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Antenna Modification Advice
I built a so called "Super J-pole" which is really two stacked
collinear half wave antennas with a quarter wave phasing stub between them. The design is by N7QVC and it can be seen at http://www.n7qvc.com/amateur_radio/ copper.html. I also placed a photograph of a representative build in the photo section of this reflector at http://groups.yahoo.com/group/amateu...um/1449680574/ pic/3362107\ 0/view?picmode=&mode=tn&order=ordinal&start=1&count= 20&dir=asc. I have had good results with this antenna but I wanted to make it into a dual band antenna that would work on seventy centimeters as well as on two meters. I was wondering if I could use a technique developed by Edison Fong, WB6IQN on this antenna in the same way he used it on an ordinary single half wave J-pole. What he did is to place an open quarter wave stub at the top of the two half wave lengths for seventy centimeters. That stub limits the UHF signal to the two half waves length at UHF of the antenna. It seems like it would work from having read his paper on the combination two meter and seventy centimeter J-pole antenna. That would give the antenna the same gain on seventy centimeters as a J-pole that is just for that band. For reference I have placed WB6IQNs paper in the files section of the newsgroup at http://f1.grp.yahoofs.com/v1/ kL6QTZLk1DQjM_Cn3vuvnsLUIuEsvRHSqUZyX2mw294a7mYKkc \ FBIXXlRY_6QxreqNWVpn0b7Dogiw9LafU63W429yoO/DBJ2_port_art.pdf. My first question is does anyone see any reason why this would not work? My second question is how would I construct the stub into a copper J- pole. Since WB6IQNs J-pole is a wire antenna, with or without a plastic radome, he used a piece of Coaxial cable as part of his two meter wire with the shield at the end of the seventy centimeter portion of the antenna shorted to the two meter radiator and the bottom of the shield, which is a quarter wave at UHF below the top of the UHF segment, open relative to the two meter radiator. Could I just use a full quarter wave stub made from a copper T, a street L, and a short length of copper pipe turned back down along the two meter radiator? (A street L is a ninety degree bend formed to allow it to be close coupled to an adjacent fitting without a pipe nipple in between.) My third question is if instead of shortening the lower VHF half wave electrically to a UHF length I found a way to short out the quarter wave phasing stub between the two VHF half wave radiators to UHF signals thus giving UHF signals three full wave lengths of radiator to use would the gain be worth the effort. To provide the shunting of the VHF quarter wave phasing stub I was GUESSING that a half wave UHF coupling stub might work. I freely admit to being out of my depth; or as a mariner might say off my soundings; here. So I'm fully prepared to hear that the UHF half wave coupling stub would not work as a shunt for the UHF signals to get past the VHF quarter wave phasing stub. I have no pretense to any expertise. I'm trying to learn. Laugh all you want but if it won't work please tell me in neophyte decipherable language why not. If you have any guidance to offer it would be most welcome but please keep the fog index down to the degree you are able. Thank you in advance. -- Tom Horne, W3TDH |
Antenna Modification Advice
On Mon, 28 Mar 2011 11:29:25 -0700 (PDT), Tom Horne
wrote: please keep the fog index down to the degree you are able Hi Tom, After looking at the mile long URL to one of your offerings - I would like to see the fog lift too. Why don't you simply tell us what performance you want to achieve from an antenna? A simple quarterwave antenna built on a SO-239 connector with four drooping radials is squat simple, cheap, and can be built and trimmed to near perfect match in half an hour or less. You want multiband? Make two vertical, slightly skewed elements (each cut for the suited band) joined at the feedpoint. You want more gain? How much more? Build two or (n-times) more and spend your effort in learning to construct feedline systems to additively join them. 73's Richard Clark, KB7QHC |
Antenna Modification Advice
On Mar 30, 2:09*am, Richard Clark wrote:
On Mon, 28 Mar 2011 11:29:25 -0700 (PDT), Tom Horne wrote: please keep the fog index down to the degree you are able Hi Tom, After looking at the mile long URL to one of your offerings - I would like to see the fog lift too. Why don't you simply tell us what performance you want to achieve from an antenna? A simple quarterwave antenna built on a SO-239 connector with four drooping radials is squat simple, cheap, and can be built and trimmed to near perfect match in half an hour or less. *You want multiband? Make two vertical, slightly skewed elements (each cut for the suited band) joined at the feedpoint. *You want more gain? *How much more? Build two or (n-times) more and spend your effort in learning to construct feedline systems to additively join them. 73's Richard Clark, KB7QHC Richard I wanted to explore whether it is practical to have my collinear dual half wave J-pole serve as a dual band antenna. If it were practical I would want the same gain on seventy centimeters that I have been getting out of the dual stacked half wave on two meters. The available testing that I was able to find says that it is 6 DB over a quarter wave vertical. What I would happily settle for would be for it to have the same gain on seventy centimeters as the dual band simple J-pole I am using now. After talking to Rol Anders, K3RA; who was the instructor for my Extra theory class and is the present chairman of the Question Pool Committee of the National Council of Volunteer Examiner Coordinators (NCVEC); last night I am taking that later approach. That is the approach that I outlined in the first paragraph of just putting an open blocking stub for UHF at thirty five centimeters ~ up the lower two meter half wave and therefore below the two meter phasing stub between the two meter half wave segments. That has the virtue of being simple and still giving me a dual band antenna that has better gain on UHF then the unmodified two meter antenna would. I wanted a dual band antenna because I only have three suitable mounting points on my home and I already have plans for a six meter J- pole and an anemometer / sensor array assembly on the other two. I have an Arrow dual band J-pole up on that mounting point right now but I wanted to return to the higher gain of the collinear dual half wave J-pole that gave me so much better real world performance on two meters. It is my hope that just adding the seventy centimeter band blocking stub to the collinear antenna's lower two meter half wave segment will do the trick. -- Tom Horne, W3TDH |
