|
On Thu, 21 Apr 2005 10:56:46 -0700, Roy Lewallen
wrote: Some years ago, John Belrose VE2CV published an article in QST about off-center fed antennas, based on a combination of modeling and building. While they look attractive on a simulation program (where the feedline wasn't also modeled), he found that in practice it was very hard to keep the feedline common mode current down to a low value. The consequence of common mode current is that the feedline becomes part of the antenna, and that makes it very difficult to duplicate or to realize the idealized performance you might expect if you hadn't taken feedline radiation into account. Multiple current baluns should be able to reduce the common mode current to a reasonable value, but they'd have to be used unless you want a lot of feedline radiation and performance that's difficult to predict. Roy Lewallen, W7EL A few years back I put up an OCFD. Feed it with open line. Seemed to work pretty well and behaved itself running less than about a 100-watts. However, when I increased my power to above 500-watts I was able to work every electrical and electronic appliance in the house! The common mode on the transmission can be a killer! Danny, K6MHE |
Nothing changes much. The radiation pattern will favor the long side of the antenna. Since the currents are unbalanced there may be an abundance of feedline radiation. Then, would the center feed be the best way to go? -- 73 for now Buck N4PGW |
Then, would the center feed be the best way to go?
For sure. I'd rather center feed, and have to use a matching device, than use OCF, and not have to use a matching device. The matching device can be as simple as a series transformer made from 75 ohm, or whatever coax. The feed of a 3/2 wave dipole is quite close to that of a full wave loop. Usually higher than 50 ohms, if clear of the ground. The same series transformer matching scheme will usually work for either one. Say if you were on 21 mhz, and the SWR was a bit high. Say 100 ohms feedpoint impedance, "more common with antennas high off the ground", you could add 7.6 ft of 75 ohm coax, from the end of the normal 52 ohm feedline, to the feedpoint of the antenna. That would give you a good match. Might mess you up on 40m a bit though....I'm not sure the exact effect it would have on the lower "1/2 wave" frequency. Maybe not a whole lot...I guess you could switch it in and out with a relay...BTW....Here, I don't have a very good match on 15 using my 40 dipole, but it's probably cuz I have 160/80/40 on one feedline...I just slap the tuner inline if I want to work 15.. I have used a series transformer on a 3/2 dipole on 20m....It worked...It was one way I could run a EDZ type antenna, but easily feed with coax. But...I used .75 wave per side, instead of the .64 wave per side of the normal EDZ. That tunes out the reactance, and all you need is the transformer. Sometimes, you don't even need that as many 40/15 dipoles users know.. MK |
A 3/2 wave fed at 1/6 or 16.67 % off one end show low fairly Low SWR's
at the follow frequencies if the wire is cut to 3/2 W @ 7 MHz 7MHz , 9.25 mhz, 11.75mhz , 21.25 MHz (about 7:1 into 50 ohms) and 26 MHz .. These are if fed with 50 ohm line. . Cheers 73 Dave kc1di |
Buck wrote:
Nothing changes much. The radiation pattern will favor the long side of the antenna. Since the currents are unbalanced there may be an abundance of feedline radiation. Then, would the center feed be the best way to go? Balanced center-feeding a balanced antenna will tend to minimize the common-mode currents but some antennas, like the Carolina Windom depend upon feedline radiation for their radiation patterns. What are you trying to accomplish? -- 73, Cecil, W5DXP ----== 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 =---- |
On Fri, 22 Apr 2005 10:17:55 -0500, Cecil Moore
wrote: Buck wrote: Nothing changes much. The radiation pattern will favor the long side of the antenna. Since the currents are unbalanced there may be an abundance of feedline radiation. Then, would the center feed be the best way to go? Balanced center-feeding a balanced antenna will tend to minimize the common-mode currents but some antennas, like the Carolina Windom depend upon feedline radiation for their radiation patterns. What are you trying to accomplish? When trimmed to SWR, I get less than 1.5:1 on fifteen meters. My plans are to build a multiband antenna, parallel elements, where each band is 3/2 wave dipoles and fed with 50 or 75 ohm coax. I haven't tried this yet, but I would hope I can have a few gain dipole antennas in a multi-band format. -- 73 for now Buck N4PGW |
I presume you realize that a 3/2 wave dipole has multiple lobes, with
