Hi Dave, The general view is that you need to reach from the antenna feed point to where ever you are going to match the open wire line to a 50 ohm conjugate match so that the transmitter sees a 50 ohm resistive load (or close to that). So the short answer is ... yes ... you can cut it shorter.
One additional consideration is that depending upon the lowest frequency you will be using, you may find certain lengths of the feed line to more easily be matched. There have been several similar articles over the years in such sources as the ARRL Antenna Compendiums which describe certain lengths of the dipole vs. certain lengths of the feedline that tend to match more easily. You would need to have more information on the length of the dipole portion to decide on the best length, assuming you have some choice in the length. As an example, I used the sizing recommendations of W6JJZ in his ARRL Antenna Compendium, Volume 2 Article "The Suburban Multibander," to make an antenna that would work on 160. His antenna was primarily designed for 40 and 20 with "near misses" at 80 and 10 meters. The design he uses is quite useful for two main bands because the sizing of the flattop and the open wire feedline is such that the impedance at the antenna feedpoint is around 190 + j740 ohms on one band and ~ 190 - j170 ohms on the other. The feedline then transforms this impedance to something close to 50 + j0 ohms for each band. This requires a specific antenna length of 88.8 feet for his design and a feed line of 46.2 feet x the V.F. of the open wire line. The V. F. (velocity factor) should be just over 90% of the speed of light. As you can see for this design he used about 42 feet of feet line. At that point he could use a 1:1 choke balun and continue from there with 50 ohm coax. What I did was double everything, so my flat top was around 180 feet and the feed line around 84 feet. This is about as short an antenna tolerable for 160 meters but still will not be optimum, but it would be quite good for 80 meters. The feed line worked well for me as it was also the right length to terminate with a 1:1 balun just outside the hamshack and I could finish the rest of the run into the shack with coax and that made it easier to take care of lightning protection. What I would try is to get your flattop portion or inverted vee configuration in the best possible location for your site and then I would cut off the open wire line at where it was convenient to do so. There are so many variables with these kinds of installations so you just have to put it up and see how it tunes up. There are cases where certain lengths on certain bands will be so highly reactive that you might have a difficult match with very high voltages (and high losses) in your tuner. I know of cases where additional open wire line was switched in and out but personally I would just tolerate the losses in the tuner:) After using open wire lines, off and on over the years, I have gone back to running mostly resonant multiple dipoles with one for 80 and one for 160 fed at a common point in an inverted vee configuration. I have a multiband vertical with about two dozen radials (70 to 100 feet) to take care of the upper bands. I have found that the vertical generally outperforms inverted vee dipoles at 30 feet at the apex for bands 40 meters and higher. The exception is the upper bands, particularly 6 and 10 meters which do not work that well on the vertical. 73, Rick, KV9U Dave wrote: >I know this is not an antenna forum, but hoping that all the experience >represented here can assist me. > >When using a dipole antenna fed with 450-ohm ladderline, does the >length of the feedline matter? The dipole I'm going to order comes with >100 feet of ladderline, but I'll likely only use 40 or 50. Is it safe >to cut off the length not needed, or will this mess things up? If I >can't cut it, what do I do with it? > >Thanks in advance & 73 >Dave >KB3MOW > > > Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
