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Filtering for co-located FM transmitters

Well sited FM transmitter locations usually want some height above average terrain. This means either a tall tower or a high hill or mountain. Once a site is developed, co-location of other FM transmitters often happens because sites are expensive to develop. A second station can save money by using existing facilities.

For all those newly permitted LPFM stations; pay attention. If you are going to be co-located at an existing FM broadcast site, you may need to do this too.

Interference from intermodulation mixing products can develop when FM transmitting antennas are in close proximity.  This is especially true with solid state, broadband PA commonly used in today’s VHF FM transmitters.  Thus, when antennas are closely placed, external filtering is required.

WUPE FM transmitter site, North Adams, MA

WUPE FM transmitter site, North Adams, MA

This is the case with a current project in North Adams, Massachusetts.  New England Public Radio is placing WNNI on the air from the WUPE-FM site.  WNNI is using one of those new Harris (now GatesAir?) Flexiva transmitters and WUPE-FM uses a Crown FM-2000A.  The antennas are on separate towers, but the towers are in very close proximity, about 30 feet apart.  In order to avoid any possible problems, a Shively 2602-3A-FB 3 pole filter was installed on each station.  The filter is a band pass for the station installed on and a notch for the other station.

The primary concern here is mixing products between the two transmitters.  Both have broad band solid state amplifiers with low pass filters before the output connector.  There are three frequencies of interest;

  1. (F1 – F2) + F1 or (100.1 MHz – 98.9 MHz ) + 100.1 MHz = 101.3 MHz
  2. F2 – (F1 – F2) or 98.9 MHz – (100.1MHz  – 98.9MHz) = 97.3 MHz
  3. F2 + F1 or 100.1 MHz + 98.9 MHz = 199 MHz

That, plus harmonic measurements out to three or four harmonics of the fundamental frequency should be enough to demonstrate compliance with FCC out of band emissions standards.

Measurements on these frequencies must meet the emissions standards outlined in FCC 73.317 (d), which states:

Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

It is also noted that this site has several cellular carriers and no doubt has or will have LTE at some point. We all know that rural LTE installations can create self induced problems, which are then conveniently blamed on the nearest broadcast station because, hey, why not?

To further complicate matters, New England Public Radio also has a translator, W266AW (101.1 MHz) on the same tower as WNNI.  The same measurements noted above will have to be preformed again for the translator.

WNNI FM transmitter and Shively filter

WNNI FM transmitter and Shively filter

WNNI equipment rack.  This is one of those new Harris (GatesAir?) Flexiva FM transmitters.

WUPE-FM Shively Filter

WUPE-FM Shively Filter

WUPE FM filter installation

wave spaced Shively antenna.  Antenna for W266AW below

WNNI 4 bay half wave spaced Shively antenna. Antenna for W266AW below

New WNNI antenna mounted on cell tower next to WUPE-FM tower. The W266AW translator antenna is directly below WNNI’s main antenna.

WUPE-FM 3 bay half wave spaced Shively antenna

WUPE-FM 3 bay half wave spaced Shively antenna

WUPE-FM antenna installed on the original broadcast tower.  I believe the tower dates from 1959 or so.

It is important to get this type of installation right the first time.  Creating interference all around or above the FM band is never a good strategy.  Going back to ask for more funds to make something right is also highly frowned upon.

The Kintronic Isocoupler

Had a problem with this Kintronic FMC-0.1 isocoupler the other morning.

Kintronic FMC-1.0 STL ioscoupler

Kintronic FMC-1.0 STL isocoupler

After an overnight drenching heavy rain and very high wind, the STL transmitter associated with this unit was having high VSWR faults.  This isocoupler crosses a base insulator of an AM 50 KW directional antenna.  This particular tower has negative impedance, which is to say, it sucks power out of the pattern and feeds it back to the phasor. An interesting discussion for another time, perhaps.

