Re: Topband: RX antenna transformer winding (pure resistancetransformation issues)
- Original Message - From: "Tom W8JI" To: "topband" Sent: Tuesday, August 20, 2013 4:00 PM Subject: Re: Topband: RX antenna transformer winding (pure resistancetransformation issues) Unless you have a system with pretty high common mode impedances, winding spacing means nothing. ** Contrary to what you have stated elsewhere The primary reason I (and DXE) isolate windings with Teflon tubes is to reduce lighting damage, and to greatly reduce assembly damage. If the system has pretty high common mode impedances there might be some small advantage in pushing windings apart, but the primary-secondary capacitance is never going to be important in Beverages or other low or modest common mode impedance antennas. Balancing a small loop might be an issue. ** Common mode is an overworked response for perceived ailments. I took your low capacitance suggestions a step further after carefully measuring C and frequency response for maintaining high directivity and F/B. Many Topband operators use their Beverages above 2 MHz. I started using Teflon sleeves in windings because I ran out of small Teflon wire wrap wire I was using. Without Teflon on the wires, and with normal mag wire, the enamel was easily scratched. Not only that, lightning would punch through from enamel to core, or from wire to wire. ** Telon is easily nicked, the type of wire used in modern vehicles is very robust but with the sleeves plain ole magnet wire works well and allows even smaller winding bundles. Since I was more interested in performance rather than production problems I took the time to evaluate a large number of variations. Carl KM1H _ Topband Reflector
Re: Topband: RX antenna transformer winding (pure resistance transformation issues)
On 8/20/2013 1:00 PM, Tom W8JI wrote: By squeezing or spreading turns to change mutual coupling between turns, a T157-2 core can have almost 2:1 impedance shift! That tells us it has considerable flux leakage. So-called "self-shielding" and it's inverse, leakage flux, also depend on permeability, which in turn depends on the material and the frequency of interest. Fair-Rite data sheets for their materials clearly show the variation of permeability with frequency. And when we squeeze or spread turns we are also changing stray C of the winding. 73, Jim K9YC _ Topband Reflector
Re: Topband: RX antenna transformer winding (pure resistancetransformation issues)
Unless you have a system with pretty high common mode impedances, winding spacing means nothing. ** Contrary to what you have stated elsewhere Where? Please quote it in enough context to see the meaning. I'll certainly clairfy it or correct it if possible, because I don't want to mislead anyone. _ Topband Reflector
Topband: Radials in the pond..?
I'm putting out 60 1/4 wave radials for my 160m Inverted L on the ground. 4-5 of those radials will end up in my pond with anywhere from 5-10 feet to 25 feet in the pond for those 4-5 radials. The pond is to the NN/E of the Inverted L. Should I: 1) Just attach to each radial a heavy enough weigh at the end of those 4-5 radials and toss the weight into the pond? 2) Coil up those 4-5 radials and lay the coil wire at the shore line ? 3) String those radials along the shore line? Thanks for any suggestions. Randy, AA8R _ Topband Reflector
Re: Topband: RX antenna transformer winding (pure resistance transformation issues)
On Tue, Aug 20, 2013 at 3:00 PM, Tom W8JI wrote: > The ARRL in some publications, for example, teaches us toroids are > "self-shielding". In fact, many toroidal cores have significant flux > leakage. > That is for sure. I was following a thread in an SWL or AM BCB group a few months ago, where someone (right or wrong) was recommending a toroid as an antenna in lieu of a ferrite rod with wire wrapped on it. The point is that the toroid was evidently doing a fair job of picking up signals! If it were self-shielding, that wouldn't happen. There is a great deal of stuff going on in a transformer. > You said a mouthful. I sure wish I understood more about that subject. I wrote Fair-Rite for some info over a year ago and they never replied. (for whatever reason). I need to learn some more basics that I don't presently understand. 73, Mike www.w0btu.com _ Topband Reflector
Re: Topband: RX antenna transformer winding (pure resistance transformation issues)
I had additional offline conversations with Rick (N6RK), and Carl (KM1H), and below is my official response to my original post about RX antenna transformer windings and complex impedance measured on the primary side of the transformer depending on how they are wound. There are several things that cause the issue you noticed, and one of them is core magnetic path length and leakage. The ARRL in some publications, for example, teaches us toroids are "self-shielding". In fact, many toroidal cores have significant flux leakage. By squeezing or spreading turns to change mutual coupling between turns, a T157-2 core can have almost 2:1 impedance shift! That tells us it has considerable flux leakage. The primary issues, however, are the generally "impure" magnetic properties of very high permeability cores. If we tightly wound a single winding on the core and measured the impedance, we would find it is not a pure lossless reactance. These