tech stuff – Page 61 – Engineering Radio

Filament Voltage Management

There are still many hollow state (AKA tube type) transmitters floating around out there in the broadcast world. High power, especially high power FM transmitters are often tube types and there are many good attributes to a tube transmitter. They are rugged, efficient and many of the well-designed tube units can last 20-25 years if well maintained.

The downside of a tube transmitter is tube replacement. Ceramic tubes, like a 4CX20,000 or 4CX35,000C cost $6-9K depending on manufacture. A well-maintained tube and last 3-4 years, I have had some lasting 8 years or more. My personal record was for a 4CX35,000C that was a final PA tube in a Harris MW50A transmitter. The tube was made by EEV (English Electrical Valve, now known as E2V) and lasted approximately 84,000 hours, which is 9.58 years. When it finally came out of service it looked like it had been through a fire, the entire metal plate body was dark blue. I took it out because the power was beginning to drop a little and it was making me nervous.

This was not an accident, I did it by maintaining the filament voltage and keeping the tube and transmitter clean. The tube filament supplies the raw material for signal amplification. Basically, the filament boils off electrons, which are then accelerated at various rates and intensities toward the plate by various control grids. The plate then collects the amplified signal and couples it to the rest of the transmitter. When a tube goes “soft,” it has used up its filament.

I had a long conversation about this one day with Fred Riley, from Continental Electronics, likely the best transmitter engineer I have ever known. At the time, the consensus was to lower the tube filament voltage by no more than 10%. On the 4CX35,000C, the specified filament voltage is 10 volts, therefore, making it 9 volts was the standard procedure. What Fred recommended was to find the performance “knee,” in other words, where the power began to drop off as the filament voltage is lowered. Once that was determined, set the voltage 1/10 of a volt higher. I ended up running that EEV tube at 8.6 volts, which was as low as the MW50’s filament rheostat would go.

The other important thing about tubes is the break-in period. When installing a new tube, it is important to run only the filament voltage for an hour or two before turning on the plate voltage. This will allow the getter to degas the tube. New tubes should be run at full filament voltage for about 100 hours or so before the voltage is reduced.

Tube changing procedure:

Remove power from transmitter, discharge all power supply caps to ground, hang the ground stick on the HV power supply.
Remove the tube, and follow manufacturer’s procedures. Most ceramic tubes come straight up out of their sockets (no twisting).
Inspect socket for dirt and broken finger stock. Clean as needed. Finger stock, particularly in the grid section, is important for transferring RF. Broken fingers can lead to spurs and other bad things
Insert new tube, follow manufacturer’s recommendations. Ceramic tubes usually go straight down, no twisting.
Make all connections, remove grounding stick, half tap plate voltage supply if possible, close up transmitter
Turn on filaments and set voltage for manufacturers’ recommended setting. Wait at least 90 minutes, preferably longer.
Turn on plate voltage and tune transmitter. Tune grid for maximum current and or minimum reflected power in the IPA. PA tuning should see a marked dip in the PA current. Tune for dip, then load for maximum power.
Turn off transmitter, retap plate supply for full voltage
Turn on transmitter and plate supply, retune for best forward power/efficiency ratio.
After the 100-hour mark, reduce filament voltage to 1/10 volt above performance knee.

Of course, every transmitter is slightly different. There may not be a dip in the plate current if the transmitter is running near its name plate rating, in which case one would tune for maximum forward power.

This system works well, currently one of the radio stations we contract for has a BE FM20T with a 4CX15,000A that has 9 years on it, still going strong.

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.

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.

Wire terminations

Radio studios involve quite a bit of wiring. Runs between the console and equipment are pretty straightforward, from whatever the connector required for the equipment to whatever the connector required for the console. When it comes to trunk runs between the rack room and the studio, however, some type of terminating block is required.

alt= — 66 block or M block insulation displacement wire termination

This particular cabling installation is for low-level signaling, contact closures, and the like. It uses a Belden cable with 37 un-twisted wires which do not follow the standard Western Electric color code. The color code can be found here. If it were audio or data, the wires would be terminated differently. That color code can be found here. For more information on color codes and pinouts, see this post.

