FW: The use of a doublet
Locked
 
|
I was asked why 90 feet of 300 ohm ladder line by the group. Ron of DX Engineering responds below. Philip LaMarche LaMarche Enterprises, Inc. www.instantgourmetspices.com www.w9dvm.com 800-395-7795 pin 02 727-944-3226 FAX 727-937-8834 NASFT 30210 K3 #1605 CCA 98 00827 CRA 1701 W9DVM -----Original Message----- From: Rod Ehrhart [mailto:[hidden email]] Sent: Tuesday, July 14, 2009 2:46 PM To: 'Phil LaMarche' Subject: RE: [Elecraft] The use of a doublet Hello Phil, Thanks for writing. **** One of the wire antenna configurations that works best for multiple band coverage with a single wire is a ladder line fed non-resonant dipole. For impedance reasons, it is preferable to use a slightly shortened dipole instead of a resonant dipole, in relation to the lowest band of operation desired. You can operate at high power across the HF bands, if this doublet or dipole it is installed with a certain length of ladder line (not a tuned length), a high quality 1:1 current choke balun (DXE-BAL050-H10-AT), and a short run of RG-213 coax to a Wide Range Antenna Tuner with a roller inductor and large air variable capacitors. This system is described by Tom, W8JI, in DX Engineering's "Choosing the Correct Balun". The ladder line length becomes important when it is connected to a non-resonant balanced dipole being used for a wide-band or multi-band application. Using an odd multiple of an electrical, velocity factor corrected, one-eighth wavelength of feedline on the lowest frequency of operation will provide a tunable impedance at a lower SWR to the wide range tuner being used for this antenna system. For example, install an 80 through 10 meter antenna of a total length of 110 to 120 feet, with 300 ohm, 0.88 VF ladder line, with a length of around 31 or 93 or 155 feet, which represents one-, three- or five-eighths electrical wavelengths. This antenna will tune more easily on all bands, especially ALL of 80 meters! However, if one were to use 60 feet of ladder line, then high voltages could develop in the tuner, since that is an electrical one-quarter wavelength for reflected energy. This has been proven to cause difficulties in tuning and erroneous meter readings. Avoiding ladder line lengths in multiples of 1/4 wavelengths electrically is the goal. To make a great 160 through 10 meter antenna system, simply double the footage figures above. This antenna system really does work well! Try one and you will see. **** Let us know if you need any additional assistance, Phil. Thanks again and 73, Rod Ehrhart - WN8R DX Engineering Customer Support [hidden email] Orders 800-777-0703 Tech Support 330-572-3200 www.dxengineering.com -----Original Message----- From: Phil LaMarche [mailto:[hidden email]] Sent: Tuesday, July 14, 2009 12:37 PM To: [hidden email] Subject: FW: [Elecraft] The use of a doublet I put this on the Elecraft reflector. Still waiting for my missing Signal Link CD. Phil Philip LaMarche LaMarche Enterprises, Inc. www.instantgourmetspices.com www.w9dvm.com 800-395-7795 pin 02 727-944-3226 FAX 727-937-8834 NASFT 30210 K3 #1605 CCA 98 00827 CRA 1701 W9DVM -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Phil LaMarche Sent: Tuesday, July 14, 2009 12:32 PM To: 'dw'; 'Elecraft_List' Subject: Re: [Elecraft] The use of a doublet DX Engineering shared an antenna with me that I put up and it works wonderful on all bands with a tuner. 120 ft and fed with 90 ft of 300 ohm line into a 1:1 balun of theirs and into the tuner with RE 213. Tuner is a Palstar AT2K. The feed line length must be 90 feet. YES, it really works. Phil Philip LaMarche LaMarche Enterprises, Inc. www.instantgourmetspices.com www.w9dvm.com 800-395-7795 pin 02 727-944-3226 FAX 727-937-8834 NASFT 30210 K3 #1605 CCA 98 00827 CRA 1701 W9DVM -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of dw Sent: Tuesday, July 14, 2009 12:26 PM To: Elecraft_List Subject: [Elecraft] The use of a doublet Great post Rick! Yes, the doublet has been over the years for me the easiest and best performing all band single antenna I've ever had. I feed mine with 450 also, but the top is only 135' long. I use the more robust SGC auto-tuner in external config. 40 feet of good quality coax and a high quality balun feed the tuner. With 20 watts applied, it finds best match in 3 seconds. It makes me feel like I'm cheating!! ;^D Duane N1BBR Hello Jim, I saw two of those tape measures once and they were actually part of a military dipole antenna system. The original antenna had 2 of them and you just pulled them ou to the coresponding frequency measurements on each side and you had a resonant dipole for that frequency. So if your friend used that to measue the wire with, then its most likely somewhere around 33 feet on each side. Being fed with 300 ohm balanced feedline makes your antenna a doublet - and a doublet is a very good all band antenna ! I use a doublet here that is 178 feet long - 89 feet per side and fed with 450 ohm feedline and it works 10 thru 160 meters via the tuner. 73 - Rick McKee, KC8AON Southern Ohio - EM88sn www.angelfire.com/electronic2/qrp With God all things are possible ~ ><(('> ~ -- [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] 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:[hidden email] 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
|
|
In the Book one studies for the Amateur Extra license are tables of what occurs to voltage and current in the feedline. The graphed waves of voltage and current in the feedline are different for coax compared to ladder line. The graphs have tick marks every 1/4 wavelength of the frequency in use - and it is at these ticks where peaks and lows of voltage and/or current occur.
