Engineering Radio

Why stealing is bad

Paul Thurst — Sat, 04 Feb 2012 02:08:53 +0000

Eventually, you will get caught, odds dictate. The local engineer for Cumulus Broadcasting in Cincinnati found this out earlier in the week. Of course, innocent until proven guilty, so I won’t assume anything.

Broadcast engineering, especially radio engineering is a small field. Sadly, when something like this happens it makes all radio engineers look bad and there is no good reason or excuse for it.

I have seen several cases where an engineer or technical person has taken advantage of their position to pilfer from a radio station. These vary from cashing in on dud tubes from a transmitters site to taking high value equipment and selling it on eBay. I recall on recent instance of backup transmitter and STL systems being sold. I cannot imagine what these people are thinking. A transmitter, STL system, console or even a dud tube has a serial number and is traceable. Anything with a serial number is likely part of a station inventory list and or will have some record of manufacture and sale.

There are instances when old equipment is getting thrown out. In that situation, I always get permission before removing anything, even from the dumpster.

I have made several trips to the scrap yard with old transmitter chassis, wire or left overs from transmitter installations. In those circumstances, I always get a receipt and write the source of the scrap on the back. This way, a record is kept and if there is any questions, I can refer to it.

Generally speaking, it is better to be overly cautious.

The Gates BC250GY transmitter

Paul Thurst — Wed, 01 Feb 2012 12:06:06 +0000

This transmitter is in service at WSBS, Great Barrington, MA as a standby. It was new in January of 1975.

Gates BC250GY transmitter, WSBS Great Barrington, MA

This was running into the dummy load for testing, which we try to do periodically.

Gates BC250GY AM transmitter audio section

The audio section is a pair of ~~8008′s~~ 810′s running in parallel. This goes through a modulation transformer to the RF section.

Gates BC250GY RF section

The RF section consists of another pair of ~~8008′s~~ 810′s running a parallel. The plate voltage for these tubes is 1,250 VDC which is fairly tame, all things considered. The transmitter is dirt simple 250 watt carrier power, 125% positive peak capable. It is not the most efficient unit under the sun but it can still be repaired with off the self parts.

Gates BC250GY Schematic

This is a somewhat faded schematic. The schematic shows a single 833A as the final, however, this particular transmitter has a pair of 810′s for the final, as shown in the above picture. Ham radio operators love these things as they are easy to convert to 160 or 80 meters for AM phone use. The bigger brother to this unit is the Gates BC1G, which is also pretty simple unit using 833A tubes in parallel with 3,500 VDC plate voltage.

How much is prevention worth?

Paul Thurst — Mon, 30 Jan 2012 11:53:31 +0000

I sometimes get the distinct impression that the corner office doesn’t understand what it takes to keep a radio station on the air and in good repair. It is most often the problems or “issues” that tend to get the most attention. The things that are working well tend to get ignored. After all, how often do you hear a news report about the airliner that landed safely.

Lightning strike, TV tower

When lightning strikes the tower and knocks the transmitter off the air causing major damage and expensive repairs, that is a problem. When lightning strikes the tower and nothing happens, no problem. What is the difference between those two situations?

Grounding strap, FM transmitter site

If the generator starts and runs during every power outage and has done so for the last five years straight, it is obviously a reliable unit, does it need all that maintenance?

Caterpillar 75 KW diesel GENSET

Money spent on preventing undesirable outcomes can be difficult to quantify as disasters and events that do not happen are ill defined. It is difficult to quantify the “amount saved” on something that didn’t or won’t occur. Using past situations is good start, but that only covers a fraction of possible outcomes. In order to invest money wisely, one has to look at the probabilities. If there is an unlimited budget, then the probability exercise should be minimal, however, there is very seldom an unlimited budget.

For example, how much does a back up STL system cost vs the risk of being off the air while the main STL system is being repaired? How often do failures occur, when are they likely to occur and for how long are all good questions. Is there an alternative to a full back up like an IP CODEC? Such a solution would cover all aspects of the STL system including antennas, transmission line, transmitters and receivers.

There are certain FM stations north of here that have neither RADOMES or antenna heaters. Once every two years or so, the antenna ices up and the transmitter folds back due to VSWR. How much of an impact to listeners notice when this happens? If it happened more often, say two to three times a year, would it be wise to invest in some type of deicing equipment?

