[BC] Re:1 5/8 foam coax

[Xmitters@aol.com]

In a message dated 2/19/07 2:15:11 PM Central Standard Time, 
broadcast-request at radiolists.net writes:

<< LDF7-50A is rated by Andrew with a 315kW peak power maximum.? The power 
max for 50kW carrier at 125% modulation is a little over 250kW; not a lot of 
safety margin, but, at least in theory, it will take it, at least temporarily.? 
The average power rating is 30.9kW according to Andrew catalog 38.? The next 
size up (and the largest LDF cable) is 2-1/4; it has a continuous average rating 
of 39.8kW and a peak of 425kW.? To get the safety you need and want you need 
to go to air dieletric 3 inch, which has an average power rating of 81.9kW.
 --
 Lamar Owen
 Director of Information Technology
 Pisgah Astronomical Research Institute
 1 PARI Drive
 Rosman, NC? 28772
 828-862-5554
 www.pari.edu
  >>

Lamar:

You have to remember that the 250 kW peak power that you mentioned is peak 
Envelope Power, which is defined as the equivalent unmodulated power that has 
the same voltage as the amplitude modulated carrier at peak crest of modulation. 

Peak Power in the general sense (assuming sine wave voltage waveform and 
unity power factor) is twice that of the average power. If I was doing this 
calculation, I would double the PEP (253 kW) and arrive at the required peak power 
handling ability of the feedline as 506  kW. Even at this figure, we have not 
considered  the required headroom for possible VSWR excursions for this system 
yet. All of the 50 kW AMs that I have seen/worked on, either use three inch 
rigid coax or four inch andrew air dielectric. 

One poster here responding to this thread mentioned filling the line with 
freon as a possible solution. I thought that the original post fas about foam 
filled line. At any rate, would freon really help? I would be tempted to use SF6 
(Sulfer Hexaflouride) assuming that there are no environmental or handling 
complications. 

It seems to me that it would be a lot easier to simply use the correct sized 
line rather than taking the risk on smaller line? Feedline should last you 20 
to 25 years or more so the amortization costs for the proper sized line would 
not make it worth it going on the cheap. 

Anyone here using 1 5/8 foam feedline for their 50 kW AM plant ? If so, you 
have more guts than I have. 

Jeff Glass, BSEE CSRE
Chief Engineer
WNIU WNIJ