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I would like to build an FM repeater using a short stack of Kenwood TK-890H rigs adjusted to 70cm USA amateur band, but the model has a 100W rating with a duty cycle of only 20%. 20% isn't nearly enough to keep up with modest repeater usage, even if I reduce the transmit power for longevity. Is there any open solution that would let me gang them together in a sort of "transmitter voting system", and have the software take turns using them (using a relay switch to swap one in as needed)? In this way the work could be divided up by round robin, or by thermistor voting, or whatever. (I realize a dedicated amp for TX is probably the most correct solution, but I already have these rigs and want to put them to use).

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    $\begingroup$ Welcome to HamSE and thanks for joining us. What benefit do you anticipate for the system you propose vs. simply combining the in-phase outputs from multiple radios operating at lower output power? $\endgroup$
    – Brian K1LI
    Dec 7 '20 at 9:14
  • $\begingroup$ @BrianK1LI I doubt the TK-890H has any mechanism to ensure they are all in phase. $\endgroup$ Dec 7 '20 at 13:57
  • $\begingroup$ @PhilFrost-W8II SPICE simulation of a (lossless) 2-way Wilkinson combiner shows about 0.1-dB difference in combined output power for two sources that differ in phase by 20 degrees. I agree there's no way to control the output phase of two off-the-shelf rigs, but I also can't think of how the same input to two rigs could cause a very large difference in output phase. $\endgroup$
    – Brian K1LI
    Dec 7 '20 at 18:14
  • $\begingroup$ @BrianK1LI These aren't just amplifiers, but whole radios that are going to take a baseband input and mix it with an independent LO. Temperature variation and differences in calibration will mean those LOs are at least some 10s of Hz apart, on a good day. $\endgroup$ Dec 7 '20 at 18:38
  • $\begingroup$ @PhilFrost-W8II You're right, of course, Phil. Hence the master oscillator(s) - LOs, XOs, etc. - that must be distributed to all components of a combined system. Thanks for the reminder. $\endgroup$
    – Brian K1LI
    Dec 8 '20 at 2:37
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It is likely that you can operate the TK-890H at 100% duty with better cooling. This would be a lot less work than the scheme you propose.

The Kenwood TK-890H has no fan - a good thing for mobile use, it's quiet and it will stay clean and last longer. But to keep it compact, the heatsink is much too small for continuous transmission.

My reasoning: The radio must however be able to tolerate a single long over without burning out or reducing power - at least 20 seconds of continuous talking. There is nothing in the electronics that can have such a long thermal time constant - the components on the PCB will heat up to full temperature in a few seconds, and the power transistors will be fully warmed up and dissipating into the heatsink. The only thing that is "slow" enough to be OK at 20 seconds, but not OK at 60 seconds, is the big heatsink itself. With a small heatsink, they're relying on the aluminium to sink all the energy from the transmission period, and dissipate it over a longer period. You can see that in the design too - it's heavy on metal, light on fins, so it can store more heat.

By adding a fan, you can dramatically improve the cooling effectiveness of a heatsink. Scratching on Digikey you'll find examples of heatsink thermal performance changing from 0.6 deg/watt (100 watts = 60 degrees rise, too hot) to 0.15 deg C/watt (100 watts = 15 degrees rise, OK) with forced air.

So I would say first test one radio with some fans blowing on it, through the heatsink in the best way you can think of. An array of fans would be better for tolerance of failure. I recommend small 10000+ RPM fans typically used in 1U servers as these move a large volume of air. They could be thermostatically controlled, or just use a timer, for the time it's transmitting plus 10 minutes. It might help to remove the case of the radio and have some air blowing over the PCB.

Finally, of course, the fans don't help if the air stays in a sealed box - they just make it worse by increasing the thermal load in the box. You'll need to have some sort of vent to the outside, with a filter to keep things clean inside.

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  • $\begingroup$ This is an interesting proposal and I'll definitely consider it, if I am unable to easily do W8II's suggestion to separate and combine the PA stages together, rather than using each rig's entire radio separately in round robin fashion. With sufficient and redundant cooling, and perhaps removing covers to vent the PCBs, how many rounds of typical consecutive 3 minute timeouts do you guess it could take before overheating? $\endgroup$
    – Jason W7DM
    Dec 13 '20 at 23:17
  • $\begingroup$ I'd hope you could run continuously, so at least 20 minutes, when all the temperatures will have stabilised. The only way would be to test it and see. You'll have to monitor the heatsink temperature, and ideally the transistors if thats possible (thermal camera?). Very likely the radio has a thermal shutdown to guard against overheating, from a long transmission in a hot cab. Eventually you'll need to test it with warm air, that it might encounter on a hot day. $\endgroup$
    – tomnexus
    Dec 14 '20 at 4:27
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Sure, I suppose it's possible. Any sort of "voting" seems overcomplicated to me. Simply have a timer that switches to the next radio every 30 seconds or so. If you have five of these radios and switch between them periodically, no one will have a duty cycle above 20%. If you have less than 5 radios then reduce power.

The complicated part I suppose is sequencing the switching. You need to:

  1. Stop transmitting on radio A
  2. Disconnect radio A
  3. Connect radio B
  4. Start transmitting on radio B

You need a little delay between each of these steps to wait for relays to react, etc. If you connect radio B while radio A is still transmitting, radio B's receiver will probably be blown out. If you transmit on radio B while radio A is transmitting, similarly bad things are likely to happen.

Of course there will be a glitch in the transmission every time a switch happens, but probably you can make it fast enough that it's not a serious problem. And I doubt you'll find any off the shelf thing to do this, so you'll have to build it yourself. The sequencing could be done by a simple microcontroller.

Alternatively, you could open the radios and extract their power amplifiers, operate each at a reduced power that could be sustained at 100% duty cycle, then combine their output power.

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  • $\begingroup$ This is one proposed approach to splitting up the TX cycles; another method could be to rely on a time-out timer that would take the active rig out of service (typically three minutes for many repeaters). I suppose it could be defeated by a 1:1 OM to rig ratio in a roundtable, and one of the OM doing all the talking while the others are just "yeah" "fine business" etc. BUT it doesn't answer my question, is there any existing repeater controller software that knows how to drive and divide multiple transmitters? or something open source that could be amended for the same? $\endgroup$
    – Jason W7DM
    Dec 13 '20 at 23:10
  • $\begingroup$ @JasonW7DM It's a pretty odd thing to do, so I'd be surprised if there was anything like it. But proving the nonexistence of a thing is difficult, and product recommendation questions are off-topic. $\endgroup$ Dec 14 '20 at 20:54

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