Antenna Modification Advice
On Wed, 30 Mar 2011 10:23:51 -0700 (PDT), Tom Horne
wrote: Richard I wanted to explore whether it is practical to have my collinear dual half wave J-pole serve as a dual band antenna. If it were practical I would want the same gain on seventy centimeters that I have been getting out of the dual stacked half wave on two meters. Which would be quite horrible. The available testing that I was able to find says that it is 6 DB over a quarter wave vertical. What I would happily settle for would be for it to have the same gain on seventy centimeters as the dual band simple J-pole I am using now. Hi Tom, It is extremely hard to accept claims for J-Poles when nothing is said about the care in choking the feed point, and further choking the section of line a quarter wave away from the feed point. Typically this discussion is arrived at with some surprise on the part of the J-Pole user who posts here (I know you have participated here before, and are thus not a newbie). Some (which means too many) respond that choking is unnecessary. They are satisfied with its performance (never daring to examine that it could be vastly improved). We also have writers here who condemn the J-Pole vehemently in equal measure. They, too, have not examined the necessity of choking and they suffer from the knowledge that things could be vastly improved. This is the pitiable lament of feeding halfwave elements in any form. Let's examine what you call "available testing." I presume this means in software, and not in the lab (never mind alternatives such as out in a field). I could be wrong and you may correct this on your response. However, moving on with whatever presumption, the reason why choking is important is that with High Z antennas, they tend to drive the transmission line into radiation. This extends the length of the radiator, and too often this raises the lobe of maximum radiation up into the sky (you are very near that with the half wave where 5/8ths is considered the limit of physical height before this trips over). These are all issues related to a vertical, and its elevation goes into the mix too to further confuse comparisons. Now, basically you are asking the same antenna to operate at roughly triple the frequency. This also means either element of the native radiator will stand like something under 3/2 wavelengths tall - truly a cloud burner (not good). You speak of stubs to fix this. You would have to start with two 3/2 halfwave radiators, one over the other. Fixing the phase for both 2M and 440 would be a miracle in achievement. I presume you would also trap the individual 3/2 wave length sections into two 5/8ths (but the ratios don't quite work out that way); or three half waves; or six quarter waves - and then do it again for the section above. Whew! This is a monumental task - but you have simpler goals as the following would suggest: just putting an open blocking stub for UHF at thirty five centimeters ~ up the lower two meter half wave and therefore below the two meter phasing stub between the two meter half wave segments. That has the virtue of being simple and still giving me a dual band antenna that has better gain on UHF then the unmodified two meter antenna would. The blocking stub I presume you to mean a trap for 440. It is going to upset the matching stub between the two halfwave 2M elements. Here you will have to juggle between tuning them both on each band. It is less than monumental, but still quite a job, and one that demands that you cut and try and fully erecting your last attempt to see how it works (doing this on the ground is going to lead to grief - especially if you ignore proper choking). 73's Richard Clark, KB7QHC |
Antenna Modification Advice
On 3/30/2011 12:23 PM, Tom Horne wrote:
Richard I wanted to explore whether it is practical to have my collinear dual half wave J-pole serve as a dual band antenna. If it were practical I would want the same gain on seventy centimeters that I have been getting out of the dual stacked half wave on two meters. The available testing that I was able to find says that it is 6 DB over a quarter wave vertical. What I would happily settle for would be for it to have the same gain on seventy centimeters as the dual band simple J-pole I am using now. After talking to Rol Anders, K3RA; who was the instructor for my Extra theory class and is the present chairman of the Question Pool Committee of the National Council of Volunteer Examiner Coordinators (NCVEC); last night I am taking that later approach. That is the approach that I outlined in the first paragraph of just putting an open blocking stub for UHF at thirty five centimeters ~ up the lower two meter half wave and therefore below the two meter phasing stub between the two meter half wave segments. That has the virtue of being simple and still giving me a dual band antenna that has better gain on UHF then the unmodified two meter antenna would. I wanted a dual band antenna because I only have three suitable mounting points on my home and I already have plans for a six meter J- pole and an anemometer / sensor array assembly on the other two. I have an Arrow dual band J-pole up on that mounting point right now but I wanted to return to the higher gain of the collinear dual half wave J-pole that gave me so much better real world performance on two meters. It is my hope that just adding the seventy centimeter band blocking stub to the collinear antenna's lower two meter half wave segment will do the trick. -- Tom Horne, W3TDH http://www.arrowantenna.info/osj/j-pole.html This design works moderately well. Drive the 19.25 inch element. The 51 inch element is the radiator on 2m, the 6.xx inch element makes the 19.25 inch one radiate on 440. The thing isn't great on 440 because the 51 inch portion is there. It is rugged though. I built a duplicate, which you can do if you look at all the parts pages on the site. The main problem, same as all J poles, is common mode current issues on the feedline. tom K0TAR |
Antenna Modification Advice
On 3/31/2011 6:43 PM, tom wrote:
