Emergency transmitter replacement

Bad weather or other disasters can strike any time of year.  Around these parts, the most dangerous weather events occur from early spring through late summer.  In the past twenty years or so, we have had tornadoes, hurricanes, micro bursts, flooding events and so on.  All of that got me thinking about what would happen if a tower came down, or a transmitter building was destroyed by fire, wind, water, etc.

If past events can predict future performance, there would ensue a mad scramble to replace damaged equipment and or get some type of temporary antenna into service.  That is what happened in great City of North Adams, Massachusetts when the tower that held the cell carriers, the 911 dispatch, and the local FM radio station came down in an ice storm.  Fortunately, we had a single bay Shively antenna at the shop that we trimmed up and installed on a temporary pole with 200 watts TPO.

That will cover the city of license, provided there is electricity…

What if there where an event that was so devastating that the electrical power would not be restored for months?  Think about hurricane Maria in Puerto Rico.   After that event, the infrastructure was so devastated that there was not even the possibility of getting a fuel truck to deliver diesel for the emergency generators at the hospital in San Juan.  It can happen.

With that in mind, I began poking around and thinking about how I would get something back on the air.  In the face of massive disasters, AM and FM radio is still the most effective way to communicate with the general public.  Radios are still ubiquitous in homes, cars and businesses.

Bext 30 watt exciter
Bext 30 Watt FM exciter

In a short period of time I came up with a couple of solutions.  First, the frequency agile Bext exciter uses a single solid state rectifier feeding 24 volts to the power supply board.  The audio input includes a mono balanced line level input which can be fed by a computer sound card or some other simple source.

Bext 30 Watt FM exciter power supply
Bext 30 Watt FM exciter power supply

From there +12, +15 and +20 VDC are created to run various circuits.  The heat sink cooling fan is the only thing that runs on 120 VAC, which is old and I might replace with a 24 VDC unit.

Bext 30 Watt exciter power supply voltage
Bext 30 Watt exciter power supply voltage

The power output is about 22 watts, which is not bad.  That will certainly get out well enough from a high spot and provide good coverage when the power is out because all the other in band RF noise generators will be off.

6 volt, 435 Ah batteries
6 volt, 435 Ah batteries

Then I though about the deep cycle batteries in my barn.  These 6 volt, 435 Ah units have been around for a couple of years, but last I checked, they still held a charge.  Other deep cycle batteries from things like golf carts, fork lifts, campers, boats etc could also be pressed into service.  The point is, 24 VDC should not be impossible to create.

To keep a charge on the batteries, this solar panel will work:

225 Watt, 36 volt solar panel
225 Watt, 36 volt solar panel

This setup would require some sort of 24 volt DC charge controller, which I found on Amazon for less than $15.00 US.  This charge controller has selectable 24/12 VDC output and also has two USB ports which would be handy for charging hand held devices.

I measured the power draw while the exciter was running 20 watts into a dummy load, it draws 120 Watts.

The final part would be some sort of antenna with transmission line.  For this situation, a simple wire center fed dipole hung vertically would work well.  This can be fabricated with two pieces of copper wire and a few insulators.

Simple dipole antenna
Simple dipole antenna

The lengths of each wire can be calculated as follows:

Approximate length in feet: 234/f (MHz)

Approximate length in inches: 2808/ f (MHz)

Approximate length in cm: 7132/f (MHz)

For the FM band, maximum length of wires needed will be 32 inches (81 cm).  Insulators can be made of anything that does not conduct RF; PVC, ABS, dry wood, dry poly rope, etc.

Emergency FM band dipole
Emergency FM band dipole, cut to 88 MHz, lowest FM frequency

I recommend to cut the wires slightly long, then trim little bits off of each end while watching the reflected power meter on the exciter.  To keep RF from coming back down the shield of the transmission line, make 8-10 turns, 6-8 inches in diameter of coax as close to the antenna as possible and secure with a wire tie.  This will create a balun of sorts.

