Re: Topband: RX splitter - ground common or not?
On 10/17/2012 2:10 PM, Tom W8JI wrote: Advice like that must assume some ridiculous amount of common mode impedance I stand by my comments. 73, Jim K9YC ___ Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more info.
Re: Topband: 1810 spur status and resolution
Very interesting and well done. Was it ever discovered what was actually causing the spurious signal? In other words, what did the station engineer do that caused the signal to disappear? Rudy is awesome too! On 10/17/2012 7:00 PM, Don Kirk wrote: Full and final details on the recent spurious signal and how it was located can be found on the following website. http://sites.google.com/site/wgymsignal/ 73's Rick K2XT & Don WD8DSB ___ Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more info. -- 73, Gary K9GS Greater Milwaukee DX Association: http://www.gmdxa.org Society of Midwest Contesters: http://www.w9smc.com CW Ops #1032 http://www.cwops.org ___ Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more info.
Re: Topband: 1810 spur status and resolution
Full and final details on the recent spurious signal and how it was located can be found on the following website. http://sites.google.com/site/wgymsignal/ 73's Rick K2XT & Don WD8DSB ___ Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more info.
Re: Topband: RX splitter - ground common or not?
The only thing I disagree with about Tom's advice is that, with the exception of a VERY large string of #73 beads (at least 100 beads), beads are useless on HF, and even those are not very useful at 2 MHz. Advice like that must assume some ridiculous amount of common mode impedance being driven by extreme voltages. As a result, the advice misleads people about the requirements of a CM choke. Attenuation added by beads, or any series CM impedance, depends on the common mode impedance at the insertion point. The beads, or any series impedance, are part of a system with the path impedance outside the added series isolation. Let's say we have a common 10-foot long lead inside the shack, between two pieces of somewhat well-grounded gear, driven by a 100 mV noise voltage source. To keep it simple, let's assume that path impedance is 20 j0 ohms. CM current would be 5 Ma. This is reasonable according to measurements made here with a nasty SMPS I have. If we added a single 1 inch long piece of 73 material, we would add 100 ohms or so additional impedance to the path. Current would now be .1/120 = 0.83 mA, instead of .1/20 = 5 mA. This is almost 16 dB attenuation from a single bead!! Now there can be extreme examples where we might need 100's of beads to produce a significant change, but those cases are much more effectively solved by the addition of a ground or altering of a cable's length. Those cases would be specific to very special cases, and not likely at all to appear in a receiving cabling system. The problem in life comes in when we focus on extreme examples as rules, when they uncommon cases. Let's look at an uncommon case. Let's assume we have a half wave of cable with a shield surge impedance of hundreds of ohms to earth, and zero external loss resistance at cable ends or along the cable. We try to add beads in the exact center. Now we have two 1/4 wave transmission lines in series at each side of the beads, so the beads are at a point where the impedance is infinite. Now no matter how many beads we add, the common mode does not change. This shows why there has to be some rational tempering of extremes we pick when we tell people what is necessary to solve a problem. There is a practical litmus test sense to the notion we might commonly need hundreds of beads. If such was a general case, very few of us would be successful adding beads to anything. Most people see very large effects even with small numbers of beads, even the WRONG beads, because the series impedance of a system is generally pretty low. This would especially be true on lower bands between pieces of desk equipment. Common mode impedance HAS to be reasonably low at source ends, or common mode current would not be an issue! The violation of this would be a very long transmission line with high shield to surroundings impedance and low loss, or a source driving the cable with hundreds of volts. In that case it would be better to fix or alter the system, rather than purchase and install hundreds or thousands of beads. 73 Tom ___ Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more info.
Re: Topband: RX splitter - ground common or not?
On 10/17/2012 6:58 AM, Tom W8JI wrote: Use beads over an unbroken shield. The only thing I disagree with about Tom's advice is that, with the exception of a VERY large string of #73 beads (at least 100 beads), beads are useless on HF, and even those are not very useful at 2 MHz. Where Tom recommends beads I would instead use either 1) a transformer having VERY low inter-winding capacitance, or 2) a common mode choke formed by winding at least 16 turns of the coax around a #31 core. 73, Jim K9YC ___ Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more info.
Re: Topband: RX splitter - ground common or not?
DX Engineering/s RSC-2 splitter appears to have all three antenna terminals connected to a metal case; i.e., the grounds are all connected together. In ON4UNs 5th edition, he appears to recommend that the common connector (the one to the braid-breaker 7:5 matching transformer) not be tied to the grounds of the two output connectors, and shows a picture of his splitter/combiner which appears to be in a plastic box. Should I construct my RX splitter with all three shields connected together, or should I isolate the common (middle) feedline shield from the other two? This is to be used in the shack to feed tor receivers. Terry, The entire issue can be pretty complex, and could be one of those ten thousand page posts where people argue a needle in a haystack of other more critical things. The only significant point of ingress for common mode is at a shield termination, such as the antenna feedpoint or a poor or improper shield connection. An exception to could be a very strong common mode RF current from a SMPS with an external ground loop for harmonics through cable shields, where a shield's -80 dB or more isolation might make am in-band birdie audible. As a general rule an unwanted signal that strong would radiate to the antenna anyway, and really needs addressed at the source. Even worse, a plastic case (or lack of a groundplane upon which connectors are mounted) just sets people up for ingress issues significantly worse than a normal shield would allow. Personally, I would not force an intentional shield discontinuity in the shack, near noise sources, or at any potentially critical location. I would use a shielded box with connectors through the wall, or a suitable well-thought groundplane with connector shells connected right at the groundplane (no wire shield leads), always. If I worried about common mode near noise sources or near potential noise sources, I'd use beads over cables and ground connectors the way the are designed and intended to be usedwith a good groundplane right at the connector common between ALL cables. The idea of switching shields, or floating a connector shell at a box entrance, when this is done near a potential noise source can in no way be considered good engineering or planning. Places where the idea of floating a shield from the enclosure or groundplane works and has logical justification is right at an unavoidable ingress point, like an antenna feedpoint. We always have ingress and egress at a point like that, and the only way to reduce it is to isolate the common mode path. This is because the cable becomes UNshielded at that point! At all other points in the system, the shield should be a shield. The connector shell should mount directly to the groundplane, and the groundplane should be as close to zero impedance between shields as practical. This even means no wired leads as a ground path leading from outside into the inside of the box. If you have common mode, fix any source first through isolation or containment at the source. Then, if you need to do something further, don't break the shield. Use beads over an unbroken shield. This is a complex issue and it varies with how a shield behaves. In practice a shield works differently at audio, for example, because of skin depth and the huge levels of common mode current at low frequencies. But for 160 meter and higher frequency signals, I'd used a closed box or a connector through a groundplane near noise sources on every lead except an intentionally balanced line. 73 Tom ___ Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more info.
