Phil, 100 feet of coax (RG-8X) has less than 1 dB loss at 40 meters - RG-213 is less loss, so I recommend using coax unless you have the patience for properly supporting a balanced feedline. What I am saying is that you will not gain very much with lower loss balanced line. Be aware when using the popular window line that it gets lossy when it is wet, in fact some have reported more loss than small coax.
You should have less than 2:1 SWR over the entire 40 meter band if you pick the center frequency correctly. 73, Don W3FPR On 1/1/2012 12:13 PM, Phil Hystad wrote: > Don, > > goals: > > 1. Lowest loss. > 2. Full 40-meter band coverage but resonant at 7.1 MHz. > 3. Assume using a tuner (like K3 or KAT500). > 4. Choke common mode currents. > > > Total feed line length will be on the order of 100 feet for this particular > dipole. For the 30 meter dipole I also plan to put up, total feed line > length is about 75 feet. My 80 meter dipole will be about 100 feet of > feedline. > > Before I posted this question to this forum, I would have gone ahead and used > the same configuration I used previously when I had my 80 meter doublet up. > That antenna was fed by 450 ohm ladder line to a 1:1 balun and then RG-8X to > the shack. > > But one motivation for my question is weighing the difference in loss of > tuner + 450 ohm ladder line and balun versus a direct run of RG8X right to > the feed point with a choke balun at that location. > The RG-8X loss is about 2 dB on that 100 foot run. I could switch to LMR-400 > (at 0.7 dB loss) but I am curious about other solutions. > > 73, phil, K7PEH > > > On Jan 1, 2012, at 5:39 AM, Don Wilhelm wrote: > >> Phil, >> >> What do you want to accomplish (other than just an antenna for 40 meters)? >> Lowest loss? Broadband? Easy- up? Etc? Those parameters will determine your >> best choice. >> >> Easiest IMHO is to just feed it with coax with a current mode choke at the >> feedpoint. That should present a less than 2:1 SWR over the entire 40 meter >> band. >> >> If you have an extremely long feedline (multiples of a half wave), then you >> might consider using balanced line cut to some multiple of a half wavelength >> (electrical length) and the rest of the way to the shack with coax - put the >> common mode choke at the transition between the balanced line and the coax. >> This latter suggestion takes advantage of the fact that an electrical half >> wave transmission line has the same impedance at both ends - it does not >> matter what the characteristic impedance of the line may be. >> >> But unless you have a very long distance to the antenna feedpoint, the loss >> of practical coax runs is rather low on 40 meters, so the easiest is just to >> use coax. >> >> 73, >> Don W3FPR >> >> On 1/1/2012 12:14 AM, Phil Hystad wrote: >>> OK, lots of discussion on open wire feeders and other ideas so I thought I >>> would throw out a question to soak up some of the Elecraft community >>> knowledge and wisdom. >>> >>> Given a 40-meter dipole antenna, up about 50 feet, what is the best feed >>> configuration for single band usage to minimize loss and minimize common >>> mode currents and radiation from the feed line? >>> >>> I am thinking of building such an antenna and I would have no plans for >>> using it on any other band but I would like to use it on the full spectrum >>> of 40 meters with primary focus on the low end for CW. >>> >>> My current idea is to cut the antenna for 7.1 MHz but I am not sure about >>> the best plan to feed it. I am thinking of 450 ohm window line (aka ladder >>> line) to a 1:1 balun and the coax the rest of the way into the shack. Or, >>> how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way >>> into the shack? Would this be a significant difference? Or, some other >>> combination? >>> >>> Or, maybe a 1:1 balun right at the antenna feed point with coax the >>> remaining distance to the shack? >>> >>> Any comments or suggestions? >>> >>> I am thinking of also raising up two other dipoles. I have room to put up >>> 80, 40, and 30 as separate dipole antennas -- it is actually a little bit >>> more awkward to do a fan dipole so I am leaning on not doing that. >>> >>> 73, phil, K7PEH >>> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > 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:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