On 3/30/2011 12:23 PM, Tom Horne wrote: Richard I wanted to explore whether it is practical to have my collinear dual half wave J-pole serve as a dual band antenna. If it were practical I would want the same gain on seventy centimeters that I have been getting out of the dual stacked half wave on two meters. The available testing that I was able to find says that it is 6 DB over a quarter wave vertical. What I would happily settle for would be for it to have the same gain on seventy centimeters as the dual band simple J-pole I am using now. After talking to Rol Anders, K3RA; who was the instructor for my Extra theory class and is the present chairman of the Question Pool Committee of the National Council of Volunteer Examiner Coordinators (NCVEC); last night I am taking that later approach. That is the approach that I outlined in the first paragraph of just putting an open blocking stub for UHF at thirty five centimeters ~ up the lower two meter half wave and therefore below the two meter phasing stub between the two meter half wave segments. That has the virtue of being simple and still giving me a dual band antenna that has better gain on UHF then the unmodified two meter antenna would. I wanted a dual band antenna because I only have three suitable mounting points on my home and I already have plans for a six meter J- pole and an anemometer / sensor array assembly on the other two. I have an Arrow dual band J-pole up on that mounting point right now but I wanted to return to the higher gain of the collinear dual half wave J-pole that gave me so much better real world performance on two meters. It is my hope that just adding the seventy centimeter band blocking stub to the collinear antenna's lower two meter half wave segment will do the trick. -- Tom Horne, W3TDH http://www.arrowantenna.info/osj/j-pole.html This design works moderately well. Drive the 19.25 inch element. The 51 inch element is the radiator on 2m, the 6.xx inch element makes the 19.25 inch one radiate on 440. The thing isn't great on 440 because the 51 inch portion is there. It is rugged though. I built a duplicate, which you can do if you look at all the parts pages on the site. The main problem, same as all J poles, is common mode current issues on the feedline. tom K0TAR Sorry, missed the part where you already have this antenna. My fault for not reading the whole post until after I responded. tom K0TAR |
Antenna Modification Advice
On Thu, 31 Mar 2011 18:46:22 -0500, tom wrote:
http://www.arrowantenna.info/osj/j-pole.html This design works moderately well. Drive the 19.25 inch element. The 51 inch element is the radiator on 2m, the 6.xx inch element makes the 19.25 inch one radiate on 440. The thing isn't great on 440 because the 51 inch portion is there. It is rugged though. I built a duplicate, which you can do if you look at all the parts pages on the site. The main problem, same as all J poles, is common mode current issues on the feedline. tom K0TAR Sorry, missed the part where you already have this antenna. My fault for not reading the whole post until after I responded. Hi Tom, Your suggestion would have been my choice too - except for Tom's first purchase choice. The open stub has always seemed to be a more natural feed method. The addition of the parasitic radiator is still an option for any design. 73's Richard Clark, KB7QHC |
Antenna Modification Advice
On Apr 1, 1:34*am, Richard Clark wrote:
On Thu, 31 Mar 2011 18:46:22 -0500, tom wrote: http://www.arrowantenna.info/osj/j-pole.html This design works moderately well. Drive the 19.25 inch element. The 51 inch element is the radiator on 2m, the 6.xx inch element makes the 19.25 inch one radiate on 440. The thing isn't great on 440 because the 51 inch portion is there. It is rugged though. I built a duplicate, which you can do if you look at all the parts pages on the site. The main problem, same as all J poles, is common mode current issues on the feedline. tom K0TAR Sorry, missed the part where you already have this antenna. *My fault for not reading the whole post until after I responded. Hi Tom, Your suggestion would have been my choice too - except for Tom's first purchase choice. *The open stub has always seemed to be a more natural feed method. The addition of the parasitic radiator is still an option for any design. * 73's Richard Clark, KB7QHC Richard I didn't ignore choking the feed line but I will readily confess that I did not choke it twice. Starting immediately below the bottom of the matching stub I followed the recommendation of the various authors and wound a multi-turn coax balun with a six inch diameter coils of coax. They call for ten turns if I recall correctly. I was not aware of the need for a second choke at one quarter wavelength away. Do you have the energy to explain why that is necessary? How critical is the length between the two chokes. Do I use the middle of the two chokes as my measuring points? Could I substitute a one to one current balun built of ferrite beads? That would have a less intrusive appearance and accumulate less ice in the winter. If you check the link that I gave for Ed Fong's dual band j-pole; available here http://f1.grp.yahoofs.com/v1/ kL6QTZLk1DQjM_Cn3vuvnsLUIuEsvRHSqUZyX2mw294a7mYKkc \FBIXXlRY_6QxreqNWVpn0b7Dogiw9LafU63W429yoO/DBJ2_port_art.pdf or just look it up in the files section of the Yahoo reflector; you will see that he did test it in the field with fairly sophisticated instrumentation. He uses a trapped radiator to obtain similar gain on seventy centimeters as a simple J-pole without the trap gets on two meters. His work was published in QST and I didn't find any authoritative repudiation, or even strong criticism, of his design. Why would the presence of the trap in the lower half wave of the two meter collinear half wave J-pole wreck the tuning on two meters. I'm only trying to learn here. -- Tom Horne, W3TDH |
Antenna Modification Advice
On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne
wrote: Richard I didn't ignore choking the feed line but I will readily confess that I did not choke it twice. Starting immediately below the bottom of the matching stub I followed the recommendation of the various authors and wound a multi-turn coax balun with a six inch diameter coils of coax. They call for ten turns if I recall correctly. Hi Tom, This sound like very common advice - so common that it begs investigation because it is common advice for HF Choking, not 2M, and certainly not 70cm. However, this common advice acknowledges the need for choking. You would be better served using ferrites (W2DU style BalUn/Choke), or, if you really wish to stick with wound coax, then use a Grid Dip Meter to test its resonance (which should reveal you can't serve both bands). Using an antenna analyzer to do this will give you measurable Z, and that may give you the data to see how well you are doing. You may wind a lot of chokes to discover that the diameter is extraordinarily huge (or so the same for turn count - one or the other or both). I was not aware of the need for a second choke at one