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The unitless coefficient of Zorch

Zorch is a term used to describe an over voltage or over current condition that usually leads to catastrophic failure, e.g. the power supply was zorched by lightning. There is also a quality to radio signals that defy and exceed theoretical definitions for service contours or power density.  That is quality defined as:

Zorch (adj): The ability of an RF signal to be received in unlikely locations; outside of predicted service contour, in steel structures, underground facilities, tunnels, etc.

It brings to mind the saying, “antennas are not amplifiers and amplifiers are not antennas.”

ERI circularly polarized 2 bay antenna

ERI circularly polarized 2 bay antenna

During the earlier stages of FM broadcasting, there was a notion that costs could be reduced by increasing antenna gain and reducing transmitter size. While theoretically, ERP (Effective Radiated Power) is ERP, broadcasters soon learned that high gain antenna, low TPO (Transmitter Power Output) installations lacked building penetration and had other reception issues.  Realizing that there is a trade off between antenna bays, transmitter power output especially in difficult reception areas, a great debate occurred and continues on what the optimal system is.  The answer is, it depends on the receiving environment.

Where this technical detail can be really important is with lower powered FM stations; Class A and LPFMs to be exact.  They are already battling against bigger stations that have tens or even hundreds of times  more power.  Certainly an LP-100 station has it’s work cut out for it.  The choice of antenna is perhaps one of the most important technical decisions to be made.  Choosing the right balance of antenna type, antenna gain, antenna height and transmitter power output can greatly influence reception reliability and thus coverage area.

A good study of this quality can be had by looking at various LPFM installations:

Station ERP (watts) Antenna Type Antenna Gain (power) TPO (watts)* Coefficient of Zorch
100 1 bay vertical 0.92 127 0.1
100 1 bay circular 0.46 253 0.4
100 2 bay vertical full 1.98 58 0.15
100 2 bay vertical half 1.40 83 0.2
100 2 bay circular full 0.99 118 0.5
100 2 bay circular half 0.70 166 0.7
100 3 bay circular full 1.52 77 0.46
100 3 bay circular half 1.01 115 0.52

*Includes 100 feet of 1/2 inch foam transmission line, Andrew LDF4-50A, loss of 0.661 dB  at 100 MHz, or 0.859 power gain.

Stations should try to get the transmitting antenna as high up as permitted without reducing ERP.  In other words, the FCC allows 100 watts ERP with 98 feet Height Above Average Terrain (HAAT) radiation center in their current LPFM rules.  Being lower in height will reduce the coverage area.  Going over 98 feet HAAT will cause the station’s power to be reduced, which will lower the coefficient of zorch accordingly.  Therefore, getting as close to 98 feet HAAT, which is different than 98 feet above ground level in many places, will net the best performance.

If a singular polarization (horizontal or vertical) is desired, vertical polarization should be chosen, as most mobile reception is by a vertical whip antenna.  For best reception performance, a circularly polarized antenna will work best, as receiver antenna orientation will not effect the signal reception.  A circularly polarized antenna has better building penetration and multi-path characteristics.  The FM broadcast circularly polarized antenna in not a true circularly polarized antenna, it is actually unpolarized.

The use of a multi-bay antenna has the effect of focusing the RF radiation outward, perpendicular to the element stack, thus limiting the radiation directly up or down from the antenna.  This is more pronounced with one half wave spaced antennas, which may be an environmental consideration in heavily populated areas.

Thus, the best coefficient of zorch for an LPFM station would be a circularly polarized, 1/2 wave spaced, 2 bay antenna.  This antenna would have some gain over a single bay antenna, take up less room on a tower than a full wave spaced antenna, offer good RF protection performance for the general public living and working under the antenna, reduce wasted upward radiation and offer good building penetration for the ERP.  It would require a slightly larger transmitter and more electricity, but that trade off is well worth the effort.

Australian Made Broadcast Equipment

Somebody working to preserve a record of past work:

Some of these have familiar looking cabinets and tube arrangements. They all look like classics to me and it is good that they are being saved. I noticed at the end of the video there is a Harris MW10A. As for the RCA Ampliphase transmitters; I maintained a BTA5J in Harrisburg PA on 580 KHz. It was reliable enough, but I could never keep it sounding good for more than a couple of days.

In any case, a worthwhile effort.  More information at: AWA Transmitters.