My emergency FM kit consists of:

  • Bext Frequency agile exciter
  • 30 feet, RG-8 coax with N male connector on one end
  • 4 ten foot RG-58 BNC male jumpers
  • 1 four foot LMR-400 N male jumper
  • Dipole antenna, cut long
  • Solar charge controller
  • Small basic tool kit; hand tools, plus DVM and soldering iron
  • Power cords, extension cords
  • 300 watt 12VDC to 120VAC inverter (pure sine wave)
  • 20 feet audio wire
  • Various audio connectors; spade lugs, XLR male and female, RCA, 1/4 TRS, etc
  • Various RF connectors; PL-259, N, BNC, etc
  • Bag of 12 inch wire ties
  • 3 rolls of 3M Scotch 88 electrical tape
  • 100 feet of 3/8 inch poly rope

This is all kept in a sturdy plastic storage bin from the Home Depot.  If needed, the batteries and solar panel are stored in the barn along with an assortment of other goodies.

Will it ever be needed?  Well,  I hope not.  However, it is much better to be prepared to restore services than wait for somebody to show up and help.  Sitting around complaining about the government does not relieve those people in need during and after a disaster.

The Temporary AM antenna

One of those things that I have written about before, but seems to be common these days as older AM towers need to be replaced. One of our clients had just such a tower. Erected in 1960, the hollow leg stainless tower was rusting from the inside out. When the tower crew came to put up the translator antenna, they discovered that there was a hole in one of the legs and climbed back down.

The tower condition was somewhat known about, there were braces installed several years ago at certain levels to keep the tower standing. The new owner had planned to replace the tower eventually, so those plans where moved ahead.

Temporary Wire antenna, WKNY, Kingston, NY
Temporary Wire antenna, WKNY, Kingston, NY

A temporary utility pole was installed near the transmitter building and a wire was strung to another customer owned pole about 170 feet away. At 1,490 KHz, that proved to be a pretty good length. The issue with these medium wave temporary antennas is always the height above ground. In order for the radiation resistance to be somewhat reasonable, the antenna needs to be at least 1/8 to 1/4 wave length above ground. That means a minimum of 78 to 157 feet at 1,490 KHz. The utility pole installed is 35 feet AGL.

WKNY temporary ATU

Thus, the wire antenna has a fairly low resistance, with loads of inductive reactance. Something on the order of 20 ohms, +j480. Since this is temporary, we reused the existing ATU that was designed for the series excited tower. With a capacitor installed on the incoming wire to cancel out some of the inductive reactance, a simple T network was configured to match the 50 ohm transmitter output to the 20 ohm antenna.

In the end, we were able to run about 400 watts into the wire, which covered the city of license fairly well. While the new tower was being erected nearby, we had to reduce that to about 100 watts to protect the tower workers from the hazards of non-ionizing radiation.

WKNY new tower build

The new replacement tower has been constructed. It is the exact same height as the old tower, but has a twenty foot pole on top instead of a normal tower section. The pole was installed to mount the translator antenna. In addition to that, there will be other wireless services installed on this tower.

WKNY will have a six wire skirt installed in the next few days. As this tower is close to 160 degrees at 1,490 KHz, the skirt can go anywhere from 60 to 120 degrees up the tower.

Differential Audio

Most professional audio facilities use differential audio or balanced audio within their plants.  The main reason for this is noise rejection, which was discovered by the early pioneers of wired telephony back in the late 1800’s.  Balanced audio is created by generating two audio signals that are 180 degrees out of phase using either a transformer or an active device.  These are usually labelled High and Low, + and – or something similar.  Those two audio signals are then transmitted across some distance and recombined at the far end, again by a transformer or some active device.

