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On Thu, 30 Jun 2005 00:22:48 GMT, james wrote
in : On Wed, 29 Jun 2005 22:45:03 GMT, "Tom Donaly" wrote: james wrote: On Wed, 29 Jun 2005 11:07:17 -0400, "Fred W4JLE" wrote: What is the reason a 2:1 SWR can cause such havoc? How can I avoid this catastrophic condition? I feed my dipoles with 450 Ohm ladder line, but the last 20 feet or so is 50 Ohm coax, I guess that makes it work ok. I haven't blown up my finals yet. Lions and tigers and bears Oh my... ***** Actually can happen if you push the finals to where there is insufficeint margin to the maximum heat dissapation. Tubes are a bit more forgiving. Transistor inadequately heatsinked and overdriven, typical CB usage, often have little of no margin for heat dissapation. If the transmitter has a refelction coefficient of zero and the load say .3, then that reflected power from the load is dissapated as heat in the output circuits and any final transistors or tubes. Now if the radio has a reflection coefficient other than zero that will lessen the heat dissapation on the transimiiter. Now you get load and source reflections convoluting within the transmission line. You ought to model a 400 Mhz square wave with source and load refelctions coefficients other than zero. It can get ugly james Consider the MRF 140, a 150 Watt 2.0 - 150.0 Mhz N-Channel linear RF power fet. From the technical data sheet: "100% Tested For Load Mismatch At All Phase Angles With 30:1 VSWR." You'd have a tough time damaging this device with a mere 2:1 VSWR. How do load and source reflections convolute within the transmission line? That's a new one on me. My old dictionary defines 'convolute' as "Rolled or folded together with one part over another; twisted; coiled." The rest of the post is pretty fanciful, too. A trip to the library would do wonders. 73, Tom Donaly, KA6RUH ****** In electrical engineering it is the instantaneous power density of two signals passing at the same spot from two directions. That is called Convolution. I'm an EE and I've NEVER heard the term used in reference to electronics, let alone used to describe standing waves. It also is a nice mathematical means of modeling SWR at any point on a transmisison line at a particular time. I know how to model standing waves, but how do you model a standing wave ratio? Well if you knew CBers, they are not satidied getting 150 watts from a transistor rated for 150 watts. But in my first paragraph I thought I made it clear but evidently I did not. I guess I must strive to better explain myslef. Agreed. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |