All other things being equal, 75-ohm cable has less loss per foot than 50-ohm cable. This assumes that the cables being compared have the same diameter, length, operating frequency, etc. This is the primary reason that cable TV systems, telco inside-plant cabling, and other similar applications use 70-ohm cable. Maximum power handling is obtained by using 30-ohm cable. 50-ohm cable represents the approximate mean between lowest loss and best power handling.
- Jim, KL7CC On 10/26/2012 10:23 AM, Bob Stephens wrote: > What is the significance of using RG11 75 ohm coax vs 50 ohm? Sorry if this > is a dumb question. > > Bob AF9W > > > On Oct 26, 2012, at 10:39 AM, N5GE wrote: > >> All, >> >> There's a trick I learned from an antenna engineer acquaintance >> recently for tuning 450 ohm feedline's with a 1:1 balun with 50 ohm >> coax feeding the TX. >> >> Here it is: >> >> 1. Assemble three RG11 coax lengths. One coax 6 feet 7 inches long, >> one coax 6 feet long and one coax 9 feet long. From these three coax >> lengths you can test with four different lengths of 75 ohm coax; >> 6' 7", 12' 7", 15' 7" and 21' 7". >> >> 2. Beginning with the 6' 7" cable and connect the coax between the >> balun and the 50 ohm feed line, repeat the test below with each of >> the lengths above. Connect the cables with double female connectors. >> >> 3. If you have an analyzer (MFJ analyzers work fine for this) that >> gives the Rs, Xs and SWR go to step 4; If you don't go to step 6. If >> you have an AIM 4170 analyzer, you are in luck. The AIM allows you >> to do single shot analysis of a single frequency, displaying more >> information than you need for this test. >> >> 4. Record the Rs, Xs and SWR of each combination at the frequency you >> are interested in. A spread sheet makes this a little less tedious. >> >> 5. After recording the calculate the SWR the tuner will see with the >> following rules: >> >> Disregard Xs. >> >> If the Rs is greater than 50 divide the Rs by 50. >> if the Rs is less than 50 divide 50 by the Rs. >> >> The result will be the SWR the tuner is working against when >> searching for a match. >> >> In my case I am looking for a calculated SWR lower than 10:1 to help >> my KAT500 tuner find an acceptable match to my KPA500 and NVIS Loop, >> which works on all amateur bands for close in Non-DX rag chew >> communications. >> >> >> 6. You can still do the test, but you will need to do it by testing >> with the tuner for the best combination of matches at each frequency >> of interest. >> >> If you are going to do the tests, remember to use the MINIMUM power >> your tuner can tune with and allow time for your rig to cool down >> between tests. >> >> 7. When you have completed your tests measure the length of the >> joined test coaxes, including the double female connectors, and make >> a piece of RG11 that length. >> >> DISCLAMER: I'm not a real technical guy and probably don't know as >> much antenna theory as many of you, but what I have described worked >> for me. Your experience may not be the same. >> >> Amateur Radio Operator N5GE >> ARRL Lifetime Member >> QCWA Lifetime Member >> >> >> >> On Fri, 26 Oct 2012 05:42:20 -0700, Ron Midwin >> <[email protected]> wrote: >> >>> I recently bought an LDG AT600 Pro Automatic Tuner. >>> >>> I seem to be able to get a low VSWR on 80 & 40 meters, but on 20, 15, & 10 >>> meters the best it will do is ~ 1.8:1. >>> >>> My "G5RV" (It's really a Van Gordon 80 meter all bander fed with 450 ladder >>> line, 33 feet long, and then transitions to coax thru a 1:1 choke balun; >>> coax is ~ 6 feet long. >>> >>> Has anyone found a configuration that works better on all the bands? >>> >>> Previously I have been using an MFJ 962D manual tuner that is able to tune >>> down below 1.2:1 on all bands. >>> >>> AE6RH >>> K3 S/N 1997 >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
