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Carolina Windom revisited: 4 to 1 balun does nothing to choke RF?
Having received some very good advice about the Carolina Windom, I now
ask a question to which I THINK I already know the answer. This particular version of the CW OCF dipole uses 300 ohm twin lead feedline terminated after 33 feet to a 4:1 balun. Richard warned of significant risk of RF on the outer (inner?) braid of the coax. I've never read of using a 1:1 unun right after a 4:1 balun to minimize RF on the outer coax. My thinking is that the 4:1 balun acts as a voltage type and will do nothing to ameliorate this. So, I will put some ferrite rings or snap on ferrites just past the 4:1 balun on the coax. Does this sound like a reasonable solution? John AB8O |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
john Wiener wrote:
... I've never read of using a 1:1 unun right after a 4:1 balun to minimize RF on the outer coax. My thinking is that the 4:1 balun acts as a voltage type and will do nothing to ameliorate this. So, I will put some ferrite rings or snap on ferrites just past the 4:1 balun on the coax. Does this sound like a reasonable solution? John AB8O If you use a 4:1 Ruthroff, it would be a voltage balun ... if you use a 4:1 Guanella it would be a current balun ... the 1:1 current balun is probably more useful behind a Ruthroff ... but hey, once you have tried all these possible combinations, you can speak from experience! grin If going from 300 ohm to 50 ohm, perhaps you would choose a 6:1. Or, just go with the 4:1 now and when you need some diversion, later, try the 6:1 to see what improvments can be had and if the loss in this design is acceptable to you ... etc. Anyway, in this document is a 6:1 (actually 6.25:1, resulting in 312ohm to 50 ohm) made from two 4:1 baluns (I would think Guanella ununs ... the 4:1 can be made from two 1:1, each wound on the opposite side of toroid core, reversing coil directions on one side. This could also be accomplished with 4 ferrite rods ... a 1:1 balun wound on each rod, two rods combined to make a 4:1 balun, then these "two units" combined to construct the 6.25:1 balun ... Regards, JS |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
John Smith wrote:
[stuff and forgot the URL, as usual :-( ] The URL for the 6.25:1 ... http://www.radioelectronicschool.net.../ocfdipole.pdf Regards, JS |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
john Wiener wrote:
Having received some very good advice about the Carolina Windom, I now ask a question to which I THINK I already know the answer. This particular version of the CW OCF dipole uses 300 ohm twin lead feedline terminated after 33 feet to a 4:1 balun. Richard warned of significant risk of RF on the outer (inner?) braid of the coax. I've never read of using a 1:1 unun right after a 4:1 balun to minimize RF on the outer coax. My thinking is that the 4:1 balun acts as a voltage type and will do nothing to ameliorate this. So, I will put some ferrite rings or snap on ferrites just past the 4:1 balun on the coax. Does this sound like a reasonable solution? A while back I did some pretty careful measurements of an OCF dipole. I found that ferrites were required at both the feedpoint and at one or more places along the feedline. The ferrites at the feedpoint suppress the conducted common mode current (which is actually forced to exist by the voltage balun). But the asymmetry of the antenna results in common mode current being induced onto the feedline by mutual coupling to the antenna. This isn't a problem in a symmetrical dipole if the feedline is positioned symmetrically relative to the antenna, since the currents induced by the two equal halves cancel. But the OCF dipole can result in quite a lot of induced common mode current. Ideally, you'd put at least a second bunch of snap on cores about a quarter wavelength from the feedpoint. But one of the main reasons people use OCFs is for multi-band operation. So the thing to do is to place the cores for maximum effectiveness on the band(s) where you have the most trouble -- the common mode current also depends on the feedline length and position, and will vary considerably from band to band even if you do nothing. My opinion is that users of OCF dipoles are just about always going to have to deal with some amount of common mode current, and the best you can do is reduce it to a level you can tolerate. Roy Lewallen, W7EL |
Carolina Windom revisited: 4 to 1 balun does nothing to choke RF ?
