tech stuff – Page 7 – Engineering Radio

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

AM station downgrade

I have been working on another formerly directional class B AM station, this one is in Rutland, VT. WSYB has been on the air since 1931 with the same call letters serving the east-central part of Vermont. In 1931, it was operating on 1500 kc with 100 watts of power. In March 1941 it moved to 1490 kc with 250 watts before settling, a few months later, on 1380 with 1,000 watts, directional night time protecting CKPC in Brantford, Ontario, Canada.

The transmitter site was first located at 80 West Street (now known as BUS US 4), in Rutland. It was moved to its current Dorr Drive (Formerly Creek Road) location in 1938, when the station was requesting a power upgrade to 250 watts. Whilst cleaning out the old transmitter building, a copy of an operating log, dated December 7, 1945 was discovered in the attic above the transmitter room:

Back from the time when readings were required every 30 minutes.

In 1956, WSYB was allowed 5,000 watts daytime non-directional with 1,000 watts nighttime directional.

At some point in the early 1990s, the original towers were replaced with solid-leg Pirod towers, each 195 feet tall.

After that, things went the way things do; AM steadily declined in favor of FM, local programming was mostly replaced by syndicated satellite stuff, there were several transfers of ownership, etc.

A translator on 100.1 MHz was added in 2016; the two-bay Shively antenna was installed at the top of the South West tower. There is local programming on the station from 6 am to noon on weekdays. There may also be some gardening shows and other such programming on weekends.

The current owner has decided like they have done in other markets, that AM directional antenna systems are a maintenance nightmare, the risk of FCC sanctions are high for an out-of-tolerance antenna array, and the ratings and income from the station do not justify the risk/cost. Thus, non-directional nighttime operation was applied for and granted. The station is now a Class D with 25 ass-kickin’ nighttime watts.

WSYB had a two-tower nighttime antenna system. The tower closest to the building (SW) was also the daytime, non-directional tower and it now holds the FM translator antenna and STL antenna. Thus, it was decided to ground that tower and keep those antennas in service. The far tower (NE), which was the second tower of the nighttime array would become the AM antenna. The nighttime ATU was built for less than 1,000 watts of input power, so several components needed to be upgraded for 5,000-watt operation.

I had available these nice vacuum capacitors that came out of another decommissioned antenna system. The vacuum capacitors are great because the voltage/current ratings are much higher than the mica capacitors that were in the circuit before. You can see black goop where one of the Sangamo mica capacitors on the input leg failed several years ago. These vacuum capacitors are rated at 15 KV and the current rating at 1.38 MHz is probably in the 70-80 amp range. I had to move the base current meter from the former daytime (SW) tower out to the NE tower. The day-night switch was taken out of the circuit. The transmission line to the far tower was replaced with 7/8 inch foam dielectric cable. A slight touch-up of the coil on the input leg of the T network was all that was required to bring it into tune.

The electric lines to the tower have been temporarily disconnected. As soon as they are reconnected, I will vacuum out all the mouse crap and other debris. The ATU building also needs some work sealing it up against the elements.

The tower base impedance is 75 ohms, +j95 making the base current 8.6 amps daytime and 0.58 amps night time.

For me, the magic of radio exists at that boundary between the real objects (towers and antennas) and the ether. The transference of electrical voltages and currents into the magnetosphere is something that still fascinates me to this day. Coupling a 5,000-watt medium wave transmitter to a tower and watching it work is something that I will never grow tired of.