impure impedances modify the impedances seen through the transformer. There is a great deal of stuff going on in a transformer. Unless you have a system with pretty high common mode impedances, winding spacing means nothing. The primary reason I (and DXE) isolate windings with Teflon tubes is to reduce lighting damage, and to greatly reduce assembly damage. If the system has pretty high common mode impedances there might be some small advantage in pushing windings apart, but the primary-secondary capacitance is never going to be important in Beverages or other low or modest common mode impedance antennas. Balancing a small loop might be an issue. I started using Teflon sleeves in windings because I ran out of small Teflon wire wrap wire I was using. Without Teflon on the wires, and with normal mag wire, the enamel was easily scratched. Not only that, lightning would punch through from enamel to core, or from wire to wire. I'm glad the isolated primary transformers took over from the auto-transformers that were so common, but I think the need for low capacitance is being a bit overplayed in the vast majority of cases. 73 Tom _ Topband Reflector
Re: Topband: RX antenna transformer winding (pure resistance transformation issues)
On 8/20/2013 11:31 AM, Don Kirk wrote: But I saw the complex impedance phenomena even with smaller diameter cores (using different grade of material), and here is my actual FT 50-43 data as an example. One missing piece in understanding what you're seeing is the fact that mu (permeability) is a complex number, u' +ju'', where u' represents the inductive component and u'' represents the loss component for series equivalent circuit elements. Here's a link to the Fair-Rite data sheet for #43 material. http://www.fair-rite.com/newfair/materials43.htm Note that at 2 MHz, u' is about 800 and u'' is about 180. The fact that mu is complex means that coupling between the windings will have phase shift! Not only that, but loss in the core is coupled into the winding as resistance, which, because it is magnetically coupled, is multiplied by the square of the turns. Here's a link to the listing of all their materials, with links to corresponding data sheets for each. http://www.fair-rite.com/newfair/materials.htm 73, Jim K9YC _ Topband Reflector
Re: Topband: RX antenna transformer winding (pure resistance transformation issues)
If I can add my $0.02 worth here... I have never found a better ferrite core for RX antenna transformers than the Amidon BN-73-202 (Fair-Rite 2873000202). I also used to use side-by-side stacked ferrite beads (eight FB-73-202 in two tight stacks of 4) to make "binocular cores", but that never worked as well. I immediately noticed an improvement in the performance of my Beverages when I switched to those little multi-aperture 2873000202 cores. Newark/Element 14 has those Fair-rite cores cheep. 73, Mike www.w0btu.com On Tue, Aug 20, 2013 at 1:31 PM, Don Kirk wrote: > > When I used two FT50-43 cores configured to make a binocular core, ... > > _ Topband Reflector
Re: Topband: RX antenna transformer winding (pure resistance transformation issues)
I had additional offline conversations with Rick (N6RK), and Carl (KM1H), and below is my official response to my original post about RX antenna transformer windings and complex impedance measured on the primary side of the transformer depending on how they are wound. Rick N6RK said > The way out of this dilemma is to use a smaller toroid core than the 1.14 inch diameter core you currently have. But I saw the complex impedance phenomena even with smaller diameter cores (using different grade of material), and here is my actual FT 50-43 data as an example. FT50-43 measured data as follows (windings on opposite side of the core from each other) : Pri = 5 T, Sec = 20 T, 1K load resistor R= 42, X = 25, 1.8 Mhz, Capacitance = 5 pf R= 51, X = 24, 3.5 Mhz R= 57, X = 32, 7 Mhz When I used two FT50-43 cores configured to make a binocular core, I obtained the following results : Pri = 3 T, Sec = 12 T, 1K load resistor R= 38, X = 21, 1.8 Mhz R= 50, X = 1, 3.5 Mhz R= 52, X = 0, 7 Mhz Rick also said : > the small amount of leakage inductance you are seeing even with the large core with separated windings is nothing to worry about. I agree with this unless you are trying to phase RX antennas, and then the complex impedance becomes a critical factor to deal with, and that's why I feel it's important to understand (know) that this phenomena exists. Rick also said : > I would rather tolerate that and get the benefit of the reduced capacitance from winding to winding. Appears everyone agrees that a very low capacitance between windings (between primary and secondary winding) is near (if not at the very the top) of the list of desired properties of the RX antenna transformer. This is also supported by statements in the ON4UN Low-Band DXing book. Carl (KM1H) goes to great lengths to reduce the capacitance between windings on the binocular cores that he uses on his RX antennas per his following statement : "Teflon tubes for each winding and that brought the C way down by compressing each winding in the smallest tube that I could get the wires thru and then forcing a toothpick thru to force the most separation." Thanks to Rick and Carl for the technical discussions we had. 73, Don Kirk (wd8dsb) _ Topband Reflector