nulls between. At many or most angles, a half wavelength dipole has greater gain. You can get just two narrow lobes from a 3/2 wave dipole by rotating the wires 30 degrees to form a horizontal "V" having a 120 degree included angle. I've used this trick with a 40 meter dipole being used on 15 meters -- it doesn't have much effect on the 40 meter pattern. (You'll see some TV antennas made this way for the same reason -- the high TV channels are about 3 times the frequency of the low channels.) But again, if a station isn't in the right direction, you'll do better with a half wave dipole than a 3/2 wave one. A "gain" antenna isn't of much use if the gain is in the wrong directions. In fact, it's worse than a lower gain one, since the higher gain it gets in a few directions comes at the expense of lower gain in the remainder. Roy Lewallen, W7EL Buck wrote: When trimmed to SWR, I get less than 1.5:1 on fifteen meters. My plans are to build a multiband antenna, parallel elements, where each band is 3/2 wave dipoles and fed with 50 or 75 ohm coax. I haven't tried this yet, but I would hope I can have a few gain dipole antennas in a multi-band format. |
Interesting comments Roy
But it does utelize the use of dipoles that are not straight but angled. It is well known that pointed /angled dipoles does have advantages such as reducing rotational area, reduction of reactances that can lead to as much extra gain as 1 Db together with a broader banded antenna. In this particular case he has achieved an extra 3 db by stacking. Your point regarding direction of main lobs is quite important but lessens in importance if the antenna is rotatable. Unfortunately I searched the web page of this new antenna but was unable to find any antenna patterns, which puts one in a "believe it or not" situation regarding its capability versus claims. Art "Roy Lewallen" wrote in message ... I presume you realize that a 3/2 wave dipole has multiple lobes, with nulls between. At many or most angles, a half wavelength dipole has greater gain. You can get just two narrow lobes from a 3/2 wave dipole by rotating the wires 30 degrees to form a horizontal "V" having a 120 degree included angle. I've used this trick with a 40 meter dipole being used on 15 meters -- it doesn't have much effect on the 40 meter pattern. (You'll see some TV antennas made this way for the same reason -- the high TV channels are about 3 times the frequency of the low channels.) But again, if a station isn't in the right direction, you'll do better with a half wave dipole than a 3/2 wave one. A "gain" antenna isn't of much use if the gain is in the wrong directions. In fact, it's worse than a lower gain one, since the higher gain it gets in a few directions comes at the expense of lower gain in the remainder. Roy Lewallen, W7EL Buck wrote: When trimmed to SWR, I get less than 1.5:1 on fifteen meters. My plans are to build a multiband antenna, parallel elements, where each band is 3/2 wave dipoles and fed with 50 or 75 ohm coax. I haven't tried this yet, but I would hope I can have a few gain dipole antennas in a multi-band format. |
On Sun, 24 Apr 2005 15:12:26 -0700, Roy Lewallen
wrote: I presume you realize that a 3/2 wave dipole has multiple lobes, with nulls between. At many or most angles, a half wavelength dipole has greater gain. Yes, I believe it has four major lobes and a few minor lobes as well. None are exactly perpendicular to the wire (IIRC). You can get just two narrow lobes from a 3/2 wave dipole by rotating the wires 30 degrees to form a horizontal "V" having a 120 degree included angle. I've used this trick with a 40 meter dipole being used on 15 meters -- it doesn't have much effect on the 40 meter pattern. (You'll see some TV antennas made this way for the same reason -- the high TV channels are about 3 times the frequency of the low channels.) You got me on this one. I haven't looked into the 'V-Beams'. I was thinking they were 1/2 wave dipoles. TV antennas are also a form or Log-Periodic aren't they? But again, if a station isn't in the right direction, you'll do better with a half wave dipole than a 3/2 wave one. A "gain" antenna isn't of much use if the gain is in the wrong directions. In fact, it's worse than a lower gain one, since the higher gain it gets in a few directions comes at the expense of lower gain in the remainder. Yes, this is a form of compromise antenna. I guess if I were looking for the perfect (omni) antenna, it would be a loop. Many hams use G5RVs or tuned dipoles cut for their lowest desired band. While they all work, they all have their lobes on certain bands, probably each band with different lobes. I realize that the antenna will have nulls and lobes. If possible, I would want to arrange the antenna in such a way as to use the lobes to my benefit. Primarily, it is an experimental antenna as much for the fun of building it as for using it. I also plan to build a multi-band beam, but not right away. Thanks for the input. I'll check into the 'V' antenna. Roy Lewallen, W7EL -- 73 for now Buck N4PGW |
| All times are GMT +1. The time now is 06:24 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com