tech stuff – Page 62 – Engineering Radio

Broadbanded AM antennas

Partly for my own edification, partly just because, here is some information about AM antenna systems and their bandwidth. An AM tower is a radiator that, simply by the physical constraints of the tower structure itself, is pretty narrow-banded, even under the best conditions. Add to that, antenna tuning units, transmission line phasing, antenna phasing units, diplexing units, and things can get very squished outside of the immediate carrier frequency. This seems to be a particular problem with directional antennas, which most AM stations employ.

WGY 810 kHz, Schenectady, NY transmitting tower w/open feed line — WGY 810 kHz, Schenectady, NY transmitting tower with open transmission line

As an engineer, you can get some idea of how narrow an antenna system’s bandwidth is by looking at the base impedance measurement. Every AM station is required to keep the latest impedance measurement on file. When looking at these measurements, there will be one curve that indicates base resistance (R) and another curve that indicates reactance ( X, although often noted as + or -j). If the resistance and or reactance curve is slopped steeply at the carrier frequency and out to 20-30 kHz, it is a narrow tower. Add to that the different phase shifts of an ATU and or Phasor and things will be compounded. That is why it takes a professional to design and tune up these things, a poor design will never sound right.

Another way to get some idea of bandwidth requires a field strength meter. Modulate the transmitter with a 10 kHz tone at 50% modulation. Then, away from the near field, measure the carrier and 10 kHz +/- the carrier frequency on the log scale. The sidebands should be symmetrical and about 1/4 the carrier level.

Generally speaking, antenna systems need to be designed for low VSWR across the entire side band range (+/- 10 kHz from the carrier) as well as symmetrical distribution of radiated energy across the lower and upper sidebands. Several factors influence these conditions:

Electrical tower height is perhaps the hardest thing to change once a tower is constructed. Short towers (less than 80 electrical degrees), or very tall towers, (taller than 200 electrical degrees) present problems. If one were constructing an AM station and could choose any tower height, something between 120 to 190 electrical degrees would be ideal. Existing towers can be top-loaded to add electrical height for an additional 30 degrees or so. Beyond 30 degrees it becomes difficult to physically attain and therefore impractical in most situations. Top loading and bottom loading of a tower can reduce bandwidth if done improperly. Bottom loading an AM tower is almost never done due to the very high voltage and current as the electrical length approaches 180°.
Antenna matching networks can greatly improve or degrade bandwidth, depending on how they are designed. A T-matching network has more parts and is more expensive, however, it allows for optimum control over the R and jX phasing. This becomes much more difficult with directional antenna where phase considerations are a part of the station’s antenna field pattern development.
Phasors present the biggest challenge, particularly in the power divider sections. A tank circuit power divider is the worst choice, and a shunt circuit power divider is the best bandwidth choice, however, it is the hardest to conceptualize.

Obviously, the more complicated the antenna system, the harder it will be to keep the bandwidth open over 20 kHz of spectrum. This is especially true on lower-frequency AM signals, where the bandwidth is a much larger percentage of the frequency. Multiple patterns, multiple tower DAs are a nightmare. Single-tower non-directional stations are the easiest to modify.

As far as the circuit itself, higher Q circuits have smaller bandwidths. Simply stated, in an alternating current circuit, Q=X/R. The better the reduction of X, which also has a lot to do with the relationship of the current and voltage phasing, the better the Q will be. This is why a T network is the best design for an ATU. With a 90° or 180° tower, this is relatively straightforward. In towers that are shorter or taller than that, it becomes more difficult as the value of R becomes less friendly.

In most cases, some sort of L/C network can be deployed to decrease the Q of an antenna system at the base of the tower. Directional stations also need to have the phasing equipment looked at, because, as noted above, certain designs can create bandwidth bottlenecks. All in all, it is usually an expensive proposition for a multi-tower directional station to broadband its antenna system. This is another reason why IBOC on AM is destined to fail, many AM towers cannot pass the extended sidebands adequately.

I will miss the CBC when they turn off their last AM transmitter

I am listening to CBC Radio One’s Saturday Night Blues program. Yet another good radio show that comes in here via skywave from CBE in Windsor, Ontario. It is in the process of migrating to the FM band, channel 248B (97.5 MHz) after which the AM station will be turned off after a three-month simulcast. I know I can stream this program, but that is a pain because it slows my computer down, plus, it does not sound as good coming from the computer speakers as it does from the AM radio even with the occasional fade.