What is the ownership and management opinion on off air conditions? I have often heard tell “Oh, its only the AM, we don’t mind if it goes off the air.” That is, until it actually goes off the air, then it is a big problem.

Based on my and others experiences, these are the things that will happen at an average transmitter site:

The electric will go off at least once per year for several hours.
The main transmitter will fail at least once every two years.
Lightning will strike the tower at least once per year.
The STL system will fail, at unknown intervals.

At studio sites, these things will occur:

The file server will crash depending on the operating system
The telephone lines and or T-1 service, internet service, ISDN etc will go out
The electric power will go out for several hours
The satellite dish will fail once every two to three years
If there is a tower, it will get struck by lightning

Other site specific things can occur like floods, blizzards, earthquakes, fire, etc.

Money spent on backup systems for those items is good insurance. Not only will the station stay on the air, the on call engineer’s phone will ring less often, which, if you are the on call engineer, should make you happy.

If a full backup is not available, a second transmitter for example, having a good stock of spare parts on hand can mean the difference between an early evening and an all nighter. Keeping good maintenance logs and well documented repair records can point out trends and give a good basis for ordering spare parts.

Repair trends are important. If the same part seems to be going bad over and over, it is time to dig deeper and find the cause of failure.

The old adage “An once of prevention is worth a pound of cure,” still holds true.

Work Ethic

Paul Thurst — Sat, 28 Jan 2012 13:45:32 +0000

We have this guy that works for us who is atypical. We call him Pete because that is his name. The other day, he was slacking off on the job again, this time figuring out how to take a nap in a transmitter:

Pete working on a Harris FM25K

What are we going to do with him?

Actually, he is rebuilding the grid tuning section (AKA input tuning section), which is no small matter. Soon, we will have this 26 year old transmitter running good as new, or better than new. It already sounds much better on the air than it did before, the input tuning is broader and there is much less AM noise.

Currently, it is running about 70% power while we wait for a replacement amplifier from Silicon Valley Power Amps.

The Harris MW1A

Paul Thurst — Wed, 25 Jan 2012 21:41:35 +0000

They say the first thirty years are the hardest, perhaps it is true. This Harris MW1A transmitter turns 31 this year:

Harris MW1A AM transmitter, WINE, Brookfield, CT

It is on the air as the main transmitter for WINE-AM in Brookfield, CT. These are not necessarily bad transmitters, although they do seem to require regular infusions of MOSFETS to stay at full power. This is Harris’s first solid state AM transmitter design, based on the work of Himmler Swanson. This is not a PDM transmitter, rather, each module has RF MOSFETS and audio transistors. The output of all twelve modules are combined for a carrier output of 1,ooo watts with +125% modulation. Thus, I would call it low level AM modulation.

This is also the only transmitter that I know of where blown fuses can cause damage to the RF devices.

The RF output transistors and audio transistors are still available from Harris. Non-OEM parts for this include the 2N5038 for the RF MOSFETS and the MJ15011 for the audio transistor. Inside the front of the transmitter is a row if incandescent light bulbs that glow increasingly as the various MOSFETS go bad. At 1,000 watts carrier power, the ratio of PA volts to PA amps is 52.5/22.5 respectively. If that ratio is off by any measure, there is a problem.

The other thing with this transmitter is it is very sensitive to any kind of VSWR. Any change in the output impedance will quickly make itself apparent. My Harrisburg MW1A had two ATU settings, one for winter and one for summer. It was a slightly tall tower on 1230 KHz, thus any change in the ground system (e.g. snow cover) would upset the tower base impedance.

The other thing that goes bad is the large Rotron fan in the bottom of the cabinet. They go bad about every 10-15 years or so.

The owner has spent some money on this particular unit, rebuilding and replacing several modules with new MOSFETS etc. Will it last another thirty years? Depends on if the RF and audio devices remain in production.

The never ending HD radio debacle continues to not end

Paul Thurst — Sat, 21 Jan 2012 16:02:21 +0000

Especially on the AM band.

Radio World, bless them, has yet another article about the public’s lack of awareness regarding HD Radio®. Calling it “lack of awareness,” is overly kind and I think they are missing the point. It would be better phrased “apathy” or “indifference.”

