Engineering Radio – Page 177 – One Hundred percent Human Generated Content for your Reading Enjoyment

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.

FCC authority to conduct warrentless searches of Private Property

I was this interesting tidbit on the Radio World website the other day. The question is, how much authority does the FCC have to conduct a search of a private residence? The Electronic Frontier Foundation wanted to know therefore they sent a FOIA request to the FCC seeking documents supporting this claimed authority.

The documents received seem to be redacted and some are mostly blank, such as the training module on how to obtain permission to enter private property is supposed to take 6 hours to complete, but consists of 3 paragraphs and 2 questions. Hopefully, that is redacted and does not reflect on the quality of agents the FCC is employing these days. The upshot seems to be the agent either needs a warrant or permission.

It may be surprising to some citizens, however, the FCC does have the authority to investigate radio signals, whether they are intentionally generated, as in a pirate broadcaster, or unintentionally generated, as in a piece of gear gone bad.

According to federal regulations, an FCC agent may request entry to inspect a private building anytime he/she believes there may be a device emitting radio frequency energy. This includes anything with an FCC part 15 sticker, which can be computers, TV remote controls, garage door openers, WiFi network routers, etc. This basically covers every house in the US as well as most businesses. Those rules were written when most homes and businesses did not have any RF generating devices and there was little to indicate that they ever would.

The consequence of failure to allow an FCC field agent into a residence or business appears to be the issuance of a citation in the form of a threatening letter. Continued intransigence would be met with a NAL (Notice of Apparent Liability) followed by a forfeiture notice also known as a fine. The typical FCC fine these days seems to be $10,000.00. If it is a repeated and willful violation, the equipment can be ceased and the perpetrator arrested.

In instances where the safety of life is in question, then every step necessary to disable the offending device needs to be taken. Things like transmitters spurring into aircraft frequencies or TV antenna amplifiers running wide open, also interfering with aircraft frequencies come to mind.

One of the examples given details a field agent trying to track down a noisy cable TV amplifier. From the FCC field agent’s perspective, the homeowner appears to be a royal pain in the ass. In this day and age when everything has a computer and most of them generate RF, tracking down interference can be a painstaking process, especially where housing is dense. Uncooperative homeowners, especially dumb ones, who have no idea what they have plugged in only make things worse.

Still, there is the issue of fourth amendment rights, which, if the above law was misinterpreted, misused, or applied with the wrong standard would likely be trampled. In these days of extra-constitutional activity, giving Los Federales entry into one’s house might invite unwanted scrutiny by other agencies. As far as changing the rules, with the current group of scoundrels and rouges in the legislative branch, one might end up with something ten times worse than before.

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.