High Power FM Transmitter Fluid Heat Sink/Radiator Experiment

By: Tim McGuire
McGuire Broadcast, Inc.
Revised September 5, 2019



While working at many transmitter sites since the early 1980's a common problem experienced was the ability to control air quality, temperature and humidity. Some installations exhausted the transmitter air directly outside. This seems ok at first and initially saves on energy and site construction cost in the short term. However, there can be

long term problems with this type of setup. The transmitter is taking air in from the outside at an equal rate to its exhaust flow. This air can come in from anywhere, and is from the outside. Unless it is filtered and mixed with conditioned air, it is often humid and unfiltered when it reaches the transmitter. This humid air enters the transmitter and is heated by the final tube often passing low level sensitive circuitry and high voltage components along the way and slowly, over time, kills the transmitter with corrosion and resulting down time.

One method commonly used today to provide better air quality and temperature stability is the “closed loop system.” The “closed loop system” is costly in terms of energy used since it takes large amounts of energy just to cool the hot air back down to building temperature much less the building itself. This is typically done with large carefully sized air-conditioning systems employing variable speed air handlers low temperature cut off controls and other methods. Despite its energy intensive nature, this “closed loop system” is preferred by many engineers due to the elimination of humidity and dust into sensitive electronic equipment increased equipment reliability and a related long term broadcast equipment cost benefits.

While I am not an expert in the field of heating and air conditioning, I have designed and installed various mix boxes and other air circulation methods to attempt a reduction of a heat load that a typical broadcast transmitter places on existing air-conditioning equipment while trying to take advantage of often cooler air from outside. Some of these methods worked better than others.

The system I am about to describe uses liquid and is not unlike what is commonly used on high power UHF TV transmitters. This system is intended to be used on large tube type FM broadcast transmitters to reduce site energy consumption and to complement existing and well designed closed loop air-conditioning systems. This “Heat Sink” in effect, is just a radiator running backwards and sinking not radiating the heat. The system is based on what was learned from my previous trials and mistakes. It to, is experimental!


Experiment Objective

The conservation of energy by using commonly available parts and materials to assemble a simple and reliable system to move the high temperature heat exhausted from larger tube type broadcast transmitters outside and reduce heat loads to existing air-conditioning systems without the introduction of outside air to the transmitter air intake while at the same time, minimizing system cost permitting and environmental concerns of liquid cooling systems and potential spills by use of other methods.


Planning, Construction and Operation

If you have a 20KW or larger transmitter and this system makes sense for your station and enjoy the hands-on aspect of radio engineering, please continue doing so at your own risk! If you like to experiment like I do, you will find that the savings you get are proportional to: Your installed cost, the temperature differential between your stack temperature presented to the heat sink and the outside temperature presented to your radiator.

To save time and energy, measure and sketch your duct work and installation on routine trips to the site and think it through making sure location of the Heat Sink is at least 6-8 feet away from the transmitter exhaust with a 2-inch water ledge/barrier or drop in the galvanized duct in case of water leaks that would love to ruin your day yet high enough to safely walk under.

Design and build the Heat Sink core and duct support frame to fit your Heat Sink of choice. Include inspection doors near the transmitter exhaust to have access to the top of the transmitter and to check back pressure from the Heat Sink. Use self tapping screws to connect ducts, frame and Heat Sink core together. Mount the Heat Sink in a manner conducive to easy removal for maintenance or leak repairs as needed. Tape duct joints with aluminum tape not duct tape and apply mastic. Check the back pressure and stack temperature at the inspection doors. Install insulation duct board, tape, mastic and check back pressure and stack temperature. Run the PVC supply and return pipes to the outside radiator. The supply will be the bottom inlet and the return will be the top outlet which carries the heated water outside to the radiator. Insulate both pipes and seal the holes in the wall with a minimum three degrees drop to allow drainage during maintenance. Install the outside radiator in a shaded area with the pump just below it. Install the fan shroud and fans on the outside radiator. Install a power supply for the fans. Fill the system slowly with tap water at first with the pump on low and check for leaks until most bubbles are eliminated on the pump inlet. The heat should immediately appear at the outside radiator and get hotter as the water is heated. Operate the electric fans (at low speeds) to remove heat on the outside radiator using the excess heat to perhaps heat your diesel generator block if practical. Run the system for a few days to make sure there are no leaks than empty the system and add distilled water and coolant as needed for your climate.









Suggested, but not imperative materials list

I used two common three-core Ford and International Harvester vertical flow truck radiators from the 1970's made of brass and copper because they can be rebuilt in my area and will have a charmed life in a transmitter heat sink compared to a truck radiator, feel free to try others.


Use a low pressure radiator cap on the outside radiator around 6-psi and 10 or higher cap on inside (pressure is not a critical part just make sure they seal)

60-70 feet of 1.25" schedule 40 PVC pipe (hot water PVC pipe if you feel the need)

Rubber grommets (for PVC to radiator transitions)

Pipe insulation foam for 1.25 PVC pipe

Duct board insulation for transmitter to radiator ducts

Aluminum duct tape

Duct mastic to keep tape secured

Custom 20 Gauge galvanized steel ducts (to be field determined and made at your local metal shop)

A radiator core and duct support frame (sized to the radiator of choice and made at your local welding shop)

Use steel struts, threaded rods, and support hardware as needed to hang the ducts from your rafters.

Use a common two speed pool pump that can be operated on low speed. (These are available for around $250.00 including shipping)

Use two automotive 12 volt 12 inch radiator fans operated in series (at six volts) for the outside heat radiator. These are used since they are common, durable, and energy efficient using PM type motors. Summit racing or Jeggs has these.

