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Longwire magnetic UnUn: Turns on T68-2
Hi. I've seen construction details of a longwire magnetic UnUn made with a T130-2 toroid. The T130-2 is trifilar wound, each of the 3 windings are 9 turns. The windings are connected in series, so that the turns in total are 27: Turns ratio is :1:3 and impedance ratio is 1:9.
Okay, but do I keep the same number of turns for each winding (9) if I use a T68-2 toroid? Thanks. EDIT:Ahem: Also asked on QRZ.com forum. Last edited by richard8123 : June 26th 14 at 08:14 PM |
#2
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Longwire magnetic UnUn: Turns on T68-2
richard8123 wrote:
Hi. I've seen construction details of a longwire magnetic UnUn made with a T130-2 toroid. The T130-2 is trifilar wound, each of the 3 windings are 9 turns. The windings are connected in series, so that the turns in total are 27: Turns ratio is :1:3 and impedance ratio is 1:9. Okay, but do I keep the same number of turns for each winding (9) if I use a T68-2 toroid? Thanks. In theory, you have to calculate the exact impedance of the primary or secondary and match it to where you want it to go. For example, if you are connecting a 50 ohm coax to the primary, you have to match it to 50 ohms, which would make the secondary 450 ohms. In reality, there is not much that a few turns or an impedance mismatch will do, so wind it the same way you would on the other toroid. While you are at it, save yourself a lot of money and use RG-6 coax. Also in reality, you may not notice any difference at all using the transformer. The famous article floating around usenet said that it was not needed for "any modern receiver" and that was written 20 years ago. You may also try a straight 1:9 transformer with the primary (high number of turns) connected to the antenna and the other side to ground, and then the secondary connected to the coax. This reduces noise by having a DC path to ground and no DC connection between the antenna and the receiver. Geoff. -- Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379 |
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Longwire magnetic UnUn: Turns on T68-2
In message , Geoffrey S.
Mendelson writes richard8123 wrote: Hi. I've seen construction details of a longwire magnetic UnUn made with a T130-2 toroid. The T130-2 is trifilar wound, each of the 3 windings are 9 turns. The windings are connected in series, so that the turns in total are 27: Turns ratio is :1:3 and impedance ratio is 1:9. Okay, but do I keep the same number of turns for each winding (9) if I use a T68-2 toroid? Thanks. In theory, you have to calculate the exact impedance of the primary or secondary and match it to where you want it to go. For example, if you are connecting a 50 ohm coax to the primary, you have to match it to 50 ohms, which would make the secondary 450 ohms. In reality, there is not much that a few turns or an impedance mismatch will do, so wind it the same way you would on the other toroid. While you are at it, save yourself a lot of money and use RG-6 coax. Also in reality, you may not notice any difference at all using the transformer. The famous article floating around usenet said that it was not needed for "any modern receiver" and that was written 20 years ago. You may also try a straight 1:9 transformer with the primary (high number of turns) connected to the antenna and the other side to ground, and then the secondary connected to the coax. This reduces noise by having a DC path to ground and no DC connection between the antenna and the receiver. I can't totally disagree with you, but you seem to have totally failed to answer the OP's question, which was "Do I keep the same number of turns for each winding (9) if I use a T68-2 toroid?" I think that the answer could be a "maybe". From what I can quickly see, the T68-7 is twice the diameter of the T30-2. See the data at http://toroids.info/T30-2.php and http://toroids.info/T68-7.php I don't think that the diameters (and hence the magnetic path lengths) will matter too much, but the permeability (AL) of the T68-7 is somewhat greater than the T30-2, and its cross-sectional area three times greater. I would therefore guess that with the same number of turns, the T68-7 would noticeably more inductance than the T30-2, and would therefore be expected to work down to lower frequencies, and roll off more at high frequencies. However, a more scientific approach would be to compare the characteristics of the two inductors by using calculators below the two sets of data information. In the calculator for the T30-2, enter (say) 10MHz and 27 turns, then 'calculate'. Note the resulting 3.13uH, 81pF and 197ohms. [I presume that the pF is the capacitance required to resonate it at 10MHz, and the ohms is the shunt loss resistance.] Next select the T68-7, and repeat. You get 3.79uH, 67pF and 238.2ohms, so there's not a lot of difference in the results. [I'm rather surprised that the inductance is only x1.2 greater than the T30-2.] However, now try reducing the inductance to match the T30-2. Try 24 turns (3x8) instead of 27 (3x9). You get 3.00uH, 85pF and 188.2ohms Hey! that's pretty close! Next try (say) 1Mhz and 100MHz. You will see that the match is maintained pretty well. So I guess that the performance of the two cores is (or can be) very similar. As I said, with 27 turns, the T68-7 will probably work to a somewhat lower frequency, and have a lower HF roll-off, but it seems that with 24 turns, it should be almost identical. -- Ian |
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Longwire magnetic UnUn: Turns on T68-2
Ian Jackson wrote:
I can't totally disagree with you, but you seem to have totally failed to answer the OP's question, which was "Do I keep the same number of turns for each winding (9) if I use a T68-2 toroid?" I think that the answer could be a "maybe". While I had written: In reality, there is not much that a few turns or an impedance mismatch will do, so wind it the same way you would on the other toroid. So we had both burried the same answer in a lot of other text. :-) Geoff. -- Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379 |
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Longwire magnetic UnUn: Turns on T68-2
In message , Geoffrey S.
Mendelson writes Ian Jackson wrote: I can't totally disagree with you, but you seem to have totally failed to answer the OP's question, which was "Do I keep the same number of turns for each winding (9) if I use a T68-2 toroid?" I think that the answer could be a "maybe". While I had written: In reality, there is not much that a few turns or an impedance mismatch will do, so wind it the same way you would on the other toroid. So we had both burried the same answer in a lot of other text. :-) But in the end, I decided that the answer was a probable 'yes'. As to the efficacy of 9:1 un-uns, the augment in their favour is that over the HF spectrum, they give a better match to a typical long wire over more of range. However, there are relatively narrow parts where the match is made worse. -- Ian |
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Longwire magnetic UnUn: Turns on T68-2
Ian Jackson wrote:
As to the efficacy of 9:1 un-uns, the augment in their favour is that over the HF spectrum, they give a better match to a typical long wire over more of range. However, there are relatively narrow parts where the match is made worse. In my experince the ability to separate the DC path of the antenna to ground did far more to improve readabilty than any slight improvment in matching. This was both with modern (Kenwood R5000 and similar radios) and Drake SPR-4s. Geoff. -- Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379 |
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