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.
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.
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: https://vhf.dxview.org/
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: https://www.swpc.noaa.gov/products/solar-cycle-progression
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.
Several companies make variations of this antenna; Scale FM-CL is a lower-power version that is used mostly by translators. They are highly directional and can be installed in a vertical, horizontal, or cross-polarized (45-degree slant) manner. This model input power is 5 KW per bay and the manufacturer’s specification is for 1.28:1 or less VSWR across the entire FM band. In the slant configuration, which Shively states is right-hand circularly polarized, the gain is 4.03 dB.
I recently did some work onsite for WXMD, California, Maryland. They were having some issues with high reflected power readings on their transmitter and suspected an antenna or transmission line problem. The station has been on the air for about 10 years and began having issues late last year after a thunderstorm passed through the area.
WXMD California, MD South East Bay Shively 6025 antenna
The main issue was that the transmitter was showing 243 watts of reflected power with 9800 watts of forward power, while the inline watt meter showed 37 watts. As part of the repairs, a new 1 5/8 transmission line was run up the tower replacing the old line which was damaged at the power divider input connector. A new power divider was also installed. Was the antenna still defective? Was the new transmission line and/or power divider defective? Was there an issue with the inline watt meter? Questions, questions, questions…
Thus, several sweeps were needed to verify things:
1 5/8 inch line terminated at the power divider with known good load
This antenna has a power divider that splits the power between a southeast-facing antenna bay and a southwest-facing antenna bay. To be sure that we were not dealing with a bad connector or transmission line, the line was swept in isolation from the input of the inline watt meter to the input of the power divider. This showed that the transmission line, connectors, elbows, and inline watt meter were all good.
Southeast Antenna SWRSoutheast antenna return loss
Next, each antenna bay was swept individually. The power divider port going to the disconnected antenna was terminated with a known good 50-ohm load.
Southwest antenna SWRSouthwest antenna return loss
Once the individual bays, jumpers, and power divider tested good, the entire antenna system was swept.
Full antenna SWR
With everything connected, the SWR showed 1.19:1. Not ideal but not terrible either. The inline watt meter readings were verified with a precision watt meter and the final SWR calculated by hand was 1.16:1.
Full antenna return loss
Therefore, the antenna system is performing within the manufacturer’s specifications.
Network analyzer
The American Amplifier Technology inline FM watt meter was then checked with a precision power meter. The readings on that device were more or less in line with the precision power meter, thus the transmitter directional coupler is out of calibration.
Mini-Circuits Precision Power meter, Forward PowerMini-Circuits Precision Power meter, Reflected Power
The transmitter shelter is just large enough for one rack. Thankfully, the weather was cooperative, we were able to work outside. Overall, it was a productive trip and an enjoyable experience.
