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.