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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 |
#2
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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 |
#3
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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 |
#4
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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 |
#5
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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 |
#6
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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. |
#7
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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 |
#8
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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 |
#9
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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 |
#10
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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 |
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