"Roy Lewallen" wrote in message treetonline... john Wiener wrote: Having received some very good advice about the Carolina Windom, I now ask a question to which I THINK I already know the answer. This particular version of the CW OCF dipole uses 300 ohm twin lead feedline terminated after 33 feet to a 4:1 balun. Richard warned of significant risk of RF on the outer (inner?) braid of the coax. I've never read of using a 1:1 unun right after a 4:1 balun to minimize RF on the outer coax. My thinking is that the 4:1 balun acts as a voltage type and will do nothing to ameliorate this. So, I will put some ferrite rings or snap on ferrites just past the 4:1 balun on the coax. Does this sound like a reasonable solution? A while back I did some pretty careful measurements of an OCF dipole. I found that ferrites were required at both the feedpoint and at one or more places along the feedline. The ferrites at the feedpoint suppress the conducted common mode current (which is actually forced to exist by the voltage balun). But the asymmetry of the antenna results in common mode current being induced onto the feedline by mutual coupling to the antenna. This isn't a problem in a symmetrical dipole if the feedline is positioned symmetrically relative to the antenna, since the currents induced by the two equal halves cancel. But the OCF dipole can result in quite a lot of induced common mode current. Ideally, you'd put at least a second bunch of snap on cores about a quarter wavelength from the feedpoint. But one of the main reasons people use OCFs is for multi-band operation. So the thing to do is to place the cores for maximum effectiveness on the band(s) where you have the most trouble -- the common mode current also depends on the feedline length and position, and will vary considerably from band to band even if you do nothing. My opinion is that users of OCF dipoles are just about always going to have to deal with some amount of common mode current, and the best you can do is reduce it to a level you can tolerate. Roy Lewallen, W7EL Roy, What's your opinion on the 4: or 6:1 balun between the 300 Ohm line and the coax? I see no reason whatever to think that the impedance coming off the 300 Ohm line is anywhere near 300 Ohms. Also, where is it written that a 50 Ohm balun will work at, say, 2000 Ohms. The ferrites as you suggest will clearly work if you use enough of them. The reason for asking this is that a friend is in the process of putting up a 75 m dipole, which he only plans to use on 75 m. Everybody is telling him to feed it with ladder line going to coax through a balun. Why in the world would you do that? Tam/WB2TT |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
john Wiener wrote:
Having received some very good advice about the Carolina Windom, I now ask a question to which I THINK I already know the answer. This particular version of the CW OCF dipole uses 300 ohm twin lead feedline terminated after 33 feet to a 4:1 balun. Richard warned of significant risk of RF on the outer (inner?) braid of the coax. I've never read of using a 1:1 unun right after a 4:1 balun to minimize RF on the outer coax. My thinking is that the 4:1 balun acts as a voltage type and will do nothing to ameliorate this. So, I will put some ferrite rings or snap on ferrites just past the 4:1 balun on the coax. Does this sound like a reasonable solution? John AB8O Ouch! Sorry about multiple posts! Not sure what happened there. |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
Roy Lewallen wrote:
My opinion is that users of OCF dipoles are just about always going to have to deal with some amount of common mode current, ... Isn't the section between the voltage balun and the choke designed to radiate? -- 73, Cecil http://www.w5dxp.com "According to the general theory of relativity, space without ether is unthinkable." Albert Einstein |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
Cecil Moore wrote:
Roy Lewallen wrote: My opinion is that users of OCF dipoles are just about always going to have to deal with some amount of common mode current, ... Isn't the section between the voltage balun and the choke designed to radiate? Cecil In the version I am putting up, the 4:1 balun is at the bottom of the twin lead feedline, so the feedline can radiate. John AB8O |
Carolina Windom revisited: 4 to 1 balun does nothing to choke RF?
Is this OCF dipole a single band antenna or a multiband antenna?
- 'Doc |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
wrote:
Is this OCF dipole a single band antenna or a multiband antenna? - 'Doc It is alleged to be a multi-band antenna. Assuming that the standing-wave current is 90 degrees out of phase with the standing wave voltage, one can actually calculate an approximation for the feedpoint impedance by dividing the standing-wave voltage by the standing-wave current at the feedpoint. With an OCF, the feedpoint impedance will always be lower than Vmax/Imin and always higher than Vmin/Imax and thus provide a reasonable SWR for 450 ohm ladder-line. The free demo version of EZNEC available from http://www.eznec.com will provide a reasonable guess at the feedpoint impedance. Here are what EZNEC 4.0 says about the feedpoint impedance of the following: ------------83'----------FP-----47'-------- 3.8 MHz, 85.5+j98.5 ohms, SWR(300)= 3.9, SWR(450)= 5.5 7.2 MHz, 177-j181 ohms, SWR(300)= 2.5, SWR(450)= 3 10.125 MHz, 3569-j102 ohms, SWR(300)= 12, SWR(450)= 7.9 14.2 MHz, 96-j357 ohms, SWR(300)= 7.7, SWR(450)= 7.7 18.14 MHz, 797-j911 ohms, SWR(300)= 6.3, SWR(450)= 4.4 21.3 MHz, 933-j1239 ohms, SWR(300)= 8.8, SWR(450)= 6 24.95 MHz, 218-j652 ohms, SWR(300)= 8.5, SWR(450)= 6.7 28.4 MHz, 757+j1019 ohms, SWR(300)= 7.4, SWR(450)= 5.1 As you can see, the myth that this antenna has a 300 ohm feedpoint impedance is just an old wives' tale but it does have reasonable SWRs when fed with 450 ohm ladder-line. -- 73, Cecil http://www.w5dxp.com "According to the general theory of relativity, space without ether is unthinkable." Albert Einstein |
Carolina Windom revisited: 4 to 1 balun does nothing to choke RF?