Noise rejection, differential signaling. "DiffSignaling" by Linear77 - Own work. Licensed under CC BY 3.0 via Wikimedia
Noise rejection, differential signaling. “DiffSignaling” by Linear77 – Own work. Licensed under CC BY 3.0 via Wikimedia

When an interfering signal is picked up, it is transmitted along both sides of the balanced audio circuit until the signals are recombined.  During the re-combining process, and common mode interference is cancelled out, as it becomes 180 degrees out of phase with itself during the re-combining process.

Differential signaling is used in analog audio, digital audio (AES/EBU), HDMI, Display Port, USB, Ethernet, POTS lines, ISDN, T-1/DS-1, E-1, etc.   It is a fairly simple concept, but one of the basic building blocks in broadcast studios.

When a studio project was completed at a disused studio/transmitter site location, a certain amount of RFI was being induced on the studio microphones by the unassociated FM transmitter in the next room.  The problem with microphone level audio is the relatively low level of  microphone output, which requires a good deal of amplification.  The amplifiers in this console have active balanced inputs, which might not be exactly 180 degrees out of phase.  In this installation, microphone level audio was run about 20-25 feet on standard microphone cable then it was converted to Cat 6 cable before going into the console.  It may have been better to use the shielded Cat 6 cable for the longer runs as it likely has better common mode rejection than standard mic cable. Another option might have been Star Quad cable.  However, none of those things were done.

Western Electric was the manufacturing arm of Bell Telephone.  In their day, they made some really good equipment.  One such piece is the WE-111C repeat coil.  These can be configured for either 600/600 ohms, 600/150 ohms, 150/150 ohms,  or 300/300/300/300 ohms impedance ratios.  Since this is microphone level audio 150/150 ohms is the appropriate setting.

WE 111 repeat coil, one of the best such transformers ever made

Over the years, I have found many of these transformers discarded at various transmitter sites and studios. There are five microphones feeding this console. I mounted five of these coils in a sturdy metal enclosure and wired them with RJ-45 jacks to be compatible with the Studio Hub wiring equipment used in this studio installation.  I also grounded each unit to a piece of copper strap, which is connected to a grounding lug on the side of the unit.

Western Electric 111C repeat coils mounted in box
Western Electric 111C repeat coils mounted in box

I swept the coils from 20Hz to 20kHz:

WE 111C coils, 20Hz sweep
WE 111C coils, 20Hz sweep

WE 111C coil 20kHz sweep
WE 111C coil 20kHz sweep

This shows a 0.4 dB difference from 20 to 20,000 Hertz, thus they are all nearly flat which is a pretty cool feat of engineering.  I would estimate the age of these transformers is between 50 to 60 years old.

These coils isolate each microphone from the microphone preamp in the console.  This completely eliminated the FM RFI and solved the problem.

The Answer to Ailing Copper

I don’t know how things are in your neck of the woods, but here in the Northeastern US, our old copper TELCO networks are on their way out.  This is a problem for broadcasters who still rely on POTS lines (Plain Old Telephone Service) for transmitter remote controls, studio hot lines, etc.  The vast majority of my transmitter site access is through dial up remote controls.  There are a few locations that have web based remote controls, but to be honest; the phone part of my smart phone still gets a lot of use.  There are several locations where the old copper is just failing outright and not through a lack of effort by the repair techs.  Generally, the copper pairs get wet and develop a loud hum, which makes the remote control unit either hang up or become unresponsive to touch tone commands.

The best course of action is to get some type of VOIP line installed.  Here is the rub; many transmitter sites are nowhere near a cable system.  Several times, I have contacted the cable company to see if they will provide a VOIP phone line at a certain site.  The response is usually; sure, we can do that!  However, it will cost  you (insert some ridiculous amount of money) to extend the cable to your transmitter site.