Using a dummy load, we isolated the problem to the isocoupler by first connecting the load to the output on top of the unit (problem still exists) then to the transmission line prior the unit (problem went away).  Of course, the AM station had to be taken off the air to do this work.

Once the issue was confirmed as the isocoupler, I opened the unit up and found that water had entered and pooled in the top of the bottom half of the isolation transformer.

Kintronic isocoupler transformer

Kintronic isocoupler transformer

The isolation transformer consists of two loops to ground capacitively coupled through air dielectric. The issue is with the opening around the top of the unit, under the lip of metal lid. Apparently, this allowed water in.

Kintronic isocoupler isolation transformer

Kintronic isocoupler isolation transformer

It is difficult to tell with the lighting in this photograph, however, the bottom part of this isolation transformer has water pooled around the center insulator.  Using a rag, I cleaned out the water and dirt from the center insulator.  After reconnecting the antenna and transmitter transmission line, a quick check revealed the problem was much better, but still not completely gone.  I suspect water seeped further down into the bottom half of this unit.  The repair work was good enough, however, to return both stations to the air.

Glad to get that bit of work done while it was still relatively warm out.

Happy New Year

I wish everyone a Happy New Year and hopefully, a prosperous 2014.

Another year has gone by, and there were few things remarkable about it. Among those are:

From the digital radio front; HD Radio continues to be a non-factor in the bigger broadcasting picture.  FM HD Radio continues to make very small inroads, especially with public radio groups who’s HD Radio expenditures are mostly tax payer subsidized.  AM HD Radio continues to backslide slowly from it’s high water mark of 310 stations in 2007.  It is difficult to nail down the exact numbers of AM HD Radio broadcasters, however, Barry McLarnon notes that 177 stations are currently transmitting AM HD Radio.  No official numbers are available from either the FCC or iBiquity itself.

The great 2003 translator log jam (Auction 83) was finally fixed so that the FCC could move ahead with the LPFM application window in October.  In the end, some 1,240 translators were granted, with more conflicting applications still in the works.

The LPFM filing window opened in October amid the government shutdown.  Many groups were predicting 10,000 new applications for 100 watt LPFM licenses.  The actual number is closer to 2,800.  The final number of Construction Permits issued with likely be somewhat lower as defective and competing applications are dismissed.  This number seems low to some LPFM proponents.  When I approached a local interest group about launching a low power radio station, I was basically met with indifference.  With a very complex set of application guidelines and operating rules, plus very low power levels, it is not surprising at all.

The NAB and the FCC have been working diligently on revitalizing the AM broadcasting band.  Results of these efforts are yet undetermined as the proposal works it’s way through the regulatory process.  The so called “analog sunset” still lurks in the background somewhere, waiting to be trotted out at the most opportune moment.  I remain skeptical of the current proposal.

Cumulus Broadcasting purchases Dial Global and renames it West Wood One.  Some people lose their jobs.

Nielson buys Arbitron rating service and renames it Nielson Audio.  Some people lose their jobs.

Clear Channel tries to fly under the radar with “staff reductions.”  Some people lose their jobs.

Long time online radio forum “Radiodiscussions.com” ceased existence.  Starting out as Radio-info.com in the mid 1990’s, radio discussions was largest, longest running radio forum in the country.  It held tens of thousands of posts on almost every radio topic under the sun.  Unfortunately, it was bought and sold a few times over the last few years and the new owners could not figure out how to monetize it.  The end.

Bernie Wise passed away on December 13th.  This is truly unfortunate as Bernie was a character perfectly suited to the radio business.  He started working for RCA and is responsible for UHF television broadcasting in the US.

On the blog front, we continue to grow in page views and readers.  As of this date, Engineering Radio gets approximately 540 page views per day and has 227 RSS subscribers.  The split is 60/40 percent domestic/international readers.  The top five international traffic sources are; Canada, UK, India, Germany and Brazil.

