Paul Thurst – Page 176 – Engineering Radio

E-skip, tropospheric ducting and other VHF propagation phenomena

While the FM frequency band (88 to 108 mHz) is mostly line of sight, there are things that cause long-distance reception hundreds or sometimes even thousands of miles from the transmitter. For a radio engineer, this can lead to all sorts of problems. Some are serious like STL cutouts, and some are quite funny, such as the general manager panicking when several new stations suddenly pop up in town. One of the many jobs of a broadcast engineer is to avoid problems and fix them if they show up (preferably the former).

The first and most common of these phenomena is Tropospheric ducting. This happens in warmer weather when there is a high-pressure system nearby and is more prevalent over flat terrain. What happens is a warmer layer forms in the atmosphere above a cool layer. That is why it is also known as “temperature inversion.” This causes a higher refractive index, which means that normally the signal would carry on out into space, however, upon encountering this warm layer it is bent back to Earth. It can last a few minutes to several hours. It affects all frequencies but is most prevalent above 100 mHz.

In some more severe cases, FM stations can travel 500 or more miles and override the local station’s transmitter site 15 miles away. In the age of digital STLs, co-channel, and adjacent channel interference can cause the STL receiver to unlock and mute. Analog STLs will become hissy or drop out altogether. It can be a big problem.

Unfortunately, not a lot can be done about main channel interference. It will go away eventually, and no, the station causing the interference is not operating illegally or any other thing. One consolation, if the duct is open in one direction, it is also open in the other, so say hello to all your new temporary listeners in East Podunk.

As far as STL paths go, the best defense is to have a good strong signal at the receive site. Boosting the signal with a preamp at the back of the STL receiver will not do anything. Larger, higher gain antennas at the transmit and receive will help, and more transmitter power will help. Sometimes diversity receiving antennas will help because at the 950 frequencies, 100 feet or so of altitude may make all the difference. Other than that, things like a backup RPU path using a lower frequency, a backup T-1, a backup ISDN line, a Comrex Matrix, basically anything to restore programming.

There is a tropospheric ducting prediction site called Worldwide Tropospheric Ducting Forecasts. They produce daily maps and predictions based on weather patterns.

The next propagation type known to abnormally affect VHF frequencies is called Sporadic E or E skip. This happens went ionized particles appear in the E layer of the ionosphere and it is more prevalent during the high period of the sunspot cycle when the atmosphere is unsettled due to solar storms. It is more likely to affect frequencies below 125 mHz, so main channel interference may be noted, but STLs and other broadcast auxiliary services will not likely see any effects.

This can happen any time of the year in any terrain and in any weather condition although it seems to be more prevalent in summer and for some unknown reason, around Christmas.

Ionospheric propagation is also known as skywave and is responsible for long-distance communications in the MF (AM broadcast band) and HF (Shortwave broadcast band).

During sunlit periods, the Ionosphere breaks down into several layers; the D layer, which is responsible for the absorption of AM signals during the daytime. The E layer, which normally reflects signals less than 10 MHz. The F1 and F2 layers, which primarily affect HF and lower VHF, from 10 – 40 MHz or so.

During sporadic E events, the E layer becomes heavily ionized in specific small thin areas, sometimes called clouds. This can last a few minutes or up to several hours. The effect is normally more pronounced with lower frequencies.

In this internet age, there is, of course, a website that can predict or at least define sporadic E, DXMaps.com has maps similar to the tropospheric ducting maps above.

Ionospheric Map — Ionospheric propagation map

Occasionally, solar storms will affect communications on all frequencies. The last time I heard this was in the last sunspot peak around 2000 or so. I was listening to the radio and all the stations faded for several seconds. It turns out a huge solar flare had erupted and sent a stream of particles through the Earth’s atmosphere. I happened to be driving down the road and immediately my cell phone started ringing. Listening to the panicked program director on the other end, you’ve thought the earth has stopped spinning on its axis. Anyway, it does happen once in a while.

Proper termination of long audio wire runs

This is standard telephone company stuff, however, it would seem that several radio engineers have forgotten this. I was reading on one forum where an AM station was using 1000 feet of 12 gauge Romex to send audio from the studio to the transmitter out back. The owner was complaining that the audio sounded bad.

Longer wire runs need to be terminated with the characteristic impedance of the cable being used, normally 110 ohms or so for typical audio wire. This is because impedance mismatches can cause return loss just like in an RF circuit. Exactly what the effect of the mismatched impedance depends on the length and frequencies involved. On shorter cable runs of less than 100 feet or so, this usually is not an issue.

The result of return loss is part of the audio energy gets reflected back to its origin (a standing wave), where it mixes with newer audio. This can cause out-of-phase issues and usually, the result is high tinny sounding audio with distortion in the mid-range frequencies. In other words, it ain’t pretty. This can really become an issue with digital audio because of the higher bandwidth requirements for high sample rates. It has always struck me as odd that AES/EBU audio uses XLR-type connectors. An XLR connector does not maintain the characteristic 110-ohm impedance of most digital cables and itself can cause pretty significant return loss. But anyway…

There are a number of options for proper termination:

1. Transformers are often used to match the impedances of circuits. A transformer converts alternating current at one voltage to the same waveform at another voltage. The power input to the transformer and output from the transformer is the same (except for losses). The side with the lower voltage is at low impedance, because this has the lower number of turns, and the side with the higher voltage is at a higher impedance as it has more turns in its coil. Western Electric 111C audio transformers were often used in equalized TELCO circuits sending audio over long distances on copper pairs.