Training up the younger set

Math Textbook

Math Textbook

Much ink has been spilled on the aging Broadcast Engineer. While it is generally true, many of the old RF engineers are getting older, there are some younger guys and girls entering into the broadcast technical field.  While this is a good development, I look on with a bit of disappointment and a jaundiced eye.  The newer broadcast engineers have fewer mentors around as there are fewer broadcast engineers.  In addition to that, those broadcast engineers that are still at it are likely very busy trying to fill all the rolls they have been assigned.  I have also noted a certain reluctance to impart information to the newer engineers.  Perhaps this is some sort of subconscious preservation instinct.   Thus, when a young guy that works with us admitted that he didn’t know that much about RF, I was not surprised.

I remember my first mentors in the broadcast engineering field.  They were mostly older, near retirement and wanting to pass on their knowledge to the next generation.  Several times, Don Porter would sit down at the work bench and draw out some basic schematic diagram on a broken piece of gear and let me try to fix it.  It took time and patience because I know I asked many silly questions and made many silly mistakes.  Sometimes he would chastise me and sometimes he would laugh and say “I did the same thing once,” which would lead to an interesting story.

Most of the younger people entering the broadcast engineering field (by younger, I mean less than thirty), have some type of computer background.  Since there are numerous computers in the studio fulfilling many different roles, having a technical computer person on staff is a good thing.  However, those people are often tasked with going to the transmitter site to do maintenance and trouble shooting.  That can lead to a dangerous situation.  Transmitter sites are and should remain the domain of well trained engineers.  Those that know the operating characteristics of a tube transmitter, if there is one present.  Those that know the basic principals behind and automatic transfer switch, if there is one present.  The real danger of an untrained person at a transmitter site is they don’t know what they don’t know.

Then there are trouble shooting skills, which are only developed with time and experience.  It takes experience to recognize that a tower crew has applied the wrong type of connector to an STL transmission line.  It takes experience to recognize the failure mode of a Harris transmitter.  It takes experience to know when a situation is too dangerous to proceed and wait for help.  Formal education is very important, but nothing can replace the education received on the job.  The field of broadcast engineering is so diverse and complex that it would be nearly impossible to learn everything in a classroom.

To be a well rounded Broadcast Engineer, one has to have knowledge in many areas:

  • Basic electronics and electricity:  Being able to read schematic diagrams, know what the components do and trace out signal paths.  Understanding basic RF amplification by solid state and tube devices, understanding TTL logic, data buses, power supplies, etc
  • Radio Frequency Principles:  Understanding the relationship between frequency and wavelength, antenna theory, antenna operation (MF, VHF, UHF), the relationship between power density and log functions, transmission line theory, propagation types and free space loss.
  • Audio engineering:  Best practices for analog and digital audio wiring, microphones, processing basic studio acoustics and sound, audio levels, analog and digital playback systems and recording.
  • Computers and IT: Computer networking, structure wiring, operating systems, servers, automation software.
  • Emergency Power: Basic functions and repairs for UPS, generators and transfer switches.
  • HVAC: Basic HVAC principals and operations.
  • Maintenance: How to maintain the facilities broadcast and broadcast related equipment.
  • FCC regulations:  Part 11, 15, 17, 73, 74, 101 and other FCC regulations pertaining to any broadcast operation.
  • Other regulations: OHSA, NEC, fire code, ADA, local zoning, etc

And that is simplified list.  Many Broadcast Engineers will gravitate toward one or two of the larger categories listed above, e.g. either Computers or RF.  Most will know something about both.

The SBE offers several on line courses and webinars with there Education Program.

In addition, many equipment manufactures offer courses, technical publications and white papers:

This is just a brief list, I am sure there are many others available on the internet.

Of course, nothing beats mentoring.  Taking an inexperienced, willing to learn person aside and showing them some of the things not taught in school or written in a manual is a rewarding experience.  There are still those that get bitten by the radio bug and are worth the effort to bring along.

Details

I found this small, yet very important detail on a DB-37 connector attached to the back of a Nautel V-1 transmitter:

DB-37 connector for Nautel V-1 transmitter

DB-37 connector for Nautel V-1 transmitter

The black wire is the ground wire and the orange wire is the remote RF off command.  A closer view:

DB 37 connector from Nautel V-1 transmitter

DB 37 connector from Nautel V-1 transmitter

The transmitter had been shutting down unexpectedly since it was installed.  When these shutdowns occurred, there was no overload, no fault, no power interruption or other indication of a problem.  When the RF on command was issued, the transmitter would turn back on and run with normal readings until it shut down again.  It was a bit of a mystery; the transmitter was removed from its mountain top home and hauled back to the shop to be repaired.  It was connected to all sorts of test equipment and studied intently for many days.  Still, the problem could not be replicated in the shop.