LAN extensions to the transmitter site are a useful for a number of reasons.  More and more transmitters are equipped with web interfaces as are processors, UPSs, transmitter remote controls, security cameras,  etc.  LAN extensions can also be used for backup audio in case of STL failure.  Finally,  an inexpensive ATA (Analog Telephone Adaptor) and DID line can replace a POTS line for a lot less money.  One example; voip.ms has the following plans as of this writing:

Plan type Per month per DID number (USD) Incoming call rate (USD) per minute Outgoing call rate (USD) per minute
Per minute $0.85 $0.01 (USA) $0.009
Unlimited $4.25 $0.00 $0.009
Toll Free (800) $0.99 $0.019 $0.009

Any of those plans surely beats the standard TELCO rate of $40-50 per month per line.

Design criteria for a wireless LAN system needs to take into account bandwidth, latency and reliability.  Each VOIP phone call takes anywhere from 28-87 Kbps depending on the protocol being used.  If the wireless LAN is being used for other things such as back up STL service, access to various GUI’s, etc then the total bandwidth of all those services need to be considered as well.  Do not forget ethernet broadcast traffic such as DHCP requests, ARP, SMB, etc which can also take up a fair amount of bandwidth.

For LAN extensions, I have been using a variety of equipment.  The older Moseley 900 MHz LAN links still work, but are slow in general.  The Ubiquiti gear has proven to be both inexpensive yet reliable, a rarity to be sure.  There are several links to various transmitter sites running on various types of Ubiquiti gear, usually without problem.  One simply needs to remember to log into the web interface once in a while and make sure that both ends have all the firmware updates installed.  They are cheap enough that a couple of spares can be kept on the shelf.

The following diagram shows how I replaced all of the copper pots lines at various transmitter sites with VOIP:

Diagram of LAN extensions to various transmitter sites
Diagram of LAN extensions to various transmitter sites

List of equipment:

Nomenclature Amount Use New or used
Ubiquiti Rocket M5 3 AP and station units New
Ubiquiti AirMax 5G-2090 90 degree sector antenna 1 AP point to multi-point antenna New
Ubiquiti Rocket Dish 5G-30 2 Station antennas New
Ubiquiti ETH-SP-G2 3 Lightning protection New
Trastector ALPU PTP INJ 6 Lightning protection out door units New
Cambium PTP-250 2 Point to Point link Existing/Used
Motorola Canopy 900DA PCDD 1 AP point to multi point Existing/Used
Motorola Canopy 900DA PCDD 2 Station Existing/Used
Microwave Filter #18486 diplexer 3 Diplexer 900 MHz ISM band and 944-952 STL band Existing/Used
Cisco SPA122 ATA 9 Dial tone for remote controls New

The main studio location has the gateway to the outside world. This system is on a separate subnet from the automation and office networks. From that location a point-to-multipoint system connects to the three closest transmitter sites.  This setup uses the Ubiquiti Rocket M5’s with various antenna configurations.  Then, from one FM transmitter site, there is an existing 5.8 GHz path to another set of transmitter sites.  This uses Cambium PTP-250s.

The next hop rides on the STL system, using Motorola Canopy 900 MHz radios and Microwave Filter Company #18486 dilpexers.  These are long paths and the 900 MHz systems work well enough for this purpose.  The main cost savings comes from reusing the existing STL system antennas which negates the cost of tower crews to put up new antennas and or rent on the tower for another antenna.

There is a smaller sub system many miles away that is connected to the outside world through the cable company at the AM transmitter site.  Unfortunately, due to the distances between the main studio and those three stations, there was no line of site shots to these sites available on any frequency.

When installing the 5.8 GHz systems, I made sure to use the UV rated, shielded cable, shielded RJ-45 connectors and Lightning Protection Units (LPUs).  Short cuts taken when installing this equipment eventually come back in the form of downed links and radio heads destroyed by lightning.

Regardless, I was able to eliminate seven POTS phone lines plus extended dial tone service to two sites that previously did not have it before.  In addition to that, all of the transmitter sites now have Internet access, which can be useful for other reasons.  All in all, the cost savings is about $310.00 per month or $3,720.00 per year.