WE 111 repeat coil, one of the best such transformers ever made

2. Resistive network impedance matches such as H or T or L pads are the simplest to implement. They limit the power deliberately and are used to transfer low-power signals, such as unamplified audio or radio frequency signals. Almost all digital circuits use resistive impedance matching which is usually built into the structure of the switching element.

3. Active balanced converters using opamps with high input impedances (10 Kohm bridging resistance) that first greatly reduce the voltage, then amplify it are often used an audio circuits. They have the advantage of active gain control and are often used in conjunction with gain reduction and limiting circuits.

The above diagram shows an active unbalanced to balanced audio converter. The advantages of such a circuit are active gain controls can be added to set levels. With additional feedback circuit elements, it can also be used for automatic gain control, gain reduction, limiting, and so forth.

For most inter and intra-studio wiring, professional audio equipment is designed for 0 dBm 600 ohm balanced audio (AKA line level audio). Audio cables such as Belden 8451 or multi-pair cables terminated on punch blocks or connectors works well. Cable impedances and matching are generally not design considerations. Long cable runs, longer than 150 feet or so, do need to be terminated in a high-quality audio installation.

Studio Builds, the never ending cycle

The lease is up, it’s time to move! Yay, we get to rip apart the old place and redo it! Again! It seems to be a matter of course that every few years a radio station will move. Such is the case with WKZE in Red Hook (the town, not the area in Brooklyn). Their lease is up on the “Grotto” location, so the owner has decided to move to a new location, closer to the center of town.

The new location was the former thrift shop. I know this because while I am working there, a constant stream of older people stop by and tell so. Once, while working alone doing some pre-move work punching down wires and computer network cables, I had to use the facilities. There I sit, on my porcelain throne, when I hear, “Hello?” in an old shakey voice. A quick glance at the door reveals it is not locked. Oh, NOs! Okay, don’t say anything, she’ll go away.

“Hello?”

“Hello, is anybody here?”

“Hello? Very strange, the doors are open but nobody is here. Hello?”

Oh for the love of Pete, “I’m in the bathroom,” I finally said.

“Where is the bathroom?” said the interloper.

I refused to say anything else and she finally left. She could have taken all my tools if she wanted to.

Anyway, the studios themselves are pretty simple, one production studio and one air studio. A T-1 line to the transmitter site, turned out the be the hardest thing about the entire operation. We moved the old Radio System consoles rather than purchasing new equipment. Radio Systems has a program called a Millennium upgrade, where you buy a new control surface, which replaces all moving parts, for something like $2,300.00 or so. For that, basically, a new console is had.

Radio Systems Former RS-12 now Millennium 12 console

The new production room is long and narrow.

The air studio is large and spacious. They often have live music from this studio, which is really cool. The station uses Prophet Systems automation equipment, although it is live most of the time.

WKZE air studio before furniture is installed — WKZE air studio before the furniture is installed

The main office area is one large room where desks will be located.

We are moving in stages:

Prep work, installing all the computer network cable, phone system cable, pulling all the audio and control wiring. Then the contractor finished up the drywalling and painting. Nice Colors!
Ordering phone lines and T-1 line. Ahhh, the phone company, such a pleasure to deal with, we had to pull a new cable through the underground conduit from the street to the building because the old cable did not have enough pairs. The conduit length is about 75 feet or so.
Removed the old production room console and took it to the shop to rebuild. It was not that difficult really, although a little cumbersome. I throughly cleaned out all the dust dirt and other detreious materials from the console frame and install the new control surface. I also checked all the power supply voltages with an oscilliscope to make sure there was no ripple. The original consoles were made in 1992, not bad for an 18 year old board.
Built a new production room with the rebuilt board.
Tested all computer jacks, audio wiring, etc prior to move.
Move T-1 circuit and all office and studio telco lines to the new location. Fortunately, the phone company is a local company not the big V we have in other cities. They were able to work with us and get things paralleled to the new location, something a large company might not have understood.
On the air from the production room at the new location
Remove the main rack, intact and move it to new location
Remove office phone system and install at new location
Remove and rebuild old air studio console
Install rebuilt air studio console in new studio, wire
Transfer operation to new studio

Right now, we are on step #6. That is going to be done next Tuesday (the day after memorial day) morning I believe. We should have the move completed by the end of the week. I’ll post updates as they become available.

Should I renew my free subscription to Radio World?

When I left my former place of employment and set out as a contractor, I also left my Radio World subscription behind. After I was let go, or quit or whatever it was, I was thinking that my days in radio were over or at least winding down and I had bigger things to worry about than trade magazine subscriptions.

I have been checking the Radio World website and find that it gets updated from time to time, usually several weeks behind the magazine. So the question is, what am I missing? As one commenter on the radio info board put it, the magazine keeps getting smaller and smaller, soon they’ll have to rename it Radio Pamphlet.

I’d like to stay abreast of things in the technical end of the radio business, but is Radio World the way? There was a time when it was a technical magazine full of good timely articles well written by fellow engineers. Occasionally I still find something interesting to read, and that is true. Interspersed with that are lots of advertisements, useless information, and many pro-HD radio articles. Is it worth the price? I don’t know, I am still pondering that one.