quarter wavelength away. Do you have the energy to explain why that is necessary? [Do I have the energy.... With all the junk cluttering this space outside of this thread, I have plenty of energy.] The antenna fields will try to excite the transmission line's common mode (the shield of the coax that it sees). This will induce currents that will become radiative (just like stacked elements in a vertical multi-element radiator - sound familiar?). The use of chokes at quarterwave intervals snubs these currents. Sometimes more chokes are needed. I use a 20' long line with a bead every four inches or so. The intent is to create a very long resistor (very wide bandwidth) with very short leads (very high frequency) between its distributed resistance. How critical is the length between the two chokes. Not particularly, you would be well served to attempt to make it at the interval of quarterwave at the highest frequency used (70cm). This, then, would snub lower frequency (2M) common mode currents. Do I use the middle of the two chokes as my measuring points? This sounds deceptively exact (and probably a consideration for those enormous chokes of common advice vintage). No, such exactitude is going to be lost at the 70cm scale anyway which will be perturbed by other factors (have I talked about environment?). Could I substitute a one to one current balun built of ferrite beads? That would have a less intrusive appearance and accumulate less ice in the winter. If you check the link that I gave for Ed Fong's dual band j-pole; available here http://f1.grp.yahoofs.com/v1/ kL6QTZLk1DQjM_Cn3vuvnsLUIuEsvRHSqUZyX2mw294a7mYKk c \FBIXXlRY_6QxreqNWVpn0b7Dogiw9LafU63W429yoO/DBJ2_port_art.pdf or just look it up in the files section of the Yahoo reflector; I would prefer not to add yet one more account registration to simply view this, sorry. However, the narrative here should tell you if we agree. you will see that he did test it in the field with fairly sophisticated instrumentation. He uses a trapped radiator to obtain similar gain on seventy centimeters as a simple J-pole without the trap gets on two meters. My background was working in the standards laboratory system of NBS (my speciality was RF measurement to the highest accuracies). I know that sophisticated instrumentation and quality results don't necessarily track each other. My point was about environment, not instrumentation anyway. You don't need $1000 meters, but you might need $1,000,000 environment. The description of Apple's echoless environmental chamber used to test their iPod is a marvel of engineering, and cost a stack of dollars. His work was published in QST and I didn't find any authoritative repudiation, or even strong criticism, of his design. The same could be said of publishing it in Playboy. QST needs content to offer subscribers. Vanity articles satisfy that need. Think of these articles as the introduction to a topic, not the final word. Why would the presence of the trap in the lower half wave of the two meter collinear half wave J-pole wreck the tuning on two meters. The trap's tuning is heavily influenced by the geometry of the elements because they are also part of the tuning. The trap disturbs that symmetry. You have also introduced new L and C components that further upsets the total circuit. These things are not super-critical when you consider that moving a resonance (say) 145KHz out of 145MHz is only a shift of 0.1%. That isn't super-critical either. Or maybe it is. I'm only trying to learn here. Let me know if this helps. 73's Richard Clark, KB7QHC |
Antenna Modification Advice
On Sat, 02 Apr 2011 16:04:27 -0700, Richard Clark
wrote: On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne wrote: Richard I didn't ignore choking the feed line but I will readily confess that I did not choke it twice. Starting immediately below the bottom of the matching stub I followed the recommendation of the various authors and wound a multi-turn coax balun with a six inch diameter coils of coax. They call for ten turns if I recall correctly. Hi Tom, This sound like very common advice - so common that it begs investigation because it is common advice for HF Choking, not 2M, and certainly not 70cm. However, this common advice acknowledges the need for choking. You would be better served using ferrites (W2DU style BalUn/Choke), or, if you really wish to stick with wound coax, then use a Grid Dip Meter to test its resonance (which should reveal you can't serve both bands). Using an antenna analyzer to do this will give you measurable Z, and that may give you the data to see how well you are doing. You may wind a lot of chokes to discover that the diameter is extraordinarily huge (or so the same for turn count - one or the other or both). This thread has shown me that I don't know enough about Common Mode Chokes. I think they might also be referred to as transmission line chokes. What I need to know is 1- When to use them. RF in the Shack is pretty obvious 2- When not to use them. There must be some negatives. 3- "Compare and Contrast" the Inductor-capacitor choke with the Ferrite Choke(W2DU). Are they electrically equivalent? 4- Are either of them a multi band solution? 5- Are there Upper and lower frequency limitations for using them. As usual, a Google search provides an overwhelming set of responses that most likely will cover the subject in greater detail than I can digest. Early in the list provided by the search I discovered http://www.audiosystemsgroup.com/RFI-Ham.pdf That may have all the answers but I have only glanced at the beginning of the 66 page document. John Ferrell W8CCW |
Antenna Modification Advice
On Apr 2, 7:04*pm, Richard Clark wrote:
On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne wrote: Richard I didn't ignore choking the feed line but I will readily confess that I did not choke it twice. *Starting immediately below the bottom of the matching stub I followed the recommendation of the various authors and wound a multi-turn coax balun with a six inch diameter coils of coax. *They call for ten turns if I recall correctly. * Hi Tom, This sound like very common advice - so common that it begs investigation because it is common advice for HF Choking, not 2M, and certainly not 70cm. *However, this common advice acknowledges the need for choking. You would be better served using ferrites (W2DU style BalUn/Choke), or, if you really wish to stick with wound coax, then use a Grid Dip Meter to test its resonance (which should reveal you can't serve both bands). *Using an antenna analyzer to do this will give you measurable Z, and that may give you the data to see how well you are doing. *You may wind a lot of chokes to discover that the diameter is extraordinarily huge (or so the same for turn count - one or the other or both). I was not aware of the need for a second choke at one quarter wavelength away. *Do you have the energy to explain why that is necessary? * [Do I have the energy.... *With all the junk cluttering this space outside of this thread, I have plenty of energy.] The antenna fields will try to excite the transmission line's common mode (the shield of the coax that it sees). *This will induce currents that will become radiative (just like stacked elements in a vertical multi-element radiator - sound familiar?). *The use of chokes at quarterwave intervals snubs these currents. Sometimes more chokes are needed. *I use a 20' long line with a bead every four inches or so. *The intent is to create a very long resistor (very wide bandwidth) with very short leads (very high frequency) between its distributed resistance. How critical is the length between the two chokes. * Not particularly, you would be well served to attempt to make it at the interval of quarterwave at the highest frequency used (70cm). This, then, would snub lower frequency (2M) common mode currents. Do I use the middle of the two chokes as my measuring points? * This sounds deceptively exact (and probably a consideration for those enormous chokes of common advice vintage). *No, such exactitude is going to be lost at the 70cm scale anyway which will be perturbed by other factors (have I talked about environment?). Could I substitute a one to one current balun built of ferrite beads? *That would have a less intrusive appearance and accumulate less ice in the winter. If you check the link that I gave for Ed Fong's dual band j-pole; available here http://f1.grp.yahoofs.com/v1/ kL6QTZLk1DQjM_Cn3vuvnsLUIuEsvRHSqUZyX2mw294a7mYKk c \FBIXXlRY_6QxreqNWVpn0b7Dogiw9LafU63W429yoO/DBJ2_port_art.pdf or just look it up in the files section of the Yahoo reflector; * I would prefer not to add yet one more account registration to simply view this, sorry. *However, the narrative here should tell you if we agree. you will see that he did test it in the field with fairly sophisticated instrumentation. *He uses a trapped radiator to obtain similar gain on seventy centimeters as a simple J-pole without the trap gets on two meters. * My background was working in the standards laboratory system of NBS (my speciality was RF measurement to the highest accuracies). *I know that sophisticated instrumentation and quality results don't necessarily track each other. *My point was about environment, not instrumentation anyway. *You don't need $1000 meters, but you might need $1,000,000 environment. The description of Apple's echoless environmental chamber used to test their iPod is a marvel of engineering, and cost a stack of dollars. His work was published in QST and I didn't find any authoritative repudiation, or even strong criticism, of his design. The same could be said of publishing it in Playboy. *QST needs content to offer subscribers. *Vanity articles satisfy that need. *Think of these articles as the introduction to a topic, not the final word. Why would the presence of the trap in the lower half wave of the two meter collinear half wave J-pole wreck the tuning on two meters. The trap's tuning is heavily influenced by the geometry of the elements because they are also part of the tuning. *The trap disturbs that symmetry. *You have also introduced new L and C components that further upsets the total circuit. *These things are not super-critical when you consider that moving a resonance (say) 145KHz out of 145MHz is only a shift of 0.1%. *That isn't super-critical either. Or maybe it is. I'm only trying to learn here. Let me know if this helps. 73's Richard Clark, KB7QHC Richard Ed Fong's article can be found on the ARRL website at http://www.arrl.org/files/file/Technology/tis/info/pdf/ 0302038.pdf. I was able to access the above link without any password or other challenge. I just reread it and found that he does address the changes in the two meter element brought on by the inclusion of the seventy centimeter trap. For one thing it shortens the two meter element somewhat. So let me propose a modification process for my collinear dual half wave two meter j-pole and you can tell me if you can think of ways to rearrange or modify the steps so as to move towards by hoped for results. What I hope to achieve is to maintain the gain of the stacked, out of phase, dual half wave antenna on two meters AND trap the lower have wave to a single half wave on seventy centimeters. Here are the steps I was thinking of: Fabricate a trap from 1/2" copper pipe coming out of a tee that has been cut in half through a plane perpendicular to the foot of the tee. Remove the top half wave and the phasing loop from the existing collinear dual half wave antenna Attach the stub to the lower half wave so that the open bottom end of the stub is one half wave on UHF from the top of the matching stub. Measure the SWR on 446 MHz. of the newly trapped two meter radiator. Adjust stub location for minimum SWR at 446 MHz. Measure SWR at 146 MHz. Shorten that radiator to lowest SWR on two meters or until the two SWR readings for VHF & UHF are equal. Reattach the upper two meter half wave and phasing stub. Measure antenna for SWR on both VHF & UHF again Adjust length of of upper two meter half wave if necessary. The antenna analyzer that I will use will be an AEA Technology SWR Meter: 140-525. It happens to be what I have available to me. Does my process seem to be along the right line or would you suggest a different order of operations? -- Tom Horne, W3TDH |
Antenna Modification Advice
On Sun, 03 Apr 2011 10:19:34 -0400, John Ferrell