There is a general misconception in the world that one either loves or hates something. That is not true, the opposite of love is indifference, not hate. The public has voted, with their wallets, for things like 3 and 4G wireless devices, satellite radio, iPods and other entertainment venues. Why? Because HD Radio® is not an advance, it is a repackaging of old ideas with slick marketing. The general public has viewed the great digital radio conversion with a jaundiced eye, opting to sit on the fence and wait for something better. What has iBiquity given them?

The technology itself is a step backwards with many band aids needed to affect the same coverage area as analog FM. A technology that has poorer building penetration, less coverage area , mobile reception issues with no appreciable difference in sound quality or program material offerings. A power increase from 1% to 10% analog carrier power (20dBc to -10dBc) hasn’t really made a difference. Now, studies are underway looking at asymmetrical sidebands and same frequency repeater networks for FM IBOC. All of these things, not to improve radio reception, but rather to achieve the same coverage as analog FM.

The AM HD Radio® has even greater issues.

There is nothing at all surprising about the public indifference toward HD Radio®.

SOPA/PIPA protest

Paul Thurst — Thu, 19 Jan 2012 14:08:56 +0000

Yesterday, January 18, 2012, I blacked out engineeringradio.us for the day in protest of the internet censorship bill working its way through congress colloquially known as SOPA or PIPA. There were some 17,000 or more others that did the same.

SOPA PIPA protest screen shot

If the internet is indeed the new media, destined to replace the old media, then having in place draconian restrictions that allow the government to block websites and content with no due process for the website owner is censorship, plain and simple. Imagine a country where the government can come in and shut down any newspaper, TV station or Radio station, give no reason other than some weak statement about copy write laws. See also: China, North Korea, Cuba, Soviet Russia, etc.

It is important to check the corporate power in this country. It is widely reported that Congress has a 9% approval rating. It is also hard to imagine their approval rating is actually that high. While signing petitions and writing senators and congressman may provide some relief, the shortest path to ending this is to boycott the corporate sponsors of the legislation. Hitting companies bottom line will speak louder than any internet protest, petition, letter writing campaign, etc. Thus, if so inclined, here is a list of producer companies that like the idea of internet censorship.

WRKI WINE transmitter move, update 2

Paul Thurst — Thu, 19 Jan 2012 02:22:16 +0000

Man, this is taking longer than I though it would. We moved the Harris FM25K last week, all went well. The only hangup, as you can see, is the harmonic filter and the height of the racks next to the transmitter. The transmitter had to go on a 4×4 to get the filter up over the racks. The output from the transmitter to the harmonic filter cannot be changed in any way, shape or form (e.g. adding a little bit of line section to the top of the transmitter), else the transmitter will not run. So, up on 4×4′s it is.

WRKI WINE transmitter room

There we were, all ready to turn the transmitter on. Press the high voltage on button, lots of volts but no current and no power output. Seems something is wrong with the outboard IPA driver (over in the bottom of the rack, that thing pulled out with the manual on it).

The IPA is a Silicon Valley Power Amplifier 500 watt unit, which replaced the internal IPA driver about ten years ago. The tube in the Harris FM25K needs at least 390 watts to drive the transmitter to full power. Unfortunately, this particular amplifier was not in the best environments prior to the recent move. It was sitting in an unconditioned building on top of the backup transmitter in high heat and humidity. According to the manufacture, such abuse is bound to take it’s toll sooner or later. The later being, of course, the night we want to turn the thing back on and go home.

Time to drop back and punt. I found an old RVR 250 watt amp at a sister station nearby, which was also in pretty bad shape but repairable. That unit was pressed into service temporarily and with 200 watts drive, the old 25K put out about 11 KW. We need to affect permanent repairs to the RVR power amp before we place into temporary service. I don’t want any 2 am phone calls. The Silicon Valley Power Amp needs to have the amplifier module sent back to the manufacturer and rebuilt. They will refurbish the entire thing for something like $900.00 plus shipping. Considering what it does, that is worth it.

This is a little short cellphone video of the turn on at half power. This is a very loud transmitter, as such, I think the audio is a little distorted.

When this beast gets up to full power, I will update this, again.

Low Pass Filter design

Paul Thurst — Sat, 14 Jan 2012 15:56:54 +0000

Every good transmitter, tube transmitters in particular, require harmonic filtering. The last thing any good engineer or broadcaster wants is to cause interference, especially out of band interference to public safety or aviation frequencies. All modern transmitters are required to have spurious emissions attenuated by 80 dB or greater >75 Khz from carrier frequency. In reality, 80 dB is still quite high these days, especially in the VHF/UHF band, where receivers are much more sensitive than they used to be. A good receiver noise floor can be -110 dB depending on local conditions.

