AM – Page 13 – Engineering Radio

Transmitter repair

Sometimes it is obvious and relatively easy, other times not so much. This summer we have had wave after wave of afternoon thunderstorms. It is almost like living in Florida; almost, but not quite. Anyway, with the storms occasionally comes some lightning damage. At most of the transmitter sites we service, every step has been taken to ensure good grounding and adequate surge suppression. This is especially true of sites that have been under our care for a few years. Even so, occasionally, something gets through. After all, those five-hundred-foot steel towers do attract lightning.

Broadcast Electronics AM5E output tuning section

This is the output section of the BE AM5E transmitter at WROW. The transmitter got pretty trashed; a bad PA module and power supply and this capacitor in the output section. This particular transmitter is 14 years old and this is the first major repair work we’ve had to do it.

Broadcast Electronics AM5E output tuning capacitor

The capacitor was fairly easy to change out. As a general precaution, both capacitors were changed. There was a spare PA module and power supply on the shelf, thus the transmitter was returned to full power relatively quickly.

Broadcast Electronics AM5E output forward and reflected power meters

The rest of the antenna system and phasor were inspected for damage, a set of common point impedance measurements taken, which showed that no other damage was sustained.

Next, the 30 year old Harris SX2.5 A transmitter at WSBS. This failure was slightly more exotic; the transmitter started randomly turning itself off. The culprit, in that case, was this:

Harris SX2.5 remote control interface bypass capacitor

Literally, a two cent part. The transmitter remote control uses optoisolators. The inputs to these opto-isolators are RF bypassed to ground on the back of the “customer interface board.” After determining that the remote control was not malfunctioning, it was down to either a bad opto-isolator or something really silly like a bypass capacitor. This capacitor was on the ground side of the remote off terminal. It shows short on the capacitance meter and 4.1 K on the ohm meter, just enough to randomly turn the opto-isolator on and shut down the transmitter. Being a Harris transmitter, removing and replacing the “customer interface board” was no easy matter. Overall, it took about three hours to find and repair this problem.

Troubleshooting an AM array

Today, there will be a quiz.

Recently, we had an AM antenna array go out of tolerance by a good margin. This has been repaired, however, I thought I’d post this information and see if anybody could identify the problem and the solution. Unfortunately, I don’t have prizes to give away, however, you can show off your AM engineering prowess.

All of the information is pertinent:

The station has two directional arrays (DA-2) using the same towers; the nighttime array is out of tolerance, and the daytime array is not affected and is performing normally.
There were no weather events connected with this event; no electrical storms, no major temperature changes, no rain events, no freezing or thawing, etc.
The problem happened all at once, one day the array was performing normally, and the next day it was not.
Station management reports that some listeners were complaining that they could no longer hear the station.
The ATUs and phasor were inspected; all RF contactors were in the proper position, no damaged or burned finger stock and no evidence of damaged components (inductors or capacitors) was observed. Several mouse nests were cleaned out of the ATUs, however, this did not change the out-of-tolerance antenna readings.
The towers are 1/4 wave (90 electrical degrees) tall.

Readings:

Tower	Phase angle as licensed	Current ratio as licensed	Phase angle as read	Current ratio as read
1	147.2	0.583	149.5	0.396
2 (reference)	0	1.00	0	1.00
3	-137	0.493	-125.8	0.798
4	107.5	0.481	92.7	0.355
5	-38.1	0.737	-60.2	0.623
6	-178.7	0.382	142.8	0.305

Licensed values for common point current is 13 amps, impedance is 50 ohms j0 and there is normally no reflected power on the transmitter. On this day, the common point current readings were 8.9 amps, impedance 38.5 ohms +j5 the transmitter had 340 watts of reflected power.

This is the overall schematic of the phasor and ATU:

WDGJ overall RF schematic diagram — WGDJ overall RF schematic diagram, click for higher resolution

Aerial view of the transmitter site, oriented north:

WGDJ aerial view showing towers as identified in schematic diagram — WGDJ aerial view showing towers as identified in the schematic diagram

So, where would you begin? Ask questions in the comments section.

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?