wrote: What I need to know is 1- When to use them. RF in the Shack is pretty obvious Hi John, Sometimes it is not always that obvious, but it can be made obvious. I say this because the symptoms do not always draw our attention to the problem arising from Common Mode (CM) current. One possible obvious symptom is that you can feel the current on the shielded components of your system - like a microphone shell when your lip brushes along it (which can sometimes give a nasty shock); or when you pull a connector and get that tingle of feeling (which might kill you if conditions were worse). "Obvious" can vary from subtle to deadly. Another possible obvious symptom is you are moving a transmission line while keyed down, and you notice your SWR shifts at the same time. Is it because of a faulty connection? Sometimes yes, but we are talking about CM; and when it is CM, then that shift in SWR is because a tuned circuit (the line is now part of a dynamic system) has been changed. * * * * * * * * * Let's set aside in-the-shack symptoms and look at the antenna end of what might be obvious. For beam antennas, through careful testing with known remote stations (beacons, for instance) you find that you have high sidelobes, almost no back rejection, and a broad response off the front. The transmission line coming down is acting like a vertical antenna, and is feeding its drive into the antenna feed point (two antennas in parallel). That extra antenna is picking up stations that would normally be suppressed by beam design. Some folks never notice this - and it shows how benign CM can be (it is there full blown, but it is NOT obvious at all). For wire antennas, you cut and cut and trim and trim your antenna to frequency, but even though it is book perfect (using the usual formulas for length), it is wildly out of whack and a bitch to tune with your tuner. Your dipole without choking is actually a tripole: two horizontal elements with a third vertical element (which is variable in length and relationship to ground/environment as it swings in the wind). You raise it higher to see if that does anything, and the tune shifts to the moon because the third element has gotten longer. Again, some people never notice this, we have seen them; others do notice it and fulminate against the dipole as being a useless invention where their endfire (which suffers the same issues, but is magical by comparison) antenna is the cat's meow. More could be said.... 2- When not to use them. There must be some negatives. That is a natural question, but I cannot think of any down-side at all. 3- "Compare and Contrast" the Inductor-capacitor choke with the Ferrite Choke(W2DU). Are they electrically equivalent? They are NOT electrically equivalent. The ARE functionally equivalent - that is, they both suit the same purpose of introducing a very hi Z into the current path of CM. The coil of transmission line is, on first pass approximation, an inductor whose Z climbs with applied frequency. The presumption is that it can only get better for the higher bands. This works to a point, but the same coil of TL also has loop-to-loop capacitance, and end-to-end capacitance. These two capacitances, at some frequency, become resonant and offer you a tank circuit. This works even better to isolate the current at the drive point from the rest of the line. However, a tank is not a wide band device by definition. It can exhibit an extremely hi Z at one frequency, and often high enough Z in neighboring bands (hence it is useful if you understand this). As you go higher in frequency, and for the very large coils suggested earlier, then the capacitances begin to dominate and actually kill any sense of choking. The ferrite choke is a resistor, plain and simple. This means it is wide-banded - although its frequency characteristic is not universally applicable (this widebandedness usefully covers two, maybe three octaves). The formulation of the ferrite determines the coverage, and so you can either throw more beads into the mix, or use beads of different formulations to create a more flattened frequency response (over decades of frequency change). 4- Are either of them a multi band solution? Yes, as described above. 5- Are there Upper and lower frequency limitations for using them. That too is described above. As usual, a Google search provides an overwhelming set of responses that most likely will cover the subject in greater detail than I can digest. Early in the list provided by the search I discovered http://www.audiosystemsgroup.com/RFI-Ham.pdf That may have all the answers but I have only glanced at the beginning of the 66 page document. It is a very good and broad discussion. The downside is its enormity. However, this is not a simple topic and understanding comes slow. The reason for that is that Common Mode analysis as distinct from Differential Mode analysis immediately brings up the specter of WTF? There are TWO currents flowing at the same time in the same circuit? This perspective is usually very foreign and takes deep meditation to come to grips with. It is difficult for the majority of the Pros too. I am going to prompt readers for the next question: "What is common mode current?" If any one cares to indulge me by asking, we can go further, but the material in this posting is enough to digest in one sitting. 73's Richard Clark, KB7QHC |
Antenna Modification Advice
On Sun, 3 Apr 2011 09:43:27 -0700 (PDT), Tom Horne
wrote: What I hope to achieve is to maintain the gain of the stacked, out of phase, dual half wave antenna on two meters AND trap the lower have wave to a single half wave on seventy centimeters. Hi Tom, Right off the bat (yes I can see the article now) you offer a conflict. Out of phase blurs gain (lowers it). Your statement suggests you are losing from the start. However, it could simply be wording. Let's make this simple: 1. State the gain at the frequencies of interest before the change; 2. State the gain at those frequencies you expect after the change. Here are the steps I was thinking of: Fabricate a trap from 1/2" copper pipe coming out of a tee that has been cut in half through a plane perpendicular to the foot of the tee. Lost already - sorry. Remove the top half wave and the phasing loop from the existing collinear dual half wave antenna Attach the stub to the lower half wave so that the open bottom end of the stub is one half wave on UHF from the top of the matching stub. 1Measure the SWR on 446 MHz. of the newly trapped two meter radiator. 2Adjust stub location for minimum SWR at 446 MHz. 3Measure SWR at 146 MHz. 4Shorten that radiator to lowest SWR on two meters or until the two SWR readings for VHF & UHF are equal. Probably not. You are going to be repeating steps 1-4 to even approach this desired end condition. 