XPost: sci.electronics.design, sci.electronics.misc   
   From: spam@spam.com   
      
   On Sun, 10 Mar 2013 08:15:56 -0400, "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.   
      
   No, not at 10kW. At 100W or so it is fine. On a long run I would   
   always opt for waveguide - tower dimensions permitting.   
      
   d   
      
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