There are a few FM stations around here that intentionally broadcast in mono. One is an FM talker, which from a technical standpoint makes a certain amount of sense since any particular human voice is a single-point sound generator.
The other FM station broadcasting in mono, WKZE, has a music format with a very eclectic playlist. It is a full Class A located in northwestern Connecticut. The idea with this station is to garner a larger and more reliable coverage area.
It comes down to a simple physics discussion about free space loss. The basic equation for free space power loss is:
That formula works for a single frequency, say the carrier frequency, for example. As the signal gets spread out by modulation, the power density on any given frequency is reduced as the energy is divided between many other frequencies.
First, free space loss takes into account the spreading out of electromagnetic energy in free space is determined by the inverse square law, i.e.
where:
is the power per unit area or power spatial density (in watts per meter-squared) at distance ,
is the total power transmitted (in watts).
Second, with Frequency Modulation (FM), the power spectral density is a function of the differences in the highest and lowest frequency:
Therefore, the narrower the bandwidth of a signal, the higher the density of the received signal will be in relation to the transmitted power. An unmodulated FM signal will have a better, more reliable coverage area than a modulated one. Of course, we need to modulate the signal, otherwise, there is no point in having the transmitter on.
A baseband or composite FM signal has several components:
FM baseband signal
An FM station transmitting a mono signal will have a much lower bandwidth. With wideband FM, the modulation index is generally 2fΔ or two times the maximum audio input frequency. Thus, a mono FM broadcast station will have an approximate deviation of approximately 30 kHz (plus any ancillary services like RDS) vs a stereo FM station, which has a 75-80 kHz deviation using the same carrier power.
For higher power FM stations, FCC Class C and B, this is not much of an issue. Those stations tend to have a great deal headroom when it comes to power density, building penetration, multipath (picket fencing and capture effect). For Class A and LPFM stations, it is a different situation. For those stations, unless FM stereo broadcasting is truly needed, it should be turned off. On low power stations, stereo can be a great detriment to reliable coverage.
We are currently working with one of our clients who need to rebuild an FM transmitter site. The site is an old house that used to function as a studio. The transmitters are wedged into various places and the whole place looks like a fire trap. We are working on moving the transmitters to a new building at the base of the tower and installing all ancillary equipment according to good engineering standards.
The transmitter site design has changed somewhat over the years. What may have been good engineering standards in the past have changed with newer transmitter designs and needs. Up until about 1990 or so, most transmitter sites were cooled with outside air. As such, there was often a “filter room” or “air mixing room” with associated blowers and fans for moving air through the building. Older sites often had these features built-in as part of the transmitter installation. WPTR’s GE BTA-25 was a good example of this.
Modern solid-state transmitters are a little more delicate than their older tube-type brethren. Tubes were designed to run hot and had no trouble with temperatures up to 110 to 120 degrees or so. Continental transmitters were famous for this. As Fred Reilly once told me “We’re Dallas and it gets hot here. The manufacturing floor is not conditioned. It doesn’t matter, 100 degrees, 105 degrees, they just keep on working.” I think he was talking about the assemblers as well as the transmitters.
Solid-state transmitter switching power supplies are also somewhat finicky.
A good transmitter site design will incorporate the following:
Good air conditioning. Calculating the AC load for the transmitter waste heat, other installed equipment, as well as the building solar gain. Waste heat is a function of AC/RF transmitter efficiency, which is found in the owner’s manual. If unknown, 50% is a good design standard, in other words, waste heat equals TPO.
Good grounding. A good grounding system is a must for all transmitter sites. This includes lightning and RF grounds. Low impedance paths to a single point ground is a must.
Good power conditioning. Mountain top transmitter sites are susceptible to all sorts of utility company irregularities. Surge protection is a must. Series types are better than parallel.
Good lighting. Nothing is worst than fumbling around in a half-lit transmitter room trying to make repairs.
Adequate workspaces and clearances. Electric panels require three feet of clearance from the front. Cabinet doors should be able to swing fully open. All-access panels should be, well, accessible.