Then the transmitter was hauled back to its mountain top transmitter site and re-installed.  It ran well for about a month and then started going off again.  This time somebody looked at the event log and noticed that a “Remote RF off command” was being issued at the same time the transmitter would shut down.  Ahhh, the missing bit of critical data.  That prompted me to take apart the DB-37 connector used for the remote control interface.  The problem was obvious as soon as I removed the hood.

Sometimes the most valuable piece of test gear is the venerable Mk I, Mod 0, EYEBALL.

I unsoldered the ground lead and put some heat shrink over the connection to the DB-37.  Hopefully, that will take care of it.

Out in the Trenches

We fight for every scrap we can get. Sometimes it is not a fair fight. Sometimes the most frustrating thing can be the suits in the corner office.  We eat, drink and sleep RF.  I have transmitter dirt permanently embedding in my skin.  If a thunderstorm passes by, I get my shoes on.  Last time I was home during a blizzard, I was in high school, and believe me, that was a long time ago.  I’ve been hot, cold, soaking wet, dirty, dusty, hungry and dehydrated all in the same day.  Those days can be 8-36 hours long or longer.  240 volts AC is low voltage.  Arcs and sparks are a diagnostic tool.

I have clip leaded things, used non-standard parts to get a transmitter back on the air, employed fans on power supplies, filed, cut, bent, tightened, burnished relay contacts, put plate transformers up on a block of wood and crossed my fingers while turned the plate supply on.

My DVM looks like this:

Fluke 111 DVM

Fluke 111 DVM

But the best part is, when I walk into a radio station studio, the DJ says “THANK GOD YOU ARE HERE!”  I don’t drive a fancy car or wear a fancy suit, but the respect I get is there, even with the young too cool for school guys on the CHR station.

I am a broadcast engineer and I am here to fix your shit.

Now where is that BNC male to N female adaptor

Working with RF can produce some head scratchers.  Most transmitter manufactures tend to use the same type of connector for things like exciter RF outputs and composite inputs.  Over the years, I have become well stocked with all sorts of BNC and Type N connectors.  Satellite equipment uses Type F connectors, Analyzers use Type N, Oscilloscope uses BNC,  GPS equipment uses SMA and so on.  Except when they don’t.  As any good engineer will tell you, when they don’t will be in the middle of the night at some mountain top location while the station is off the air.

After one such incident, I invested in a TPI-3000A adaptor kit.  This kit has both the male and female versions of Type N, F, SMA, BNC, TNC, UHF, UHF mini and RCA.  They can be mated in any combination using the Universal interface.

TPI 3000A adaptor kit

TPI 3000A adaptor kit

On more than one occasion, this little kit has meant the difference of between being back on the air or driving down the mountain to look for an in between series adaptor.  A couple of recommended additions include a 7/8″ and 1 5/8″ EIA flange to type N male.

TPI-3000A inside

TPI-3000A inside

They can be a little pricey, however, I have seen several for sale on eBay for less than $100.00.  The key to not loosing the various little parts to this kit is to write a little note detailing the date and location where the adaptor was used,  then stuff it in the empty hole.  Hopefully, when permanent repairs are made, the adaptor will be retrieved.

Bench Work

Almost every broadcast engineer has to do some type of bench work. While I enjoy a certain amount of bench work, it is not my strong suit.  I suppose if I had to do it more often, I would become more proficient.  Truth be told, I would rather be at a transmitter site than sitting work chair studying schematic diagrams. It is becoming increasingly difficult to make repairs in the field due to surface mount components.  The company I work for has a repair and rework shop where almost anything can be repaired.  There is one bench tech, who is pretty proficient with power supplies and RF amplifiers among other things.  There is a complete set of test equipment including several Tektronix spectrum analyzers and oscilloscopes.

Likely the most versatile piece of equipment is the IFR 1500 service monitor.

IFR 1500 communications service monitor

IFR 1500 communications service monitor

The bench itself is fairly large:

Shop work bench

Shop work bench

There is also a good stock of spare equipment that can be rented out while repairs are being made:

Shop spare equipment

Shop spare equipment

Repair work includes by is not limited to:

  • RF repairs; Moseley STL systems, Marti STL and RPU systems, TFT STL systems, most exciters, IPA modules, etc
  • Transmitter repairs and retuning
  • Mechanical devices like transmission line dehydrators, transfer switches, etc
  • Switching and linear power supplies
  • Uninteruptable power supplies
  • Remote control equipment; Gentner VRC-2000, Burk ARC16, Moseley MRC-1600
  • Audio Processing; All Orban equipment, Symetrix, Valley, DBX
  • Audio equipment; Amplifiers, consoles, reel to reel machines, cassette decks, CD players, DAT machines, etc

I am sure there are many other things that I am leaving out.

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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