Installing a 60 KW FM transmitter

Recently, I installed this very nice GatesAir FAX60HD.

This project was for WPGC, Washington, DC. WPGC (Prince George’s County) is an Audacy station with a Hip-Hop and R&B format. I was listening to The Good Morning Show on my drive from the hotel to the transmitter site, and those guys were hilarious! It’s nice to hear a well-programmed radio station.

It is always fun to accept new and interesting challenges. This is, to date, the largest transmitter either AM or FM that I have ever installed. Previously, I installed several FLX-40 units, which is quite a bit of power for the FM side of things.

MSC unit with touch screen pad controls both transmitters and exciters

This transmitter combines two FAX30 transmitters and is controlled by an MSC unit. The content stream for HD comes from an FMXi4g, which has several great features.

BDI inline watt meter

This station’s TPO is 45.7 KW with the HD carriers at -14dBc. While this is a class B station with an ERP of 50 KW, the four-bay half wave-spaced antenna requires a lot of wattage to make that TPO. This is a largely residential neighborhood, which is, I surmise, the reason for the half wave-spaced antenna.

WPGC main and backup antennas, Capitol Heights, Maryland

I was told that this is not the greatest part of town. The station has had some theft of outdoor air conditioner equipment in the recent past. That being said, it is much nicer than many areas we normally work in the NYC metro area. The transmitter site has been here since the station signed on in 1948.

WPGC transmitter site
FAX60HD power supplies and power amps installed

It took a bit of time to install the 42 power supplies and 48 power amps. The power amps were installed in the same slots as during the factory test cycle. Thus the data on the test sheet matches the data seen on the transmitter GUI when we turn it on.

FAX60HD cabinet interconnects completed

All of the cabinet interconnects; RF plumbing, grounding, AC supply, sample lines, and various control lines were completed.

Transmitter hybrid combiner for the two FAX30 transmitters
FAX60HD, WPGC-FM Washington
4 Inch Dielectric coax switch with 60 KW load
WPGC 4th harmonic

Most of the harmonics (2-10) looked like this. However…

WPGC 5th harmonic

This is something interesting that came up during the proof. When measuring the harmonics, most of them were in the -130 dB range. This one is slightly higher than that, which is due to the proximity of WFDC-DT on channel 15 (476-482 MHz), 1000 KW ERP about 10.6 miles away. Their signal was coming back down the transmission line from the FM antenna. This is a good demonstration of how other unwanted signals can get into the final sections of transmitters which can cause intermodulation mixing products. In this case, the FAX60 has several low-pass filters that remove this and other signals before that happens.

This is replacing a pair of combined BE FM25-T transmitters that were getting a little bit long in the tooth. The air staff has commented on the noticeable improvement of the station’s sound. The downside of tube transmitters is the delicate tuning procedure to reduce the AM noise. High-powered transmitter tubes are also getting more expensive and, for some types, harder to source.

All About the 7/16 DIN connector

These connectors appear on the backs of various FM and lower-power TV transmitters. I thought it would be interesting to explore them a little bit.

First, according to Wikipedia, DIN is an abbreviation for Deutsches Institut für Normung. If you enjoy a lager or two then squint at that name you might see The German Institute for Standardization. 7 is the size of the inner conductor contact in mm and 16 is the size of the inside diameter in mm of the outer conductor. The thread is 29 x 1.5 mm.

From what I can find, the connector was originally developed for the German Military, which needed a rugged, efficient connector that was easy to install properly.

This style of connector is very popular with cellular antenna manufacturers due to its relatively high power handling capability and excellent passive intermodulation performance.

According to Amphenol, their 7/16 DIN connectors have the power handling capability of 3000 watts/1 GHz @ 25C (77F). That is a fairly low temperature. They give the working voltage as 500 V RMS continuous, which is 5000 watts at 50 ohms impedance. That, of course, assumes a 1:1 match, which is never the case. The connector is probably safe to ~2 KW in normal operating conditions for VHF and UHF TV operation.

Amphenol 7/16 plug for LMR-400 and Belden 9913

Also, keep in mind that RG-8, RG-213, RG-214, LMR-400, and Belden 9913 look similar but they use different connectors because the inner conductor of the latter two is larger than RG-8, RG-213, or RG-214. This happened on a project recently. It came time to crimp the pin on the center conductor and, oh no! The pin was too small!

A properly installed connector will be outdoor water-resistant in most weather conditions.

Marconi would be impressed

I finished up another GatesAir FLX-10 install recently. This one was way out in Provincetown, MA at the end of Cape Cod for WOMR (Outer Most Reaches). That is a community radio station that has an eclectic mix of programs. The studios are on the second floor of an old church.