Adequate electrical system. Pole transformers and service entrances are properly sized for the load. Backup power. Plenty of work outlets around the room.
Some of these may seem like no-brainers, however, one would be surprised at how transmitter sites have grown over the years. An FM site that may have started with one 5 KW transmitter in 1950 will have been greatly upgraded over the years. Today, that same site may not employ a 30 KW transmitter, full air conditioning, several tower tenants, etc.
WHUD transmitter site diagram
This is a transmitter site that we redesigned about four years ago. The original site was built in 1958 and had a Gates FM5B as the main transmitter. The electrical service consisted of two 200 amp panels which had been greatly altered over the years. It had an old Onan 65 KW propane generator inside the building. Grounding, Air Conditioning, lighting, and workspace were all substandard.
The first thing we did was replace the generator with an outdoor unit. That allowed us to remove an interior partition, freeing up a good deal of floor space. The next thing we did was upgrade the electrical service and replace the generator transfer switch. Much of the interior wiring had been altered or added to in non-code-compliant ways. All of those modifications were removed or bought up to the current electrical code.
A safety grounding ring was installed around the outside of the building and all grounding points were bonded together. Nautel has an excellent guide for transmitter sites which includes lightning grounding and protection for AM and FM transmitter sites. Recommendations for Transmitter Site Preparation (.pdf) and Lightning Protection for Radio Transmitter Stations (.pdf) are available for download from their site. All RF cable outer jackets are bonded to the ground at the base of the tower and the entrance to the building. All the interior equipment is bonded together. Ferrite toroids are placed on all cables going into and coming out of the transmitter cabinets.
With the electrical service upgrade, we added the series LEA surge protector.
Inside view of LEA surge suppressor
This site as at the very end of the utility company line and has always suffered from power issues. This unit greatly smooths out the various nasties that get sent our way.
I decided that it was easier to use compact fluorescent lights (CFL) rather than long tubes. This site is as the top of a rough mountain road and it is simply easier to carry several small boxes in the cab of the truck than four foot or eight foot florescent light tubes. There is a total of ten 28-watt fixtures in the main transmitter room which light up every nook and cranny.
WHUD transmitter
All of the transmitters and electrical panels were laid out to give working room around them.
The air conditioners were also greatly upgraded and added to the generator load. Prior to this, when the power went out, which was often. the air conditioners did not run and the transmitter room would overheat unless the door was left open. What we previously the filter room became space for tenant equipment. There are a few two-way and paging companies still at this site.
Of course, all this work was done while keeping the station on the air as much as possible. There were a few instances of having to turn off to move transmission lines and so on.
The result of all this work is greatly improved site reliability.
Sorry for the prolonged absence. I have been, quite literally, out of reach for the last two weeks. In fact, for the entire month of July, I spent just five days at home. Some of the travel was for work and some for pleasure.
On the work side of the equation, WVPS in Burlington, Vermont has a new Nautel NV-40 transmitter. WVPS is the NPR affiliate for Vermont Public Radio and it’s transmitter site is located on top of Mt. Mansfield, in Stowe, VT. I will do a separate article about the Mt. Mansfield transmitter site because it is an interesting place. WVPS is a Class C FM on 107.9 Mhz. They have one HD subchannel for the VPR classical music format.
The Nautel NV-40 transmitter is greatly updated from the V-40, which was installed at WHUD. Basically, the V-40 is four ten-kilowatt transmitters combined. It is a novel approach and offers quite a bit of redundancy as entire transmitters can be switched off and worked on with the other three remaining on the air at full power.
The NV-40 is a single large chassis with internal combining networks. It uses different RF modules but the same power supplies. The entire thing is controlled by a fancy GUI on the front of the transmitter but also has the ability for manual control if the GUI fails. That is a key feature not seen in other transmitters which simply won’t work without the fancy computer. Other things that I like, are the ability to control all of the biasing and other options via the GUI and things like a spectrum analyzer and Lissajous display. The ability to look at several graphic displays at once makes it easy to configure and monitor.
The transmitter arrived at the top of the mountain via a local moving company. After unloading it on the loading dock, it took some amount of doing to get it down the hall into the transmitter room. The thing weighs in at 1,600 pounds after being uncrated.