The principle behind a low pass filter is pretty easy to understand. The desired frequency is passed to the antenna, while anything above the cut off frequency is restricted and shunted to ground via a capacitor.

Low pass RC filter

In this case, the resistor is actually an inductor with high reactance above the cut off frequency. Often, these filters are lumped together to give better performance. This is a picture of an RVR three stage low pass filter:

RVR three stage low pass filter

RVR is an Italian transmitter maker that sells many transmitters and exciters in this country under names like Bext, Armstrong, etc. The inductors are obvious, the capacitors consist of a copper strip sandwiched between teflon insulators held down by the dividers in between the inductors.

Schematically, it looks like this:

Low pass filter schematic diagram

For the FM broadcast band, a good design cutoff frequency would be about 160 MHz. This will give the filter a steep skirt at the first possible harmonic frequency of 176 MHz (88.1 x 2 = 176.2).

Values for components:

Capacitors	Value	Inductors	Value
C1	20 pf	L1	74.7 nf
C2	54 pf	L2	75.1 nf
C3	54 pf	L3	73.9 nf
C4	20 pf

The inductors are wire, or in this case copper strap, with an air core. It is important to keep the transmitter power output in mind when designing and building these things. Higher carrier powers require greater spacing between coil windings and larger coil diameters. This particular filter is rated for 1 KW at 100 MHz.

Update: WINE WRKI transmitter site move

Paul Thurst — Fri, 06 Jan 2012 20:07:18 +0000

I have been spending my days in Brookfield, Connecticut, dragging transmitters around and reconnecting them in various ways. The WRKI-FM WINE-AM transmitter site is finally moving into the “new” transmitter building at the base of the tower. Today, we moved WINE.

WINE was first signed on in 1963 on 940 KHz from a 170 degree non-directional tower on top of a pretty high hill. That same tower serves as the antenna support for WRKI, which signed on in 1957. The station runs 680 watts daytime, however since it is non-directional, it has some pretty serious power reductions at night. The post sun set power drops in two steps, 450 watts for the first hour, then 189 watts for the second hour, followed by 4 watts night time.

The 4 watt night time signal goes about 2-4 miles before it becomes unlistenable. The Post Sun Set Authority (PSSA) allows the station to stay on the air with at least some coverage up to about 6:46 pm in the winter time and 10 pm in the summer, which is better than nothing.

The problem is, the Harris MW-1A transmitter goes down to 250 watts and no lower. In order to make the night time power, the station switches to a dissipation network to burn off 246 watts of RF, at 50% percent AC-RF efficiency, which just ends up being a waste of power. Further, the station engineers have been ignoring the PSSA because there are too many steps and it was easier to just switch to night power at sunset.

What we decided to do instead, was install a small low power night time transmitter, a Radio Systems TR-6000. The MW1A can then be set to use the low power level for the first step of the PSSA, then switch the dissipation network in for the second step of the PSSA, finally switching in the night transmitter at the proper time.

Harris MW1A AM transmitter, WINE 940 KHz, Brookfield, Ct

This is the Harris transmitter, new Circa 1981, was cleaned up and moved into the new transmitter building.

WINE Parallel dissipation network and dummy load

The dissipation network. This will have to be reconfigured for the proper power levels, once the night transmitter is installed. The dissipation network is on the right, a dummy load is on the left. The two large RF contactors switch the dissipation network in and out, or select which transmitter is feeding the antenna/dummy load. This is the really, really old school way of doing it. Most transmitters manufactured after 1990 or so can run at any power level, making a dissipation network unnecessary.

Before re-installing the dissipation network/dummy load, we lined the enclosure with copper mesh. I don’t want that thing interfering with any of the other equipment nearby, which would be the STL receivers, satellite receivers or Town of Brookfield police dispatch radios.

Schematically, it looks like this:

WINE 940 KHz Brookfield, CT night time dissipation network

This is the picture behind the transmitters, shows the coaxial cable feed through ports and the dissipation network on the wall.

WINE WRKI transmitter room, behind the transmitters

It is a work in progress, so forgive the mess.