Rohde Schwarz Test & Measurement Fundamentals

I found a great resource for learning about test and measurement on Rohde Schwarz’s YouTube channel. Each video is about 5 to 15 minutes long and covers the basics of RF test equipment, measurement parameters, and definitions.

Rohde Schwarz Test and Measurement Fundamentals

Measuring RF systems is an important part of Broadcast Engineering. Many folks think that RF plants are going away, replaced by all IP content distribution. I disagree; Terrestrial Broadcasting will be around for a while yet. AM and FM radios are still ubiquitous in cars, homes, businesses, etc. There is no other information distribution method that is as simple and robust as over-the-air broadcasts. That is why Federal Emergency Management is still spending money on hardening broadcast facilities.

The Internet and Mobile Data in particular are susceptible to failure in emergencies. Cellular networks were almost useless due to congestion or system outages during the 9/11 attack or a natural disaster such as Hurricane Sandy.

Radio still has a role to play.

As the older Broadcast Engineers retire, there is a dearth of qualified RF specialists who can make accurate measurements on antenna systems, filters, and other transmission system components. There are very few mentoring opportunities, especially in commercial broadcasting. Gone are the days of several engineers on staff, when there was time to teach the younger people some hard-learned lessons. One of the reasons I write this blog is to pass along some of that knowledge to others so that the industry might survive.

Summer Time Atomspherics; Why is there another FM station on our frequency!!??

That is indeed a good question. There may be several explanations; a pirate, somebody’s Part 15 device, or atmospheric ducting. If the weather is good, tropospheric ducting can cause VHF (FM broadcast) and UHF (TV broadcast as well as Remote Pickup units and STLs) signals to travel far beyond their intended reception areas.

The Troposphere is the zone in the atmosphere closest to the Earth, ranging from 0 to 15 km. It is the area where most weather phenomena take place. For VHF and sometimes UHF, refraction can bend the signal back towards the surface of the Earth. Refraction at lower altitudes (called surface ducts) can cause radio signals to travel shorter distances than normal unless they are over water. Refraction at higher altitudes (elevated ducts) can cause those same signals to travel far beyond their normal range, sometimes hundreds or thousands of kilometers.

Three things affect the tropospheric refraction index (or N); water vapor, air pressure, and air density. At higher altitudes, the air is normally cooler, less dense, and dryer than air closer to ground level. However, high barometric pressure will often bring warm, dense, moist air to high altitudes. This can create a layer of warm air over a layer of cooler air known as a temperature inversion. This can create a “duct” in the upper troposphere similar to a waveguide. These signals can be very strong, sometimes overpowering a local FM signal due to the capture effect.

There is an online source that predicts atmospheric ducting, mostly used by Amateur Radio operators, but it can also be a useful troubleshooting tool:

That site produces a map like this:

VHF Ducting map

This can happen any time of the year but is more common in summertime. Tropospheric ducting is not an effect of ionization from the sun. This phenomenon is known as Sporadic E, which will be covered below.

The good news is tropospheric ducts normally last a few minutes to a few hours. Sometimes they can last longer however changes in the width or length of the ducts will change the frequencies and distances that RF signals travel along that duct. In addition, if you are hearing a co-channel FM station from many hundred kilometers away, listeners of that station are now hearing your station the same way.

Another long-distance VHF propagation phenomenon is called Sporadic E layer propagation or simply Sporadic E. This happens when the Ionosphere is heavily affected by a solar storm or sunspot. Sunspots run in an 11-year cycle. We are approaching the solar maximum for Solar Cycle 25, predicted to happen in July 2025.

NOAA Space Weather Solar Cycle 25 progression

Sporadic E is much less predictable, more random, and short-lived. Solar storms can create highly ionized areas in the E layer of the Ionosphere, creating skywave conditions for VHF signals. These signals will skip in the same way that HF and MF signals do. Fortunately, these conditions usually last a few seconds or minutes at most. More on the solar cycle can be found here:

North American Sporadic E Map

Once again, Amateur Radio operators are interested in this as a mode of communication. There is a Sporadic E map online at:

Co-located common antenna FM stations

One of our clients needs to move to another transmitter site because their lease is expiring at the old site. We have been working on this for several months now. One of the nice features of this project is the panel antenna.

Kathrein 754154 spec sheet

This is installed in a 2-bay 3-around configuration. I don’t see this particular model in the Kathrein catalog anymore, but there are other cross-polarized panel antennas available from them.

Colocated tower

There are many existing services on this tower including two full-power FM stations, a translator, a VHF TV station, numerous cell carriers, etc. Once the installation is done we will have to check carefully for intermodulation.

Honda Track Machine

Winter in the Northeast; there was just enough snow and slush on the access road that the truck could not make it to the top of the hill. This track machine works great. We have added a Polaris Ranger 900 to our inventory (not this machine) for winter access to several of the more difficult transmitter sites. While I do enjoy the occasional walk in the snow, the key word here is occasional.

AAT branch combiner inputs

The three stations are combined into the panel antenna with this rather nice American Amplifier Technologies C-IR-3-3-30K-N branch combiner.

AAT branch combiner output side

The input filters needed a slight adjustment to compensate for the difference between the test load they were tuned to and the actual antenna load they will be running into

Touching up input filters

Two of the transmitters are Broadcast Electronics STX-10 units. We have had good service from the STX-10 which was installed on Mount Beacon a few years ago.

Pair of BE STX-10 transmitters

We are waiting for the Comrex Bric Link III to come back from the factory after their firmware update. They are to be used for the STL. Once they are returned, we should be good to go for site turn-up.