Hi Tom,
Yes, the broadcast 50 ohm load has a very low VSWR. Harris claims that with the 75 % RF efficiency, and only 25 % heat generation, it lets them to use air cooling. Individual new power supply for each module. I will have better information in a few weeks. 73 Bruce > ----- Original Message ----- > From: "Tom W8JI" <[email protected]> > To: "Bruce" <[email protected]>; <[email protected]> > Sent: Saturday, July 07, 2012 3:58 PM > Subject: Re: Topband: New Linear amp > > >> 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 >> > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
