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.

The Shively 6025 Broadband Log Periodic Antenna

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 SWR
Southeast 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 SWR
Southwest 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 Power
Mini-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.

SAS studio installation work

I receive several emails a week from interested readers. One noted that the blog seems to be focused on RF. Yes, that is what I do most, but the company does studio installs as well.

This was from a few years ago.

WDST moved out of their Bearsville studio location into the former Methodist Church in West Hurley.

WDST Studios, West Hurley, New York

We installed a new SAS audio router and console system.

Eaton UPS powers rack room and studios
Small Rack Room for NextGen servers and STL equipment
Air Studio, Under Construction
Rack Room
Ethernet Patch Panel
Office area
Live Performance Stage
Rack Room
Back of the racks
WDST Air Studio
WDST Production Studio

Pictures of their old Bearsville studio can be found here:

WDST, Woodstock, New York

If folks are interested, I will post some more pictures of more recent studio-build projects.

The Nautel J-1000

I Finished up this installation of a J-1000 in Brookfield, Connecticut for Nossa Radio. That is a Portuguese broadcaster that owns three other stations in the US.

WINE 940 KHz Brookfield, CT

These Nautels are fairly simple affairs; a controller and two RF amps with incumbent power supplies.

Nautel AM-RF and AC mains surge suppressor

Be sure to install the surge suppressor that comes with the transmitter.

The J-1000 is replacing the 43-year-old Harris MW-1A which will function as a backup. Like all new transmitter installations; some things must be done to complete the job.

Harris MW1A

Harmonic measurements out to about the 5th or 6th harmonic need to be documented and compliant with NRSC-2 (AM mask requirements). Although NRSC-2 measurements are required, I don’t see how they can enforce that specification after AM HD radio came into being. Nevertheless, it was measured and passed. With the station carrier power of 680 watts, I used the RF monitor port on the back of the transmitter to make the measurement. Otherwise, I would need to find an empty field somewhere 1 KM away and stand in the middle of it to reduce all of the electrical noise.

Spectrum Mask from a Spectrum Analyzer

The NRSC-2 mask is mainly a function of High-Frequency limitation in the audio processor—certain transmitters, like the aforementioned MW1A did make some contributions to out-of-tolerance measurements.

NRSC-2C AM mask requirements
Harmonics measurements
WINE folded unipole feed point

The antenna is a skirted tower that has many other services colocated on it. At the top is WRKI.

WINE ATU
WINE daytime coverage map

Driving away from this site, I would have to agree with the predicted contour map above, at least on the highway. I think it may be a bit different driving around in town.