5Reattach the upper two meter half wave and phasing stub. 6Measure antenna for SWR on both VHF & UHF again 7Adjust length of of upper two meter half wave if necessary. This will disturb the work done in steps 1-4. You cannot rely on that configuration being isolated from the work in steps 5-7 which will also demand repeated attention. However, proceed with 5-7 repeats until you obtain the desired end condition. NOW, revisit steps 1-4 with everything attached. I may be wrong, but I suspect you will need to trim things here again. And guess what? This means you need to visit and repeat steps 5-7 again. And guess what? You will be visiting steps 1-4 AND 5-7 again, and again. Take heart, these revisits "should" result in ever smaller adjustments. That means you will approach your desired end condition asymptotically. The antenna analyzer that I will use will be an AEA Technology SWR Meter: 140-525. It happens to be what I have available to me. Does my process seem to be along the right line or would you suggest a different order of operations? The order is quite rational and fully expected. You are entirely on the right track. Your experience will reveal how much they are interactive and how sensitive all this is. If you anticipate the amount of repeated operations and plan accordingly, it will go far more quickly. Epilogue I would note that your writer does not offer any testing of sufficient caliber to support the claims of the EZNEC model. He merely provides signal reports using a sophisticated instrument (a spectrum analyzer that is not being used for spectrum analysis). Those reports also tell me that there is at best 1.2dB gain over a 1/4 wave design - very underwhelming. The only SWR data is reported as "The SWR was low." 73's Richard Clark, KB7QHC |
Antenna Modification Advice
John Ferrell wrote:
On Sat, 02 Apr 2011 16:04:27 -0700, Richard Clark wrote: On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne wrote: Richard I didn't ignore choking the feed line but I will readily confess that I did not choke it twice. Starting immediately below the bottom of the matching stub I followed the recommendation of the various authors and wound a multi-turn coax balun with a six inch diameter coils of coax. They call for ten turns if I recall correctly. Hi Tom, This sound like very common advice - so common that it begs investigation because it is common advice for HF Choking, not 2M, and certainly not 70cm. However, this common advice acknowledges the need for choking. You would be better served using ferrites (W2DU style BalUn/Choke), or, if you really wish to stick with wound coax, then use a Grid Dip Meter to test its resonance (which should reveal you can't serve both bands). Using an antenna analyzer to do this will give you measurable Z, and that may give you the data to see how well you are doing. You may wind a lot of chokes to discover that the diameter is extraordinarily huge (or so the same for turn count - one or the other or both). This thread has shown me that I don't know enough about Common Mode Chokes. I think they might also be referred to as transmission line chokes. What I need to know is 1- When to use them. RF in the Shack is pretty obvious 2- When not to use them. There must be some negatives. 3- "Compare and Contrast" the Inductor-capacitor choke with the Ferrite Choke(W2DU). Are they electrically equivalent? 4- Are either of them a multi band solution? 5- Are there Upper and lower frequency limitations for using them. As usual, a Google search provides an overwhelming set of responses that most likely will cover the subject in greater detail than I can digest. Early in the list provided by the search I discovered http://www.audiosystemsgroup.com/RFI-Ham.pdf That may have all the answers but I have only glanced at the beginning of the 66 page document. That document certainly has a LOT of the answers... and good practical data to use, as well. |
Antenna Modification Advice
On Sun, 03 Apr 2011 10:40:45 -0700, Richard Clark
wrote: On Sun, 03 Apr 2011 10:19:34 -0400, John Ferrell wrote: What I need to know is 1- When to use them. RF in the Shack is pretty obvious Hi John, Sometimes it is not always that obvious, but it can be made obvious. I say this because the symptoms do not always draw our attention to the problem arising from Common Mode (CM) current. One possible obvious symptom is that you can feel the current on the shielded components of your system - like a microphone shell when your lip brushes along it (which can sometimes give a nasty shock); or when you pull a connector and get that tingle of feeling (which might kill you if conditions were worse). "Obvious" can vary from subtle to deadly. Been there, done that... although my particular case was trying to cathode modulate a home brew final about 50 years ago... Another possible obvious symptom is you are moving a transmission line while keyed down, and you notice your SWR shifts at the same time. Is it because of a faulty connection? Sometimes yes, but we are talking about CM; and when it is CM, then that shift in SWR is because a tuned circuit (the line is now part of a dynamic system) has been changed. I had not thought of that. I will have to check that out. * * * * * * * * * Let's set aside in-the-shack symptoms and look at the antenna end of what might be obvious. For beam antennas, through careful testing with known remote stations (beacons, for instance) you find that you have high sidelobes, almost no back rejection, and a broad response off the front. The transmission line coming down is acting like a vertical antenna, and is feeding its drive into the antenna feed point (two antennas in parallel). That extra antenna is picking up stations that would normally be suppressed by beam design. Some folks never notice this - and it shows how benign CM can be (it is there full blown, but it is NOT obvious at all). For wire antennas, you cut and cut and trim and trim your antenna to frequency, but even though it is book perfect (using the usual formulas for length), it is wildly out of whack and a bitch to tune with your tuner. Your dipole without choking is actually a tripole: two horizontal elements with a third vertical element (which is variable in length and relationship to ground/environment as it swings in the wind). You raise it higher to see if that does anything, and the tune shifts to the moon because the third element has gotten longer. You have just described my recent attempt on 160 Meters. I will put it on my "TO DO" list and try again. Again, some people never notice this, we have seen them; others do notice it and fulminate against the dipole as being a useless invention where their endfire (which suffers the same issues, but is magical by comparison) antenna is the cat's meow. More could be said.... 2- When not to use them. There must be some negatives. That is a natural question, but I cannot think of any down-side at all. 3- "Compare and Contrast" the Inductor-capacitor choke with the Ferrite Choke(W2DU). Are they electrically equivalent? They are NOT electrically equivalent. The ARE functionally equivalent - that is, they both suit the same purpose of introducing a very hi Z into the current path of CM. The coil of transmission line is, on first pass approximation, an inductor whose Z climbs with applied frequency. The presumption is that