Cecil,
I sort of thought that this OCF antenna was typically used for a multiband antenna, that's typically the 'idea' behind using them. Keeping that 'thought' in mind, how can you 'optimize' this multiband antenna for one particular band without 'de-optimizing' it for others? That seems sort of going at it from the wrong direction, why not use an antenna that works well for the particular band of interest. Oh, I know that assumes that you can have more than one antenna, which isn't always the case. So having an 'un-optimized' antenna is what you'll end up with in most (if not all) bands except for one. Which is the 'fault' for almost all multiband antennas. Isn't it? - 'Do |
Carolina Windom revisited: 4 to 1 balun does nothing to choke RF ?
wrote in message ... Cecil, I sort of thought that this OCF antenna was typically used for a multiband antenna, that's typically the 'idea' behind using them. Keeping that 'thought' in mind, how can you 'optimize' this multiband antenna for one particular band without 'de-optimizing' it for others? That seems sort of going at it from the wrong direction, why not use an antenna that works well for the particular band of interest. Oh, I know that assumes that you can have more than one antenna, which isn't always the case. So having an 'un-optimized' antenna is what you'll end up with in most (if not all) bands except for one. Which is the 'fault' for almost all multiband antennas. Isn't it? - 'Do Seems to me it's not "optimized" for any band, but is just a fudged radiator fed by ladder line, matched with a tuner or maybe a pi-net without too much line loss. You can fiddle with it to your hearts content, but in the end will it load up where you want it to and will you get out? It would be nice to have an antenna analyzer to have some indication of YOUR installation as you fiddle. Obviously you will have trouble using any of the modern internal ATUs. Years ago, I threw one up but fed it with coax. It didn't work so I rebuilt it as parallel fed Dipoles for 80 and 40m. Thanks for the NEC output, Cecil. |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
Tam wrote:
Roy, What's your opinion on the 4: or 6:1 balun between the 300 Ohm line and the coax? I see no reason whatever to think that the impedance coming off the 300 Ohm line is anywhere near 300 Ohms. Also, where is it written that a 50 Ohm balun will work at, say, 2000 Ohms. The ferrites as you suggest will clearly work if you use enough of them. The reason for asking this is that a friend is in the process of putting up a 75 m dipole, which he only plans to use on 75 m. Everybody is telling him to feed it with ladder line going to coax through a balun. Why in the world would you do that? As you suspect, the impedances encountered by the transformer on some bands are wildly different than its nominal design impedances. In the one which I carefully measured, the result was no surprise. When the antenna impedance was substantially different from 300 + j0, the transformation ratio wasn't 6:1, and the transformer added series and/or shunt reactance, sometimes a pretty large amount. And this was the case on most bands. This isn't to say that an OCF dipole can't be fiddled until, radiating feedline and all, it manages to present an acceptable SWR on several bands. But when it does, it's not working at all like predicted by a simplified analysis which ignores the strong feedline coupling and very non-ideal transformer effects. Roy Lewallen, W7EL |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
Roy Lewallen wrote:
... This isn't to say that an OCF dipole can't be fiddled until, radiating feedline and all, it manages to present an acceptable SWR on several bands. But when it does, it's not working at all like predicted by a simplified analysis which ignores the strong feedline coupling and very non-ideal transformer effects. Roy Lewallen, W7EL I have switched my balun designs to the one in this URL, Figure 4 - Improved 4:1 Current Balun, page 3: http://home.earthlink.net/~christras...k4to1Balun.pdf Can be used balanced, or forced to unun fashion ... bandwidth becomes increased (and, it "just works better for me!") But, what works for you is the most important, always ... Regards, JS |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
JB wrote:
Thanks for the NEC output, Cecil. I showed how to optimize an OCF for a single band, e.g. 40m. If the 50/120 ohm transformer is link coupled, it will cause a common-mode current node at the transformer on 40m. Any antenna system can be optimized if one knows what they are doing. For instance, a shunt 1000 pf cap at the twinlead to coax junction will optimize a G5RV for 75m. -- 73, Cecil http://www.w5dxp.com "According to the general theory of relativity, space without ether is unthinkable." Albert Einstein |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
Roy Lewallen wrote:
This isn't to say that an OCF dipole can't be fiddled until, radiating feedline and all, it manages to present an acceptable SWR on several bands. I bought the 300 ohm feedpoint myth when I was at Texas A&M in 1958 when I didn't know any better. I had an OCF fed with 300 ohm twinlead fed through a 6:1 air-core Heathkit balun driven by a DX-40. The results were amazing to me at the time. Now I know the pi-net output of the DX-40 would achieve a match to almost anything and I should have been using a 1:1 balun. The Heathkit balun didn't have much loss and the 300 ohm twinlead didn't have much loss. I have no idea what the actual impedances were, but losses were minimized and the system successfully worked the world at the height of the most active sunspot cycle in recorded history. -- 73, Cecil http://www.w5dxp.com "According to the general theory of relativity, space without ether is unthinkable." Albert Einstein |
Carolina Windom revisited: 4 to 1 balun does nothing to choke RF ?