Main studio, Wheatstone Console
Talk Studio, up/down studio furniture

The transmitter site is located about a mile away from the studios.

WOMR antenna

A few details on the installation; the antenna is mounted on the water tank, which is the tallest thing around for many miles.

WOMR antenna, Shively 2 bay 6810 mounted on a water tank


The building is less than 120 square feet, which made working a little tight.

WOMR transmitter building, Provincetown, MA
GatesAir FLX-10 transmitter placed
Pump Station


There was very little room outside for the heat exchanger because of the need to get vehicles around the water tank for maintenance. The area around the building was taken up with a generator and HVAC gear. We ended up mounting it to the side of the building horizontally. The main reason was that the wind at this location could come from any direction. If mounted vertically, there was a chance that the wind could blow into the heat exchanger against the fans, possibly causing some overheating problems. Also, the horizontally mounted unit will have a smaller surface area during Hurricanes and Nor’easters.

Heat Exchanger, mounted horizontally on the northeast-facing wall


To do this, I made a unistrut frame and attached it to the outside wall.

The liquid-cooled transmitter takes up much less floor space than the prior transmitter, a Nautel V-10. This installation also included activating an HD Radio signal for this station.

The GatesAir FMXi4G importer/exporter resides at the transmitter site. Currently, the station has HD-1 on the air, no word on any future sub-channels.

Looking North towards the Atlantic Ocean

We had nice weather on one of the days. On the last day (Friday) it was raining with 35 MPH wind guests.

Speaking of Marconi, the US’s first trans-Atlantic wireless transmission station is a few miles down the road in South Wellfleet. Nothing is left of the site; time and erosion have taken all traces away.

Postcard, South Wellfleet Marconi Station
Bronze plaque placed in 1953 by the Wellfleet Historical Society

Here we are standing on the location of the transmitting building. The last two tower bases tumbled over the cliff edge in 2011.

It’s always an enjoyable time installing a liquid-cooled transmitter!

Fordham University WFUV

Over the last several months, I have been helping out in a small way with WFUV’s new transmitter installation. George Evans, CE for WFUV, and Bill Weeks of Wolftron Electronics did most of the work. This project’s location is unique; the transmitter is in an equipment shelter on top of the Montefiore Medical Center in the Bronx. That complicated the removal of the old transmitters and installation of the new transmitters.

WFUV tower atop the Montefiore Medical Center

This is the top of the Montefiore Medical Center in the Bronx. The building has 28 floors. To access the transmitter, one needs to go up another flight of stairs to the roof. Go outside onto the roof and take two more flights of stairs to the transmitter shelter just below the tower. Those stairs can be seen as a thin wispy thing on the left of the elevator building.

WFUV tower, close up

At the top of the tower, the two-bay Shively antenna is for WVBN 103.9 Mhz. WFUV 90.7 MHz is the six-bay Shively just below that.

BE FMi 703 (FM-10S)

These are the transmitters being replaced. There are two of them running combined for a TPO of 14.2 KW (including the -14 dBc HD subcarriers). They are about 20 years old.

The before picture; combined BE FMi 703 transmitters

As you can see, there is little room to work in this space. The old transmitters needed to be removed from the room, hosted down two stories to the main rooftop, wheeled around to the door, and then lowered another flight of stairs to the 28th to the elevator.

It makes me tired just thinking about it.

BE FMi 703 (less RF modules and power supplies) being lowered to the rooftop level

Out with the old, in with the new.

GatesAir FAX15K being hoisted from the roof up to the transmitter shelter

The GatesAir FAX15Ks made the trip up safely. Able Rigging from New Jersey did all of the moving into and out of the shelter. The two transmitter chassis, all of the boxes containing the exciters, RF modules, power supplies plus a 25 KW dummy load, and the cut-up for scrap Myat variable T antenna combiner were removed in one go.

The after picture; GatesAir FAX15K transmitters, two cabinet version

The FAX15K’s are in the shorter cabinets (requires two) because the taller cabinet would not fit in the elevator.

FAX15K with back doors removed, power block, combiner, directional coupler, and RF output flange.
3-inch coax switch and RF plumbing

The output of each transmitter goes to a 3-inch coax switch. The bottom port of the switch goes to the Shively filter (thence to the antenna), and the top port goes to the 20 KW dummy load.

3-inch coax switch
Bill Weeks routing control wiring to coax switch
WFUV CE George Evans, heading to the job site
“How many more times do I have to climb these stairs today?”
But the view is great! Mid and upper Manhattan in the distance
Your author is about to tell a sea story…

I enjoyed working on this project.