February 13, 1998

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Back to the Mitreks. A subject dear to my heart is transmitter duty cycle. Mobile radios like the Mitrek do have large heat sinks on the transmitter and can take more abuse than conventional ham gear. Up to a point that is. In the old days (real old) ham gear was mostly AM on voice and was designed for what was called ICAS service. That is, Intermittent Commercial and Amateur Service. ICAS meant that the radio was designed to transmit for ten minutes and be allowed to cool for ten minutes. This explains why a 100 watt HF AM phone radio of 30 years ago was so huge! 

Todays radios are designed for a much lower duty cycle. Thus they can be a great deal smaller. The heat generated by the losses in the PA has to go somewhere. Radios with big heatsinks can transmit longer without damage than small radios with a tiny heatsink. Note that many of the really small FM mobiles now have tiny cooling fans on their equally tiny heat sinks! 

In practice a repeater is really a CCS device. That is Continuous Commercial Service. Just like a broadcast transmitter. The problem is that us hams are long winded and can just as easily talk three hours as three minutes. If your repeater is to survive, you have to build it for the worst case scenario. You have to be able to put a brick on the PTT button and walk away for a few hours. 

This means you have to think about your repeater transmitter in a different way. You have to think of it more like a piece of broadcast equipment (CCS!) than ham gear. If you don't..... Be prepared to replace the transmitter! 


The broadcast engineer handles the problem two ways. First components are derated. Often by 50% or more. Next great attention is paid to cooling. Not just the PA heatsink. The whole chassis is considered and particular attention paid to the power supply. 

Lets say for instance that you have a 60 watt Mitrek radio for your new repeater. First derate it by 50%. Connect your watt meter and dummy load and set the output at 30 watts. Than locate a accurate thermometer and attach it with some heat sink grease to the PA heatsink. Locate it as close to the final transistors as you can. 

Key the transmitter and monitor the temperature rise. Note that due to the components thermal resistance that the heatsink (or even the transistor case) temperature will be below the junction temperature of the PA transistors. The key thing here is that the transistors junction temperature should be kept below 150 degrees C. To be conservative I try to keep the heatsink below 130 degrees F. and kill the transmitter when the temperature reaches 150 degrees F. 

For repeater service you should have at least two fans on the PA fins. I would not use anything less than 50 cubic foot per minute fans and would prefer to have more powerful ones. Mount them right up close to the heatsink fins and blowing directly on them. 

Now back to our testing. When the heatsink temperature gets to 110 degrees turn your fans on. If the temperature continues to climb reduce transmitter power till it is stable at 110 to 120 degrees F. No more! If really conservative turn the power down another five watts and leave it there. This is what I do and it lets me sleep nights! 

I would NEVER let the fans run continuously! They do have limited life times and even on thermostatic control I have replaced a lot of fans over the years. There are two good ways to control the fans. The simplest and maybe the best way is to use the little Klixon thermostatic switches. You can buy these in a number of case styles and temperatures. They also come in normally open and closed configuration. The normally closed type are perfect for transmitter over temperature shut down. 

The other way is more complex. One disadvantage of the Klixon thermostatic switches is that they are difficult to mount tightly to the heatsink close to the PA transistors. I use a single IC fan controller based on a 8K thermistor and a single LM339 quad comparator IC. 

The thermistor is mounted with some heatsink grease right up close to the PA transistors. The fan on/off and transmitter over temperature on/off set points are adjustable by means of ten turn pots. There is a blinking LED for over temperature alarm and a shut down logic output to your controller. Next month I'll publish the design. 

What if you don't want to use fans? Well there is a way used by commercial two way shops that convert Mitreks to repeaters. They simply take the PA deck to a machine shop and have the heatsink fins milled off and than ground flat. The deck is than bolted to a HUGE heatsink. I've seen them as large as 19" wide by 12" high! This does work, don't ask about the cost. 


Pay attention to your power supply as well. The common Astron power supplies are ICAS rated to. Always derate them by 50% for repeater service and put a fan on their heatsink to. Try to duct some of the air into the Astrons case to vent it as well. The power transformer can get awful hot without some cooling help. While you are at it buy a set of spare regulator transistors, ballast resistors and a LM723 regulator IC. Stuff them inside your repeater for the day when the old Astron blows. This will happen! 

If you do all of this right your repeater will have a long and happy life. Keep in mind that heat damage occurs slowly as the radios components degrade. Transmitter power can fall off from partially failed emitter junctions in power transistors. Solder joints can oxidize or even melt and eventually fail. Trust me, I've seen a lot of PA's slowly die over the years. 

After your repeater is on the air it pays to check it out periodically Get a good rag chew going and check that the fans are pushing air not just spinning slowly. Feel the heatsinks, can you lay your hand on them? Check the transmitter power output and the SWR and log your results. Check the power supply voltage (this is important) both under load and when the repeater is off the air. 

While you are at it, clean the PA fins and the fans. Removing the dust increases cooling efficiency. Wiggle all the RF cables while the repeater is transmitting and listen for any noise on the receiver. Than check the connectors for corrosion. The important one is the one on the antenna feed line. Do all of this and you will have a cool happy transmitter this summer! 

David Metz, WA0AUQ

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