XPost: sci.electronics.design, sci.electronics.misc   
   From: presence@MUNGEpanix.com   
      
   In sci.electronics.misc Michael A. Terrell  wrote:   
   >   
   > Don Pearce wrote:   
   >>   
   >> On Sun, 10 Mar 2013 04:21:03 -0400, "Michael A. Terrell"   
   >>  wrote:   
   >>   
   >> >   
   >> >upsidedown@downunder.com wrote:   
   >> >>   
   >> >> On Sun, 10 Mar 2013 02:43:08 +0000 (UTC), Cydrome Leader   
   >> >>  wrote:   
   >> >>   
   >> >> >>  Not many folks making hard coax runs anymore.   
   >> >> >   
   >> >> >this stuff was pretty darn old.   
   >> >> >   
   >> >> >are there power levels where they stil use wavegides and the like?   
   >> >>   
   >> >> In countries still using analog TV, the UHF final amplifier is often   
   >> >> implemented with klystron in the 100 kW range. The waveguide is quite   
   >> >> large, due to the low frequency.   
   >> >>   
   >> >> DVB-T digital TV transmitters typically operate with only 1-10 kW,   
   >> >> consisting of multiple redundant solid state modules, so there is not   
   >> >> much need for waveguides any more.   
   >> >   
   >> >   
   >> >How do you get that to the antenna without waveguide?  Coax losses are   
   >> >much higher than waveguide, and is less likely to have problems since   
   >> >there is no dielectric to break down.   
   >>   
   >> Andrew's Heliax is pretty low loss, and good for VHF and UHF runs.   
   >> Dielectric amounts to nothing more than a thin spiral spacer - the   
   >> rest is all air.   
   >   
   >   
   >   Heliax for 10 KW?  Ever had the filter fail on the compressor and get   
   > water in Heliax?  I had a stupid SOB for a boss 30 years ago who was too   
   > sheap to replace filtes on schedule and ruined a piece of 3" Heliax used   
   > at 4 GHz.  Waveguide is better at high power, and better than Heliax.  I   
   > had over 1700 feet of it at one TV transmitter.  It carried about 195 KW   
   > of RF to the top of the tower.  We had to maintain a set pressure of dry   
   > nitrogen on the waveguide to keep from compressing the sync pulses.   
      
   what exciting things happens when you get moisture ingress?   
      
   how hot can coax or waveguides run at high powers? I've never been by   
   large transmitters, so the concept of anything but a power cord running   
   warm is just strange to me.   
      
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