Studio to Transmitter Link

KYFC-TV used a private microwave link, licensed by the FCC, to get the programming from the studio at 4715 Rainbow, Kansas City, KS to the tower site about 4.2 miles away in Kansas City, MO. This is one of those “behind the scenes” activities that viewers are unaware of.  

In the broadcast industry this is called a Studio to Transmitter Link (STL). Located at the studio is a low power transmitter that operates in the microwave frequency band. There is a receiver at the transmitter site that receives the audio and video signals from the studio and sends them to the high power television transmitter for broadcast. The antennas used for the STL look similar to the one in the photo on the right.

These parabolic antennas, sometimes referred to as “dishes” focus the energy into a narrow beam. The dishes at both ends must be lined up precisely looking towards each other.

The map below shows the path of the STL. You will see the starting point at the KYFC-TV studio and the end point is at the transmitter. It is imperative that the path not be obstructed. At the studio the dish is mounted on the roof of the building. At the transmitter site it was originally mounted at the 200 foot level of the tower.

The solution was obvious. We couldn’t move the building so we had to move an antenna. Moving the studio antenna was not an option as that would have required constructing a tower. That would not only be expensive but was highly unlikely of being approved by the City of Westwood.

However, the dish at the tower could be moved fairly easily. Path engineering showed that moving that antenna up another 200 foot on the tower would adequately clear the obstruction. This is what was done and everything was back to normal.

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© 2023 Joe Snelson

As you can see, the STL is a critical component of a television station. Nowadays many stations use fiber or even specialized Internet services to connect the studio to the transmitter. Those options were not available back in the 70’s. Some stations, however, continue to use microwave radio links for their primary STL or as a back-up.

Joe Snelson shares a second story. One day in 1987 he received a call from the then current KYFC-TV Chief Engineer explaining that the audio was getting noisy and “snow” was seen in the picture. It was fine at the studio but bad at the transmitter. It was apparent the the STL was in trouble. A visit was made to the studio with a pair of binoculars. Joe went up on the roof near the dish and looked toward the KYFC-TV tower. Lo and behold, it was seen that the path was being obstructed by a new building being built! The map below shows the west end of the STL path and the obstruction which was located at 49th and Main Street. A photo of the building is shown on the right.

The maps and equipment mentioned above were given as background for a couple of stories we would like to share that interrupted KYFC-TV from transmitting its programming due to issues with the STL.

The first story was submitted by Roger Topping, a former KYFC-TV Chief Engineer. Circa 1982, the STL started fading, but only at sundown and sunrise. This was fall, about September. It would fade out for about 30 minutes and then be good until the next cycle. Visually confirming the path was still good, I started looking at other causes. After troubleshooting, we found moisture in the STL waveguide at the transmitter site.  Apparently the fades were caused as the moisture was changing phases from gas to ice and back to gas from the sun heating the line. So, in an attempt to move all the moisture out of the line I got several bottles of nitrogen from a local welding supply, had a tower professional climb the tower and open a valve at the top, and then let the nitrogen flow through the line to push out the humid air. [Dry nitrogen is used to keep the line dry by applying positive pressure inside the hollow waveguide so moisture doesn’t seep in.]

After we got the line dried out as best as we could, the tower professional did an inspection and found that two cover flanges had been put together at the flex going to the dish.
He inserted a pressure window with gaskets to solve the issue. [The flex being referred to a short piece of very flexible wave guide that couples the regular waveguide to the dish. There was leaking occurring at this location. Think of it like a short hose going from your outside faucet to a garden hose reel that has bad washers and is leaking.]

The line that is used to connect the dish to the transmitter or receiver is called waveguide. Without going into the technical reasons you can’t just use wire for equipment that operates in the microwave band. A photo of what waveguide looks like is shown on the right. It is made of copper and hollow on the inside. The corrugated copper allow it to be easily bent. It is covered with a black polyethylene jacket.