Hi Bruce, > The new Harris Flexiva 10 KW FM band (not pulse) stereo transmitter is > not > having a heat problem with air cooling, in a reasonably small package . It > has individual modules that can be hot switched.
I'm sure Harris did a great job. The modules can be swapped while the TX is on, even power supply modules, which is a nice feature. But hot or live swapping does not mean the modules are "hot switched", as in allowed to have no load while amplifying. Generally a module is designed to remove power first when pulled, and apply power last when plugged in, which is nothing at all like some ham transmitting with an arcing tuner, or without a feedline. You will also note Harris protects the modules, which is what everyone should do. VSWR ratings in data sheet are really useless marketing hype, because near full power in longer duty cycle modes, as little as a 2:1 VSWR can blow a device out. My point is about Ham gear use where people are thinking they do not need to watch SWR or other parameters. There is little worry in a commercial transmitter that runs the device at 600 watts or less per device with adequate protection for SWR and thermal issues, and most likely current limiting. We should not translate what Harris did into something with no thermal protection, no SWR protection, and twice the power on the device!! > The Harris Salesman, Engineer, Ham, said there was already a W2 using a > pair, but had a small water cooled system. I played with a thick copper heatsink and air cooling, but concluded it was too expensive, and it would be a manufacturing PITA to get the surface machining flat enough in production. With 1000 watts of heat in such a small footprint, the surface almost has to be polished flat. The copper spreader would have cost about $30-40, plus the aluminum heatsink would need to be reasonably flat all across the surface. I'm not even sure that would work without liquid cooling. It makes sense hearing the W2 used liquid cooling. At current costs for cooling and the devices, it is better to use more lower power devices and spread the heat around. Especially when the devices are just as reliable, have a history of good SSB IM performance, and cost less per watt. My guess is the marketing blitz and creative specs are why we find a few amateur "soon to be released" products without any final specifications. They probably believed a single 1200 watt device would be linear at 1200 watts, and would actually handle high SWR without protection. Anyone who has actually worked with high power RF semiconductors knows the VSWR specs are meaningless marketing drivel. Like I said, I ran a pair of MRF150's in pulsed service, in deep class C, and they were safe and reliable at a kilowatt without any SWR protection, other than power supply limiting. Because they worked in pulse service deep class C in a low-Q system at 1000 watts without worry doesn't mean 2x MRF150's would work on CW at that power, or be reliable at 200 watts CW without SWR and thermal protection. Apples and oranges. When the device price comes down a bit, or if they make a smaller device that allows spreading the heat around, you'll probably see the Freescale device in Ham gear. 73 Tom _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