Nautel NV-40, Mt. Mansfield transmitter site loading dock
Unpacked:
Nautel NV-40 uncrated and read to move down the hallway
Moving into the final position in the WVPS transmitter room.
Movers putting transmitter into final location and removing pallet jack
The connections were made to the transmitter, including connecting grounding strap to the back, 200 amp electrical service and the RF output connection via 3 inch rigid coax.
Nautel NV-40 installed
The remote control consists of basic transmitter functions going to a dial up Gentner remote control and a Network connection going to the GUI. The network connection allows persons on the network to use a web browser to look at the GUI. The HD radio connections are made via a HD radio importer and exporter, located at the studio, which also uses the network, via a connection on the exciters, to send the HD subchannel. The analog main channel is via an AES/EBU connection from the STL.
All connections go through large toroids to help isolate the transmitter from any lightning-related surges.
Before I left, we tested it at full TPO into the dummy load. All worked well, the only outstanding issue was getting the HD radio importer/exporters to work over the network, which was out of my jurisdiction.
Author and Nautel NV-40
Here is a rather blurry picture of your author standing next to the NV-40 with the exciter and GUI turned on. There are to IEC power connectors at the top of the transmitter that go to the GUI and exciters. This allows those part of the transmitter to run on UPS’s, which is nice, being that the GUI takes about a minute to boot up after power failure.
Occasional reader Jeffery asks a good question, which I will attempt to answer here in simple terms. Phasing, when used with antennas, refers to the relationship that two or more radiating elements share with the waveform being transmitted. It is used to create an RF radiation pattern by adding energy to the wavefront in one direction by taking energy away from the wavefront in another direction.
Phasing is often described as +/- X number of degrees from a reference point. Graphically, it would look like this:
One wavelength with +/- 180 degrees notated
The reference point can be changed to any point on the waveform, in radio applications it is usually oriented around +/- 180 degrees. If the reference point is a single tower or element then this would be the end of the story. Add a second tower to this system and it would look something like this:
Double waveform
In this picture we have two waves being radiated from two separate elements. These elements are spaced 100 degrees apart and tower #2 is phased to +90 degrees. RF generator is coupled to both towers via a power divider, the reference tower (tower #1) is feed with 57% of the power that tower #2 is being feed. Thus, the ratio of power to the respective towers would be 57:42. Thus, if tower one had a power reading of 1.00, tower two would be 0.74. The towers are on a north/south line with the reference tower bearing 180° from tower #2. In the area of subtraction, the waveforms from each tower cancel each other out to some radiating less power toward the south; in the area of addition, the waveforms sum to create a more powerful waveform, radiating more power towards the north.
This is a typical two tower array, however, there are two slight differences; the reference tower is 215 degrees tall, tower two is 90 degrees tall. This is yet another use of “degrees” to relate electrical length or separations. The second, more notable distinction is that this array is Directional daytime, and non-directional night time, which is the opposite of most AM stations in this country.
Electrical height can also be described as a function of wave length, e.g. 1/4 wave, 1/2 wave, etc. Most AM towers in this country are 1/4 wave length, which equates to 90 degrees. Often, higher powered stations, and some low powered stations put up towers near 1/2 wavelength due to the better ground wave performance of those towers. At lower dial positions, a 1/2 wave tower becomes an expensive proposition due to the height required.
In theory, an unlimited number of towers can be used to create a pattern by introducing nulls (areas of subtraction) and lobes (areas of addition). In practice, the highest number of towers I’ve ever heard being used in an AM directional array is twelve; KFXR 1190, Dallas, TX. There may be others, too.
An excellent resource for AM directional antenna technical information is Jack Layton’sDirectional Antennas Made Simple, which is out of print but available from various sources.
is the signal wavelength (in meters),
is the signal frequency (in hertz),
is the distance from the transmitter (in meters),
is the speed of light in a vacuum, 2.99792458 × 108 metres per second.
is the power per unit area or power spatial density (in watts per meter-squared) at distance 
is the total power transmitted (in watts).
