcasting’s technical product have done the opposite in many cases. This is especially true of the “ratchet rule.”
SBE, du Trial, Lundin and Rackely, MMTC: MDCL (Modulation Depended Carrier Level) Likelihood: Possible. MDCL does not do much to improve AM signal quality, but it can save the station owner some money on the electricity bill.
Alabama Broadcaster’s Association, et al: Better FCC enforcement. Likelihood: Not very. This is another area where interference and AM noise problems can be fixed. Given Ajit Pai’s desire for “non-regulatory” relief, stepped-up enforcement seems to be a non-starter.
Hatfield and Dawson: Eliminate substandard AM stations. Likelihood: Not very. Getting rid of substandard stations and letting the remaining AM stations enjoy a little breathing room is actually a big step in the right direction. H&D notes that the FCC should petition congress for tax relief for those stations that choose to surrender their licenses. Unfortunately, it does not appear likely that the FCC, congress, and the current station owners would go for it.
du Treil, Lundin, and Rackely: Do away with skywave protection for class A stations Likelihood: Possible. The argument goes; skywave listening represents a very small number of mostly hobbyists (AM DXers) as other, better methods for program distribution exist for serious listeners. Sad but true.
du Treil, Lundin, and Rackely: No more new AM stations. Likelihood: Possible. There is a cogent argument to be made regarding the overcrowding of the AM band. Stopping any further crowding is a good idea.
SBE, Cohen, Dippell, and Everist, et al: Tighten regulations on electrical noise emitters. Likelihood: Unlikely. The FCC does not have the mettle to tighten regulations against powerful manufacturing and technology lobbies.
iBiquity: Do not let anything get in the way of the HD Radio rollout. Likelihood: Is it possible to get in the way of something that is standing still?

Talking amongst engineers and AM broadcasters, many of these ideas have merit. The real question is, will any of this bring more listeners?

The Kintronic Isocoupler

Had a problem with this Kintronic FMC-0.1 isocoupler the other morning.

Kintronic FMC-1.0 STL ioscoupler — Kintronic FMC-1.0 STL isocoupler

After an overnight drenching heavy rain and very high wind, the STL transmitter associated with this unit was having high VSWR faults. This isocoupler crosses a base insulator of an AM 50 KW directional antenna. This particular tower has negative impedance, which is to say, it sucks power out of the pattern and feeds it back to the phasor. An interesting discussion for another time, perhaps.

Using a dummy load, we isolated the problem to the isocoupler by first connecting the load to the output on top of the unit (the problem still exists) and then to the transmission line prior to the unit (the problem went away). Of course, the AM station had to be taken off the air to do this work.

Once the issue was confirmed as the isocoupler, I opened the unit up and found that water had entered and pooled in the top of the bottom half of the isolation transformer.

The isolation transformer consists of two loops to ground capacitively coupled through air dielectric. The issue is with the opening around the top of the unit, under the lip of metal lid. Apparently, this allowed water in.

Kintronic isocoupler isolation transformer

It is difficult to tell with the lighting in this photograph, however, the bottom part of this isolation transformer has water pooled around the center insulator. Using a rag, I cleaned out the water and dirt from the center insulator. After reconnecting the antenna and transmitter transmission line, a quick check revealed the problem was much better, but still not completely gone. I suspect water seeped further down into the bottom half of this unit. The repair work was good enough, however, to return both stations to the air.

Glad to get that bit of work done while it was still relatively warm out.

The last AM station

AM; it has a future or not? I cannot make up my mind sometimes. As some AM stations can and do make a profit, many others do not. Truth be told, the engineering effort that goes into an AM directional antenna is becoming a black science. And some people may say, “oh, but that gives you job security,” but that is not usually how it works. Instead of paying somebody more money (or any money) to maintain something, the business philosophy these days seems to be to chuck the baby out with the bath water. Because after all, if not AM then FM right? Yes, of course! Except, the very thing that happened to AM is happening to FM too. Increasing noise floors, jamming signals into every possible nook of the frequency spectrum, no thought toward technical facilities and infrastructure, and horrible, horrible programming ~~will result~~ has resulted in the decline of listening for FM too. Mark my words and the date; FM broadcasting will suffer the same fate as AM if current trends continue.

How will it end? I would hazard with more of a whimper than a bang. I imagine something like this:

One day, in the not very distant future, at an AM station somewhere, the transmitter faults and goes off the air. Chip, the computer guy, goes in the back room, moves a bunch of cleaning supplies, cases toilet paper, a garbage can and the remote gear out of the way so he can reach the ON button on that box the old guy told him about. The big box makes some clunking noises and comes on for a second, but then the fault light called “VSWR” or something comes on and the transmitter shut off again.

Chip, the computer guy, remembering what the old guy said about that big tall thing behind the building, pushes the back door open. What used to be a field is now completely overgrown with weeds, brush, and trees. He follows the pipe from the back of the building, through the prickers until he comes to an old fence, which is falling down. He pushes on the locked gate and it falls off the hinges. Inside the fence, there is a rusty tower and a white box. Finding the box unlocked, he opens it and sees a baffling array of metal coils, copper tubes, and black round things. He sees that one of the black round things is cracked in half and black goop is coming out of it. The computer guy takes a picture with his cellphone and emails it to the market manager/vice president of sales.

A few minutes later, the the market manager calls back and Chip tries to explain what is going on, stating the the transmitter went off and the black thing in the box by the tower looks broken. The market manager/vice president of sales asks “Has anyone called and complained?” Chip says no, not that he is aware of. The market manager says, “Eh, fuck it leave it off.”

The end.