1) 12 Volt 5A power supply.

Pocket thermometers for quick visual inspection of operation and calibration of remote sensors.

Use temperature sensors for the transmitter stack and after the heat sink radiator for temperature drop comparisons via your favorite station remote control system.



The system is currently in use at a site in North Florida. What is known is that the water pump uses less than 300 Watts to move the water and the fan motors running in series use 70 Watts of power to move the outside air across the radiator into a generator housing. This was done so the warm air from the radiator could heat the small steel housing the generator is located. This keeps the block heater running less. It’s estimated is that the system is removing 3-6 KWH of waste heat 24 hours a day and more during the damp cool evenings.

The system is by no means practical for all applications or locations and may even be more trouble than it is worth depending on your application. It is however, already showing promise at this location by a modest decrease in room temperature and decreased run durations on a second 5-ton AC unit. The installation of temperature sensors and remote metering are in process as of this writing. Over time it should be a worthwhile addition to the site. For conservation of travel and fuel cost, this experiment was planned and coincides with routine visits to the site.

General System Specifications

Overall system consumption: 370 Watts

Heat exchange fluid: Distilled water

Coolant: Propylene Glycol and Water

Water capacity: 10 Gallons

Estimated initial materials cost: $1000.00 to $2,000

Estimated initial labor time: 40-80 hours

Maximum estimated annual utilities cost of operation: $389.00 (based on 0.12 per KWH)

Estimated yearly power savings: $3,100-$6,300 (not including block heater savings)

Minimum Estimated yearly energy savings: $2,400.00


Spill Concerns

The system uses about 10 gallons of distilled water and coolant in a closed loop using the PVC pipe as storage. In the event of a leak the maximum spill will be just that. Make sure your location can take a water spill without any harm to your equipment. Use of a small spill pan under the inside radiator with a drain pipe to a storage tank outside is recommended. Consider placing all equipment on 2x6 pressure treated bases and secure.


What if the system pump or fans stop working?

If the system pump or fans stop working, the only thing that will change is your existing AC system will be presented with the original full heat load from your transmitter. (Use temperature sensors before and after the heat sink to monitor the system.)


System Efficiency

In a system design such as this, power consumption by the low speed pump low speed fans are relatively unaffected by operating temperatures. (Increasing pump/fan speeds will only use more energy and not significantly increase heat transfer) As a stack temperature increases, the temperature differential between the inside stack/radiator and the outside radiator ambient temperature increases. This results in an increasing efficiency ratio as the stack temperature rises.

If you have to use a higher TPO for HD radio operation or have lower final tube efficiency these are just the situations that will increase your final stack temperature! Good insulation on the duct leading to the inside heat sink radiator and the water supply and return coolant piping is a must to maintain the highest temperature differential and best overall efficiency. Remember, this higher the transmitter stack temperature, the better the heat sink works!


Just replace the transmitter

The replacement of an older tube final transmitter with a solid state transmitter would also save energy and eliminate the need for this system. However, the age of your current transmitter, pending changes in HD radio power levels, large capital expense versus long term savings for a particular station situation along with market forces would need to be carefully considered. In the meantime, this system could buy some time and reduce operating cost or, scare you enough with the water and used truck radiators to help you decide sooner.


What About my current AC system

Volumes can be written about AC units at tower sites. Here are some tips:

Fix door gaskets and caulk cracks inside of the building

Pressure clean, caulk, prime and paint the outside walls of the building using quality semi-gloss white paint. Many transmitter sites are concrete block and have minimal insulation. Using quality gloss white paint will greatly lower inside temperatures and save money!

Insulate the building attic

Use a white metal roof or white shingles.

Put up an aluminum carport on southern exposed walls for shading of exposed walls from direct sunlight or plant trees to shade southern exposed walls or roofs

Try to estimate the heat your equipment produces both constant and intermittent


Is my current AC system OK?

Run a minium of two split systems where each can carry the full load. Avoid wall/window units! Most of these units are energy intensive and seldom repairable. Other problems can be vibration, not starting after power outages, icing up, and the list goes on.

Get a good AC company that will discuss options and not just sell equipment/extended warrantees. If the estimator fails to at least ask you some very basic questions about the heat you equipment produces, use a duct sizing chart and make some basic load measurements for system sizing, move onto another company.

Ask the company where they plan to locate your outside condenser units. The outside condenser units should be shaded from direct sunlight as much as possible. If they say it does not matter, move on. (Sunlight increases corrosion on equipment shortening life expectancy.)

Higher SEER ratings can save money up to a point. Currently 13 SEER is the minimum.

Make sure the outside condensing unit is place on a concrete pad at least 18 inches above the ground to help avoid getting seeds and dust into the condenser coils. Keep the unit at least five feet away from other equipment, buildings or shrubs. The condensing unit should have hurricane straps to help discourage theft and keep it secured in high winds.

Keep all copper lines and ducts as short as possible ton increase efficiency.

Again, use two separate fully loaded systems. Each one should be capable of holding the full load. Staggered the temperature settings for the second unit to come on if needed. This will reduce short cycling and provide redundancy to save weekend overtime repair rates.

Discuss variable or 2 speed air handlers for small rooms to prevent short cycling.

Keep the outside condenser clean by washing several times a year.



Special thanks for advise and tolerance to:

K-County Radio Ocala, Florida

B&L sheet metal works Ocala, Florida

Butler Radiator Service Gainesville, Florida.

Bray Welding Ocala, Florida

Bertie AC Gainesville, Florida

Summit Racing, OH



McGuire Broadcast, Inc. 2019

Solar Shop Rebuild 2019 Before PV install