"Cecil Moore" wrote in message ... JB wrote: Thanks for the NEC output, Cecil. I showed how to optimize an OCF for a single band, e.g. 40m. If the 50/120 ohm transformer is link coupled, it will cause a common-mode current node at the transformer on 40m. Any antenna system can be optimized if one knows what they are doing. For instance, a shunt 1000 pf cap at the twinlead to coax junction will optimize a G5RV for 75m. Of course but I'm too lazy to run the numbers if it isn't my project. My favorite single band wire antenna is the Dipole. How is OCF an improvement? I also liked my Inverted L made out of a homebrew 9' bug catcher mobile antenna with 25' horizontal clip on extension for 40/80 and clip on shunt caps at the mobile mount. Works well in the campground. Keeping the antenna matched to the coax DOES make a difference, considering coax losses. |
Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
JB wrote:
... Of course but I'm too lazy to run the numbers if it isn't my project. My favorite single band wire antenna is the Dipole. How is OCF an improvement? I also liked my Inverted L made out of a homebrew 9' bug catcher mobile antenna with 25' horizontal clip on extension for 40/80 and clip on shunt caps at the mobile mount. Works well in the campground. Keeping the antenna matched to the coax DOES make a difference, considering coax losses. A "one size fits all" antenna is still VERY MUCH a pipe dream ... in my humble opinion (or, IMHO ...) Regards, JS |
Carolina Windom revisited: 4 to 1 balun does nothing to choke RF ?
"John Smith" wrote in message ... JB wrote: ... Of course but I'm too lazy to run the numbers if it isn't my project. My favorite single band wire antenna is the Dipole. How is OCF an improvement? I also liked my Inverted L made out of a homebrew 9' bug catcher mobile antenna with 25' horizontal clip on extension for 40/80 and clip on shunt caps at the mobile mount. Works well in the campground. Keeping the antenna matched to the coax DOES make a difference, considering coax losses. A "one size fits all" antenna is still VERY MUCH a pipe dream ... in my humble opinion (or, IMHO ...) Regards, JS At least the "one size" will be very large. I once saw a truly monumental commercial or military log periodic. Tam/WB2TT |
Carolina Windom revisited: 4 to 1 balun does nothing to choke RF ?
I also liked my Inverted L made out of a homebrew 9' bug catcher mobile antenna with 25' horizontal clip on extension for 40/80 and clip on shunt caps at the mobile mount. Works well in the campground. Keeping the antenna matched to the coax DOES make a difference, considering coax losses. A "one size fits all" antenna is still VERY MUCH a pipe dream ... in my humble opinion (or, IMHO ...) Regards, JS Then there are the discones and LPDAs where size matters. The bug catcher worked well and still does despite many repairs, 15-40 m by itself. 40 and 80 with the extension. It's a modified hamstick with 4" dia. by 8" hand wound #14 coil in the center with scrap plastic forms. There must be an unwritten law that says only the ugliest projects will work. It was a pipe dream of flea clips, but the goal was to do all matching on the antenna. I used it mobile with a 365 pf broadcast variable on the mount and a remote operator into the cab. Add a little series XsubL with a flea clip, matched out with the cap to raise the impedance at the mount. The coil would self resonate so 12 and 17 wouldn't work (perhaps a longer stepped pitch later). Shorting around the coil got me 10m back. I would tune for min SWR (actually ALC) and the difference between not heard with an OK SWR and heard well after a fine tuning was repeatable over and over. Even with as little as 15' of RG8x. Worked a lot of DX on the commute back and forth to school. Had no time to do it any where else. |
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