Many engineers use the venerable 66 block or M block insulation displacement termination. These terminal blocks were designed by ATT to terminate 25 pair 22 through 26 gauge solid wire. The original design was rated for category 3 (16 MHz or 10 mb/s) communications standards. Newer designs are category 5 or 5e compliant (350 MHz or 100 mb/s). Notice the part about the solid wire. Most audio wire is stranded and as such, the metal fingers on a 66 block will cause stranded wire to spread out losing contact with the terminating finger. This causes intermittent connections and audio dropouts, which I have experienced often (before I knew better, I used 66 blocks when building studios). The way to cure audio dropouts on a 66 block is to heat the termination fingers with a soldering iron. This melts the wire insulation and gets it out of the way. In the long run, it is better to use more suitable terminations.

Krone LSA-PLUS 110 type wire termination block

The 110 block is an updated version of the punch block for high-speed networks. it is also designed for 22 through 26 gauge solid wire. This is the termination used on category 5, 5e, 6 patch panels and RJ-45 jacks. They are also formed into block-type terminations the size of small 66 blocks. The 110 block is designed for 500 MHz (1 gb/s) or greater bandwidth. Krone makes a version of a 110 block called LSA-PLUS which is an acronym that stands for: Lötfrei, Schraubfrei, Abisolierfrei, Preiswert, Leicht zu handhaben, Universell anwendbar, Sicher und schnell. This translates to: no solder, no use of screws, no insulation removal, cost-effective, easy to use, universal application, secure and fast. Unlike a standard 110 block, the Krone block is designed for solid or stranded wire. 110 blocks are acceptable for use with AES/EBU digital audio at sample rates greater than 268 KHz as well as gigabit networks and analog audio.

In very old installations, I have seen Christmas trees. This is a wire wrap system where wires are wrapped around metal fingers that form the shape of a pine tree, hence the name. They were very popular in the fifties and sixties and only work with solid wire. It is also time-consuming work and requires special tools and skills. Wire wrapping is a bit of a lost art.

Christmas Tree wire wrap termination block

Screw barrier strips have been used to terminate audio cables from time to time. I wouldn’t consider this method because it is too time-consuming, takes up too much space, and is difficult to label.

ADC ICON wire termination block

ADC makes a good termination block called ICON (Integrated Cable Organization Network) which uses QCP (Quick Connect Panel) connectors. the connectors are small square devices that are insulation displacement termination (like 66 and 110 blocks) but require a special tool to “punch down.” This particular type of connector is well suited for stranded wire from 22 through 26 AWG. QCP connectors are also used on some of ADC’s patch panels and other audio products. Like any other termination technology, they are only as good as the person punching down the wires. QCP connections are small high-density devices, I have seen them get mangled by someone in a hurry who got his punch-down tool across two of the terminals by accident. ICON blocks can be used for digital audio, however, they do not maintain the 110 ohms impedance of most digital-type audio cables (neither do XLR connectors, by the way). This can lead to some return loss, which on longer cable runs can cause problems.

Radio systems prefer RJ-45 connectors with Category 5 cable, something they call Studio Hub. These are 110 blocks as noted above, but designed primarily for computer networks. Radio Systems discovered that the impedance of most audio cables is very close to that of computer network cables, audio cable is designed for 110-ohm impedance vs. computer network cable which is designed for 100-ohm impedance. Therefore, RJ-45 connectors and shielded or unshielded twisted pair work well with balanced professional audio, either analog or digital.

For analog audio wires, ICON blocks seem to be the best, most secure high-density termination system. In all my years of using them, I have never had a connection go bad. 110 block and other category 5 or 5e systems also work well. For digital audio, Krone blocks or 110 blocks need to be used in order to maintain the full bandwidth characteristics of the cable being used. Using appropriate cable and or terminations in digital audio circuits often leads to impedance mismatches and high return losses in the system.