tech stuff – Page 9 – Engineering Radio

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.

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

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.

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:

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.

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, 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’s condition was somewhat known, 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 were moved ahead.

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 wavelength above ground. That means a minimum of 78 to 157 feet at 1,490 KHz. The utility pole installed is 35 feet AGL.

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.

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 1800s. 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 labeled 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, common mode interference is canceled 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 a 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

I swept the coils from 20Hz to 20kHz:

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.

Installing a satellite dish

This is a replacement dish for the Comtech dish destroyed in a downburst event a few weeks ago. The first part of the job entailed the placement of the new dish on the ground. The town code enforcement officer was much happier with this idea than mounting it up above roof level along the back of the building as the old one was. Of course, this is possible due to the shift in satellites last year to AMC-18.

Finding a good spot on the radio station property was fairly easy. The studio is located in a business district, thus the sideyard requirements were zero feet, which is great. The building inspector required that we dig a test hole to see what type of soil was there. It turned out to be fill. That required the footing design to be changed somewhat and stamped by a licensed engineer. Not a major problem.

Satellite mount pole, waiting pre-pour inspection

The footing is 36 inches wide by 7 feet deep.

A little bit of water in the bottom of the hole

The mounting pipe has flanges welded to the side of it to prevent it from spinning in the concrete.

After the pour, we let the concrete set up over the weekend.

The dish is assembled and waiting for lift. We used a backhoe to lift the dish onto the mounting pole, unfortunately, I was not able to take a picture as I was on a ladder attaching the dish to the pedestal with U-bolts.

Viking 1374-990 3.7 Meter R/O dish installed

Here it is installed and aimed at AMC-18. I used the Satellite Buddy, which makes the aiming job much easier. Once the signal is acquired, I like to peak the Eb/No on the West Wood One carrier, which seems to be the most sensitive to any type of change.

Viking 1374-990 3.7 Meter satellite dish, back view