FM – Page 4 – Engineering Radio

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.

Measuring FM Harmonics

Anytime a new transmitter is installed or major changes to an FM transmission system are implemented, the performance measurements described in FCC 73.317 should be completed to ensure no interference to other radio services. This is becoming a larger issue with the advent of LTE and 5G mobile data. These services along with E911 and other mobile services are often co-located at FM transmitter sites.

The FCC stipulates that emissions removed from the carrier by more than 600 KHz must be attenuated 80 dB below the carrier. These days, that is not enough. We have had issues with older tube-type transmitters interfering with cellular and mobile data service, even though they met or were far below the FCC specification. The first in, first out rule also didn’t seem to matter either. Those mobile phone providers paid a lot of money when they purchased chunks of RF spectrum at auction, and the FCC will side with them if there is any dispute.

Having a record of measurements that show compliance with the FCC regulations can go a long way in heading off any future problems. I make measurements out to the 10th harmonic.

To get the best results, I have been using a couple of high-pass filters from Mini-circuits.

These attenuate the carrier power seen by the spectrum analyzer by approximately 90 dB depending on the frequency. That allows the instrument noise floor to be lowered to -130 dB which should be well below any receiver noise floor being used by other wireless services.

100.7 with two HPF-200+ High Pass Filters

The carrier is attenuated by 92.44 dB. The rest of the measurements are made with the attenuation set to zero and the preamp turned on. For the lowest FM frequency, 88.1 MHz, the filters are on the edge of their shoulder at the 2nd harmonic. I measured the return loss and found that they matched the manufacturer’s datasheet.

Mini-circuits HPF-200+ X 2, 176 – 216 MHz S21 Return loss

That loss is counted as attenuation for the second harmonic. For the rest of the harmonics, I used 0.5 dB attenuation, which represents connector loss. I could have also measured the cable loss at each harmonic, but that seems unnecessary, given several of the readings were below the noise floor.

To speed things along, I made this handy Excel spreadsheet, which does all of the calculations for me:

A copy of that spreadsheet can be downloaded: FM Harmonic template

Once completed, I printed a copy and put it with the station maintenance log at the transmitter site.

The Rhode and Schwarz THR9 transmitter

Part II of II

This thing is on the air! There are still some tidying-up things to finish, but it is up and running and sounds great! Here are some pictures of various stages of the installation work:

The filing cabinets hold manuals and spare parts. There is not a lot of room left in this building, so workspace is at a premium. The filing cabinet on the left needs some Windex and elbow grease.

Main disconnect and conduit to 400-volt transformer

3/0 cables, 240-volt input to Hammond HPS Sentinal K transformer

The transformer does not have a neutral reference to the power company. The neutral for the transmitter is derived from the Y output connection. The transformer is also designed to suppress harmonics from non-linear loads like switching power supplies.

Square D I-line panel rated for 600 volts

#2 SOOW cable feeding upper and lower sections of transmitter

Wiring to disconnect switch on transmitter

The wiring on the pump station and heat exchanger needs a little more work. The client wanted to get this on the air as soon as possible because they are in a book and were running at 50% power. Once things calm down a bit, I will put the backup transmitter on for an afternoon and properly dress the wires.

I found this FM modulation analysis function on my spectrum analyzer very useful. The station deviates slightly more than the allocated 75 KHz because of a subcarrier. Overall, it looks good. I measured the harmonics out to the 10th harmonic, most of them were in the noise floor. A few made a slight appearance, but well within FCC tolerances. It is important to document this, as this site has colocated cellular carriers and several E911 services.

FCC part 73.317 states:

(d) Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.
47CFR 73.317

WHUD fundamental with two Mini-Circuits NHP-200 high pass filters installed

The rest of the harmonics were measured down to -130 dB with the two NHP-200 high-pass filters in the circuit. The 3rd, 4th, 5th, 6th, and 8th harmonics were unmeasurable. The 8th, 9th, and 10th made slight appearances.

WHUD 8th harmonic makes a little appearance

Antenna VSWR according to the transmitter directional coupler

Pretty close, the VNA was inserted at a patch panel, which is the last thing before the transmission line leaves the building. The transmitter goes through an ERI switchless combiner, which probably gives it a slightly better load.

Aside from the finishing details, I need to keep an eye on this for a week or so and top off the Heat Transfer Fluid as needed. It takes a bit of time to get all of the air out of the coolant loop. Another thing; the operating pressure on this is 4 Bar, which is almost 60 PSI. That is higher than other liquid-cooled transmitter systems I have installed before.

The Rhode Schwarz THR9 transmitter

This is part I of II.

We are in the process of installing an R&S 40 KW liquid-cooled FM transmitter. My first comment; these are well-built units. A quick look at the machining of the parts indicates attention to detail is a key design feature.

As the price of electricity continues to rise, liquid-cooled transmitters for this power level make a lot of sense.

This installation is for Pamal Broadcasting’s WHUD, Peekskill, New York. The site has undergone major upgrades in the last few years. The original 1958 World Tower Utility 80 was replaced a year ago with this Valmont 60X394. Two cell carriers, two translators, and several E911 services are now colocated on the tower.

Valmont 60X394 tower, WHUD Peekskill, NY

The transmitter building is also the original cinder block structure from 1958. When it signed on, the station had a Gates FM5B 5 KW transmitter, an RCA BFA-7, 7-bay horizontally polarized antenna with an ERP of 20 KW. In 1970, that antenna was changed out to a 6-bay circularly polarized ERI with a Harris FM20H transmitter, increasing the ERP to 50 KW. As of now, the station has a 4-bay ERI SHP-4-A-C main antenna and the TPO is 28 KW for the same 50 KW ERP. As the station’s power increased, the building became a little bit smaller than optimal. We needed to rearrange some equipment to gain space for the pump station and step-up transformer.

Rhode Schwarz recommended installing a step-up transformer for the incoming AC mains. The power supplies run most efficiently with 400 volts AC.

Hammond HPS Sentinel K dry core transformer

The Rhode Schwarz RF connection to an ERI switchless combiner

We decided to reuse the ERI switchless combiner left over from the Nautel V-40 installation. There are two Nautel V-10 transmitters with a hybrid combiner that are to be used as a backup. We won’t be running this as a combined transmitter operation, it is a way to save money rather than install a separate 3-inch coax switch. I will build a simple control panel to move the combiner position either all the way up (THR9) or all the way down (V-10s).

2.5 inch core drilled holes for coolant supply and return

Working on the liquid cooling system. I used a core drill to make the supply and return lines to the outdoor heat exchanger. I made sure that I had the shop vac (with a HEPA filter) running while drilling so that all of the concrete dust was captured. That stuff can get everywhere and has a bad tendency to destroy motor bearings. Whatever plant made these blocks in 1958, they used some hard material. It took a while for my masonry drill to get through them.