2013 stat counter image

2013 stat counter image

There are some 634 articles with 2,640 legitimate comments and 429,600 spam comments.

Regarding site outages, there were 343 minutes of server down time.  Two DDOS attacks lasting six and three hours respectively and one incident of a corrupted .htaccess file rendered and error 500 message for six hours.  Total down time 1,243 minutes or 20:43 hours which gives a 99.87% availability for the website.  Not bad, but we can do better as the uptime goal is 99.99%.

On a personal note, my college studies are progressing well.  I have three more classes or 10 credit hours left until I am done.  My GPA is 3.90 which is not terrible considering I am working full time and going to school almost full time.  Truth be told, I cannot wait until it is finished.

Analog Sauce

A little blast from the past. This was found in a transmitter manual at one of the sites we take care of:

CCA Optomod 8000

CCA Optomod 8000

I thought I would scan it and make it available here.  As luck would have it, there is also a corresponding piece of equipment to go along with it.  I had never seen a “CCA Optomod” (.pdf) before I was working at one of the radio stations in Trenton, Florida.  This unit was rescued from under a pile of garbage out in the lawn shed.  It was full of mud was nests and mouse droppings.  Needless to say, it required a bit of TLC to return it to operation.  I replaced the electrolytics, cleaned it up and ran some audio through it.  It is probably as good as the day it left the factory.  Bob Orban made some really good stuff in his day.

CCA Optomod 8000

CCA Optomod 8000

The original Optomod 8000 was an evolutionary design that made FM radio processing what it is today.  The idea of combining broadband limiter, AGC and stereo generator in one box was a radical departure from the norm.  The audio limiter functioned as a 15 KHz low pass filter and broadband AGC.

Orban Optomod 8000 audio limiter block diagram

Orban Optomod 8000 audio limiter block diagram

The stereo generator used very modest amounts of composite clipping to reduce overshoot and transients. Many people disparage composite clippers. If done correctly, it is transparent to the listener and increases perceived loudness by stripping off modulation product that is non-productive.

Orban Optomod 8000 Stereo Generator block diagram

Orban Optomod 8000 Stereo Generator block diagram

Some thirty five or so years later, there are still many of these units in service in various stations around the world.

Somewhere, a community has lost its radio station

I am sure that this has happened in more places than one. WZAD 97.3 MHz was licensed around 1990 as part of the 80-90 drop ins.  The 80-90s were the beginning of the end for viable douopoly operations is smaller markets and triggered the huge wave of consolidation that began a few years later.  WZAD started out as a community oriented station, with a free-form format.  DJ’s often brought their own records to the studio and spun anything from classic rock to jazz to disco or whatever.  As such, the station never really caught on.  Listeners would tune in to hear their favorite Led Zeppelin song but here “Ernie’s Classic Polka Show” instead.

A few years later, the station was sold to somebody that changed formats to a satellite oldies format.

The station was sold again and again and again before finally ending up with a major consolidator.

There is a lesson there for all the would be LPFM applicants:  Nail down your programming ideas now, float ideas out among the community and see what will work and what people are interested in.

This is the WZAD studio now:

WZAD studio, Wurtsboro, NY

WZAD studio, Wurtsboro, NY

When was the last time anyone from the station was here or set foot anywhere near the community of license?  The front lobby of the studio is full of garbage and an old dot matrix printer.  It looks like there has been a leak and all the ceiling tiles have fallen down.

The station is being programmed out of the Poughkeepsie studio cluster with an automated country format called “The Wolf.”  There is a live morning show, or at least there used to be, I don’t know anymore.  How is this station serving as a public trustee?

