FM transmitter site design

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:

  1. 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.
  2. 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.
  3. 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.
  4. Good lighting.  Nothing is worst than fumbling around in a half-lit transmitter room trying to make repairs.
  5. 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.
  6. 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
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.

LEA series 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
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.

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3 thoughts on “FM transmitter site design”

  1. This post bring to mind a question I’ve had for a wile. What is long term economics of solid state plus aircon VS tube minus aircon?

  2. Always important to make sure the lights go off when you turn them off! Leaking RF can keep them on. A neon bulb can be lit by a 5W hand-held radio; a four-foot fluorescent bulb will light up if it is near a 100W transmitter.

    The failure current for the bulb is about 1000A. This is why the ballast was often built as an autotransformer or choke — to limit the current should the tube “short”. But the operation of the tube itself is a short; the gas inside is ionized by electricity, but physically contained so the ionized channel isn’t blown out like a spark in air. Ionizing gas ionizes more atoms around it, growing the size of the ionized channel until it becomes large enough for an upstream failure protection device to be triggered.

    Attach a bulb to a tower at one end and the ground on the other and you’ll find glass everywhere when it becomes a glass spark gap and the tower is hit.

  3. Looking for a recommendation. I see here that you installed an LEA Surge Protector. Would you recommend, instead, installing a power conditioner (like an Eaton EVR) if cost was no option, or is that going overboard? I am running a Harris Z10, and not much else. I do not have any protection at the moment, but I want to install something when we move to our new building this spring.


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