According to the FCC website (yes, the FCC has records of Canadian, Mexican, Cuban, and Bahamanian radio stations), it is a class A station with a 2-tower directional, 1/2 wave towers with 10,000 watts, unlimited. Pattern nulls to the south with a big fat main lobe north, east, and west.

I wonder what they are going to do with the transmitter when they turn the station off. I wonder what kind of transmitter it is, sounds like a tube unit to me. Certainly sounds good with that old blues music tonight.

Arbitron PPM; engineering speak

We have a few stations that are currently encoded with the Arbitron PPM encoders. I did a little research on the encoding method since it is not immediately apparent how they are transmitting their data.

According to Wikipedia, which can sometimes be relied upon, Arbitron used Martin Marietta to help develop the technology. Martin Marietta (now Lockheed Martin) is mostly known as a defense contractor, they have helped develop several complex military communications systems over the years.

There are no fewer than 39 US patents that cover the technology used in the PPM. The most significant of these appears to be 7,316,025 which describes the psychoacoustic masking technique employed.

It really is pretty slick, using a sample rate of 8.192 kHz, it transmits 4 bits per second in the 300-3000 Hz range by hitting specific frequencies in that range at varying intervals, adapting to the audio levels to keep the encoding below the programming content. 4 BPS is very slow and thus very robust. After all, I believe the only formation transmitted is a six-digit encoder serial number. I did not read all 39 patents to see if anything else was changed in the encoding method, so it may be slightly different.

This type of system would have fairly low overhead, not adding to the station’s bandwidth which is a consideration for FM stations, and in the correct frequency range for most AM receivers on the market today. Some people have said they have heard the encoding on one of our stations, most notably during silence or very quiet programming. Perhaps, especially in a dead air situation, one might hear in nearly imperceptible low frequency slow fluttering sound.

If anything, the encoding is perhaps too robust.

Now for the deployment of the monitor technology, which has so many up in arms. As with other Arbitron ratings methods, the main bone of contention seems to be the size and distribution of the sampling hardware. Minority groups feel they are underrepresented because the PPM is unevenly distributed.

Rating samples always seem to skew one way or another. The data samples themselves seem to be too small to accurately predict a station’s listenership. One anomaly and the entire month or quarter can be thrown off. The PPM seems to correct some if the issues with keeping an accurately written diary. One problem with the PPM however, it can also pick up incidental background noise and count it as time spent listening (TSL).

Think of the cubical environment where somebody several cubes away might be listening to a radio station. To the PPM wearer, it is unintelligible background noise, however, because of the perceptual encoding, the PPM picks it up and it counts as several hours of TSL.

A broader sample would dilute this with other more accurate representations of radio listening. A broader sample would also alleviate some of the complaints from the Minority Media and Telecommunications Council (MMTC). First-year physics students would recognize that not enough sample data can make results wildly inaccurate. Or, as one emergency room doctor stated while washing my knee out with a liter of sterile water after a dirt bike accident, the solution to pollution is dilution.

What the inside of a ceramic vacuum tube looks like

In case you have wondered it yourself:

This is an EIMAC 4CX3500A which came out of a Harris HT5 transmitter. As you can see the ceramic cracked in half. When I arrived at the transmitter site, the unit was on full plate voltage, with no plate current, and no overload lights. I figured it might be something with the tube, so I tried to pull it out, but only the top half came. One of those “Ah ha” moments.

Fortunately, there was a working spare at the transmitter site and we got back on the air relatively quickly. That, in and of itself is amazing considering the building that this transmitter lived in. One of those abandoned former studio sites with the transmitter jammed into a back room somewhere. To get to it, one has to dodge pigeons, beware of rats and wade through piles of garbage.

It is a little bit hard to tell in this photograph, but there are two “cages” which are the Screen and Grid. The post in the center is the filament/cathode and the top detached part is the plate/anode. In an FM transmitter, the exciter is coupled to the grid, and the screen accelerates electrons toward the plate and therefore controls the power, the plate collects the electrons and is coupled to the output stages and the antenna. Good stuff.