Middle Aged Iron: Cetec/Bauer 701B transmitter

In service as a backup unit at WALL 1340 KHz in Middletown, NY:

WALL 1340 KHz, Middletown NY AM1A on air, 701B into test load

WALL 1340 KHz, Middletown NY AM1A on air, 701B into test load

I believe the Cetec transmitter is from the early 70s. I wouldn’t really call it old, we have much older units in the field that are still in backup service.  WALL itself has been on the air since 1942 from this site. The tower out back was replaced in the mid 90’s and is 147 degrees tall. It broadcasts the “True Oldies Channel” and is currently owned by Cumulus, soon to be Townsquare.

Cetec 701B tube deck.  4-500As.

Cetec 701B tube deck. 4-500As.

The site is also home to sister station WRRV (92.7 MHz) which has a side mounted antenna near the top of the WALL tower. We are currently reconnecting the CCA transmitter as the backup for WRRV. That unit is also from the early 1970’s.

WRRV WALL transmitter site, Middletown, NY

WRRV WALL transmitter site, Middletown, NY

WRRV WALL transmitting tower, Middletown, NY

WRRV WALL transmitting tower, Middletown, NY

Time to consult the transmitter wall

I saw this at the WIZN transmitter site in Charlotte, VT:

WIZN FM25K  transmitter log

WIZN FM25K transmitter log

Somebody went through quite a bit of trouble to chart the transmitter readings from April of 1987 through about February of 1992.

A closer view:

WIZN transmitter log

WIZN transmitter log

I have not seen this at any other transmitter site, so I though it was an interesting way to keep a transmitter log.  It also seems to be time consuming and a bit obsessive.  Over the years, I have found my fellow broadcast engineers to be a somewhat strange group sometimes.

Modifying the old Broadcast Electronics Transmitter

In the previous post, the issue with the WVOS-FM transmitter was detailed: The PA feed through/bypass capacitor had arced to the PA cavity causing lots of unwanted off air time. When I went to order the replacement parts, of course, they were not available. It seems that Broadcast Electronics changed the design of their transmitters in the late 1980’s to use a different feed through arrangement.

They were nice enough to send us a nifty retrofit kit; BE part number 959-0272 which replaces BE part number 959-0115.  If interested, the six pages of installation instructions are available here, for your reading pleasure.

The retrofitting itself was quite the job; drilling six mounting holes and one one inch feed through hole in the PA cavity, mounting the new feed through housing, rewiring the high voltage connection to the tube and back to the HV bleeder assembly, ect.  What with all of the drilling, sawing, filing, deburring and whatnot, I began to wonder if the transmitter would ever run again.  This is the transmitter before the modification:

Broadcast Electronics FM3.5A PA cavity

Broadcast Electronics FM3.5A PA cavity

This is the old high voltage feed through hole, arc mark clearly evident.

Broadcast Electronics FM3.5A old high voltage feed though

Broadcast Electronics FM3.5A old high voltage feed though

This is the modified feed through/bypass configuration.

Broadcast Electronics FM3.5A new PA feed through capacitor

Broadcast Electronics FM3.5A new PA feed through/bypass capacitor

While doing this work, I removed the tube and put a plastic sheet in the bottom of the PA cavity and around the HV parts in the bottom of the transmitter.  Somehow, getting aluminum filings in the tube socket seemed like a bad idea.  I also thoroughly vacuumed out the entire transmitter once all of the metal work was done.

I removed the Kapton capacitor plates from the old feed through arrangement and reinstalled the Teflon insulating plates to keep the air flow out of the tube cavity going in the correct direction.  The new capacitor looks very beefy, perhaps it will never fail again.

Once the installation work was done, I brought up the transmitter first with no screen and no connection to the tube anode.  Then with the tube connected, and finally with the screen supply turned on.  The tuning needed a brief touch up but all in all, the transmitter came up and ran well with the new feed through arrangement.

The old Broadcast Electronics Transmitter

Alternate title: More blown up stuff

This Broadcast Electronics Fm 3.5A will be thirty years old in April. We should have a party!

