FM antenna – Engineering Radio

The Bext TFC2K broadband FM antenna

FM Broadband antennas are a compromise because they generally have less gain than tuned antennas, are more complicated, and take up more space. However, this antenna has none of those issues. The gain and radiation pattern appears to be almost the same as a tuned three-bay FM antenna.

We are finishing up an antenna project in Pittsfield, MA, this week.

The project involved replacing a Shively 6812 tuned to 95.9 MHz (WBEC-FM) with the TFC2K so that the W277CJ 103.3 MHz (WUPE) translator located on the roof of the 14-story Holiday Inn on West Street could be moved to the studio location. In this case, having the translator in-house will save significant rent. The new antenna will continue to serve as a backup facility for WBEC-FM when the main site is off the air for whatever reason.

The input power per bay is based on the antenna’s input connector. In this case, each bay has a 7-16 DIN connector and the power divider is a 7/8 inch EIA flange. Thus the maximum input power for this setup is 5.5 KW. The licensed output for both facilities is far below that.

According to the manual, this antenna should be spaced at 0.85 wavelength, which is frequency-dependent. I chose a frequency halfway between the two (103.3 – 95.9)/2+95.9 = 99.5 MHz. The formula from the Bext general antenna manual is:

D = (300/F) x 0.85

Where
D = the distance between center of radiating elements (booms)
F - Frequency in Mhz.

Thus, D = (300/99.5) x 0.85 = 2.56 meters (or 8′ 5″)

As this is a series excited AM tower, some type of broadband isolation coil is needed to cross the base insulator. This one is simply a large coil of 7/8 inch coax, likely with a capacitor across the outer conductor to create a resonant LC network.

To me, it looks like a water heater. Since the ground is frozen solid, we made a temporary stand. We will have to come back in the spring to create a permanent stand or perhaps a unistrut mount to the wall of the ATU building.

In the rack room, the transmitters are combined into a Bext FDCSDC2 star point combiner.

Broadband sweep shows a good match across the entire FM band. I will be interested to see how it performs with respect to the Shively single bay 6812 on the roof of the hotel (103.3 W277CJ).

Return loss, Bext TFC2K 3 bay FM antenna

The return loss looks good on both 95.9 and 103.3 MHz. The interference noted in the sweep is from local FM stations including the main transmitter for 95.9 MHz.

Weather related broadcast issues

We just finished our 3rd annual February ice storm. It is becoming somewhat of a tradition in these parts. After shoveling the driveway this morning, I sat down to enjoy my nice hot coffee. While doing that, I figured I would check some of the transmitters to see how things were going. That is when I noticed this:

The reflected power is much higher than normal indicating potential issues with the antenna deicers. I knew something was wrong after a quick call to the Burk, which stated the deicers were on. A quick double-check showed that the reflected power had increased by another 75 watts, so a nice drive to the transmitter site was in order.

Indeed, the controller had turned on the antenna deicers.

Using a clamp on amp meter, I saw almost no current on either leg of the 240-volt circuit. In the meantime, the backup antenna had 2 amps on each leg, which is normal. Then I noticed this:

The relay on the right shows signs of overheating.

I moved the Main Antenna circuit over to the aux antenna relay to get things going again. The current on each leg of the main circuit is 4.2 amps. Over the next 45 minutes, the reflected power returned to normal.

Other transmitter sites to the north in Albany have had similar issues. Unfortunately, those antennas do not have heaters or radomes. Thus, the only remedy is reducing power until the transmitter stays on.

I also noticed that when there is an antenna problem, the station does not sound as good as it normally does because of the bandwidth restrictions adding distortion in the frequency domain.

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.

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.

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.

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.

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

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

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.

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.

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.

Once the individual bays, jumpers, and power divider tested good, the entire antenna system was swept.

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.

Therefore, the antenna system is performing within the manufacturer’s specifications.

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.