Thus you will also find tables at dxengineering.com recommending particular lengths of feedline for various bands. In addition I and many others have pruned the feedline (easy for ladder line) to obtain the best SWR. I have even seen on the internet a simple Antenna-TUNER project where one switches in various lengths of feedline using DPDT switches to find the best swr. One switch adds or takes out an 8 ft length of ladder line, another switch adds or takes out a 4 ft length of ladder line, another switch adds or takes out a 2 ft length of ladder line, and another switch adds or takes out a 1 ft length of ladder line. The vast majority of people just use a random length of feedline - whatever length was needed for the particular antenna installation. Last year I put up a Cactus-Super-J-Pole 2meter antenna for my winlink rms packet station. I just couldn't get a good SWR until I added an additional 8 ft of coax cable. (I just happened to have an 8 ft jumper laying around). To some, this is amazing magic. Howard |
|
|
In reply to this post by Phil LaMarche-2
CONTENTS DELETED
The author has deleted this message.
|
|
|
Phil,
Ron speaks truth! The "magic" is that the feedline acts as an impedance transformer, and open wire line (or dry twinlead) will operate at high SWRs with negligible loss. We have formulas for quarterwave (or odd multiple) transmission line sections, but not often mentioned is that the transformation for 'in between' transmission line lengths can be beneficial to provide a decent feedpoint impedance for the tuner. The relationships for the ham bands will be such that often a compromise can be found in the transmission line length so it can be matched by commonly available tuners. Ron's homebrew link coupled tuner can match most any impedance efficiently - and I will wager that it has plug-in coils to cover the various bands, but the same is not true of many commercially available tuners - particularly the popular T match. The T match tuner can have settings that do nothing but 'heat the tuner' while indicating a low SWR at the input. The net result is that certain antenna lengths give rise to specific antenna feedpoint impedances for each of the ham bands, and the transmission line electrical length will transform that inpedance to some other value - with a lot of judicious calculations, one can come up with a combination of lengths that will provide a range of (for example) 25 ohms to 1000 ohms at the end of the transmission line, and most available tuners can handle that impedance range with low loss in the tuner itself. The antenna and parallel feedline is inherently low loss (when the 300 ohm feeder is dry). 73, Don W3FPR Ron D'Eau Claire wrote: > Tnx Phil. > > Note that using the lengths do not improve the performance, only the > impedance excursions involved and they can be handled by a wide range tuner > quite nicely. > > That said, some tuners show excessive losses at extreme impedances even > though they may show a 1:1 SWR on the coax link to the rig. I use a homebrew > fully-balanced link-coupled tuner with my doublets that I have considerable > confidence in having low losses across its range and the ability to match > everything from a fraction of an ohm to many thousands of ohms. Using many > "commercial" tuners I'd be much more concerned. > > The ARRL Handbooks have a table showing the combinations of wire length and > feeder length that produce the least extreme feed point impedances as well. > > Ron AC7AC > > -----Original Message----- > > I was asked why 90 feet of 300 ohm ladder line by the group. Ron of DX > Engineering responds below. > > > Philip LaMarche > LaMarche Enterprises, Inc. > www.instantgourmetspices.com > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ------------------------------------------------------------------------ > > > No virus found in this incoming message. > Checked by AVG - www.avg.com > Version: 8.5.375 / Virus Database: 270.13.13/2237 - Release Date: 07/14/09 05:56:00 > > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
|
|
In reply to this post by Phil LaMarche-2
--- On Tue, 7/14/09, Ron D'Eau Claire <[hidden email]> wrote: Note that using the lengths do not improve the performance, only the impedance excursions involved and they can be handled by a wide range tuner quite nicely. That said, some tuners show excessive losses at extreme impedances even though they may show a 1:1 SWR on the coax link to the rig. I use a homebrew fully-balanced link-coupled tuner with my doublets that I have considerable confidence in having low losses across its range and the ability to match everything from a fraction of an ohm to many thousands of ohms. Using many "commercial" tuners I'd be much more concerned. And well you should be. In the time leading up to the publication of my "ladder line" paper (see another post in this thread) I had considerable correspondence with Dean Straw, the ARRL antenna guru and editor. I introduced the idea that tuner losses could be much higher than amateur folklore assumed. While none of this was ever published (it may be more than coincidental) that shortly thereafter there began to be articles published on the subject of tuner losses (several by Frank Witt) and Dean's own software began to feature the capability of calculating those losses. I had also pointed out in that correspondence that the popular T-network tuner with three variable components could be easily (more than likely) mis-adjusted such that a match was achieved at a less than optimum condition as far as losses were concerned. This has since been addressed in other articles. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
«
Return to Elecraft
|
1 view|%1 views
| Free forum by Nabble | Edit this page |