Broadcast Electronics FM 3.5A, WVOS-FM, LIberty NY

Broadcast Electronics FM 3.5A, WVOS-FM, LIberty NY

Unfortunately, this transmitter is not doing too well these days. The PA high voltage feed through capacitor has arced over to the PA cavity, causing the station to be off the air.

BE FM3.5A HV feedthru capacitor

BE FM3.5A HV feed thru capacitor

Naturally, this happened over the weekend, parts will not arrive until Tuesday at the earliest, and the station is without a backup transmitter.

Obviously, trouble shooting this was a two person job.  Never work alone on HV equipment.  The symptom was the main circuit breaker was tripping after the HV on command.  Starting from the transformer end of the HV power supply circuit and working toward the anode of the PA tube, all of the components were tested by isolating each component then turning the HV on.   Special care was taken to discharge all components after each test.  The capacitors and bleeder resistors were reconnected at the same time.  There is too much risk involved with charged 8 KV capacitors and no way to bleed that charge to ground.  Everything worked up until the PA cavity was reconnected (without the tube), then the breaker tripped again.  Thus, the above feed through capacitor was removed and disassembled, revealing the damage.

The question is, how long should transmitting equipment last? After all, if one were running a freight delivery company, you would not be driving around in thirty year old trucks, would you? No, not if you wanted to stay in business. Like all electro-mechanical equipment; transmitters, consoles, STLs, antennas, computers, etc wear out.  A smart plan would be to have a replacement schedule and be putting money into a capital equipment replacement fund.   Equipment life varies with the type.  Getting twenty years out of a main transmitter is pretty good service life, going beyond that is pressing one’s luck.  Ten years on any one computer is a very long time.  Then there are certain transmitter manufactures that drop support on older units, which makes it difficult to keep them operating.  Owners and managers need to be cognizant of the age and condition of critical infrastructure.  As field engineers, how much time do we devote to keeping antiquated equipment running or should we even be servicing it at all?  As independent contractors, we incur a liability whenever we touch something.  Where does the ownership’s responsibility lay in providing safe, functional equipment for their stations?  All interesting questions.

WDST, Woodstock, New York

WDST is a well known radio station in Woodstock, NY. Formatically, I would call it Adult Album Alternative (AAA) and it is one of my favorite stations to listen to. We also do the engineering work for this station.  While I was there last week, I snapped a few pictures of the studios:

WDST air studio, Woodstock, NY

WDST air studio, Woodstock, NY

All of the studio use Audioarts R-60 consoles, which are in good condition considering their age.  Lots of guest microphones and the windows look out into a performance venue.

WDST music library, located in hallway outside of studio

WDST music library, located in hallway outside of studio

The music library is extensive.

WDST main production room

WDST main production room

The production room, another R-60 console. I don’t know where the microphone disappeared to, perhaps it was borrowed by the morning show.

WDST technical operation center

WDST technical operation center

Technical Operation Center (TOC). WDST uses NextGen from RCS for music storage, playback and automation.  Other equipment includes ISDN, POTS phone, Distribution Amps, Limiters, streaming computer, STL, etc.

WDST transmitter, Broadcast Electronics FM5C

WDST transmitter, Broadcast Electronics FM5C

The transmitter site is on Hallihan hill, across the street from the old ATT long lines site. The station uses a Broadcast Electronics FM5C transmitter.

WDST forward power meter

WDST forward power meter

Forward power, almost five whole kilowatts of flame throwing power.

WDST antenna, Hallihan Hill, Kingston, NY

WDST antenna, Hallihan Hill, Kingston, NY

The antenna is a Shively 6810 2 bay half wave spaced.

Axiom


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
~1st amendment to the United States Constitution

Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
~Benjamin Franklin

The individual has always had to struggle to keep from being overwhelmed by the tribe. To be your own man is hard business. If you try it, you will be lonely often, and sometimes frightened. But no price is too high to pay for the privilege of owning yourself.
~Rudyard Kipling

Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~Universal Declaration Of Human Rights, Article 19

...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner

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