it can only get better for the higher bands. This works to a point, but the same coil of TL also has loop-to-loop capacitance, and end-to-end capacitance. These two capacitances, at some frequency, become resonant and offer you a tank circuit. This works even better to isolate the current at the drive point from the rest of the line. However, a tank is not a wide band device by definition. It can exhibit an extremely hi Z at one frequency, and often high enough Z in neighboring bands (hence it is useful if you understand this). As you go higher in frequency, and for the very large coils suggested earlier, then the capacitances begin to dominate and actually kill any sense of choking. I thought this was a likely scenario but you are the first I have encountered that passed that along. Somewhere I read about a fellow who was using spool of coax in the shack for a choke on 160M... Well intentioned, but clearly not really a solution! The ferrite choke is a resistor, plain and simple. This means it is wide-banded - although its frequency characteristic is not universally applicable (this widebandedness usefully covers two, maybe three octaves). The formulation of the ferrite determines the coverage, and so you can either throw more beads into the mix, or use beads of different formulations to create a more flattened frequency response (over decades of frequency change). 4- Are either of them a multi band solution? Yes, as described above. 5- Are there Upper and lower frequency limitations for using them. That too is described above. As usual, a Google search provides an overwhelming set of responses that most likely will cover the subject in greater detail than I can digest. Early in the list provided by the search I discovered http://www.audiosystemsgroup.com/RFI-Ham.pdf That may have all the answers but I have only glanced at the beginning of the 66 page document. It is a very good and broad discussion. The downside is its enormity. However, this is not a simple topic and understanding comes slow. The reason for that is that Common Mode analysis as distinct from Differential Mode analysis immediately brings up the specter of WTF? There are TWO currents flowing at the same time in the same circuit? This perspective is usually very foreign and takes deep meditation to come to grips with. It is difficult for the majority of the Pros too. I am going to prompt readers for the next question: "What is common mode current?" If any one cares to indulge me by asking, we can go further, but the material in this posting is enough to digest in one sitting. It is you who are indulging me! Google and other Internet resources are very helpful but they are a poor substitute for direct answers to direct questions. As the sailor told the Captain, "I won't abandon the ship until the keel comes through the foredeck!" It seems to me that I might get an indication of current on the outside using a Grid dip meter as a detector at various points on the transmission coax. 73's Richard Clark, KB7QHC |
Antenna Modification Advice
On Sun, 3 Apr 2011 09:43:27 -0700 (PDT), Tom Horne
wrote: On Apr 2, 7:04*pm, Richard Clark wrote: On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne wrote: Richard I didn't ignore choking the feed line but I will readily confess that I did not choke it twice. *Starting immediately below the bottom of the matching stub I followed the recommendation of the various authors and wound a multi-turn coax balun with a six inch diameter coils of coax. *They call for ten turns if I recall correctly. * Hi Tom, This sound like very common advice - so common that it begs investigation because it is common advice for HF Choking, not 2M, and certainly not 70cm. *However, this common advice acknowledges the need for choking. You would be better served using ferrites (W2DU style BalUn/Choke), or, if you really wish to stick with wound coax, then use a Grid Dip Meter to test its resonance (which should reveal you can't serve both bands). *Using an antenna analyzer to do this will give you measurable Z, and that may give you the data to see how well you are doing. *You may wind a lot of chokes to discover that the diameter is extraordinarily huge (or so the same for turn count - one or the other or both). I was not aware of the need for a second choke at one quarter wavelength away. *Do you have the energy to explain why that is necessary? * [Do I have the energy.... *With all the junk cluttering this space outside of this thread, I have plenty of energy.] The antenna fields will try to excite the transmission line's common mode (the shield of the coax that it sees). *This will induce currents that will become radiative (just like stacked elements in a vertical multi-element radiator - sound familiar?). *The use of chokes at quarterwave intervals snubs these currents. Sometimes more chokes are needed. *I use a 20' long line with a bead every four inches or so. *The intent is to create a very long resistor (very wide bandwidth) with very short leads (very high frequency) between its distributed resistance. How critical is the length between the two chokes. * Not particularly, you would be well served to attempt to make it at the interval of quarterwave at the highest frequency used (70cm). This, then, would snub lower frequency (2M) common mode currents. Do I use the middle of the two chokes as my measuring points? * This sounds deceptively exact (and probably a consideration for those enormous chokes of common advice vintage). *No, such exactitude is going to be lost at the 70cm scale anyway which will be perturbed by other factors (have I talked about environment?). Could I substitute a one to one current balun built of ferrite beads? *That would have a less intrusive appearance and accumulate less ice in the winter. If you check the link that I gave for Ed Fong's dual band j-pole; available here http://f1.grp.yahoofs.com/v1/ kL6QTZLk1DQjM_Cn3vuvnsLUIuEsvRHSqUZyX2mw294a7mYKk c \FBIXXlRY_6QxreqNWVpn0b7Dogiw9LafU63W429yoO/DBJ2_port_art.pdf or just look it up in the files section of the Yahoo reflector; * I would prefer not to add yet one more account registration to simply view this, sorry. *However, the narrative here should tell you if we agree. you will see that he did test it in the field with fairly sophisticated instrumentation. *He uses a trapped radiator to obtain similar gain on seventy centimeters as a simple J-pole without the trap gets on two meters. * My background was working in the standards laboratory system of NBS (my speciality was RF measurement to the highest accuracies). *I know that sophisticated instrumentation and quality results don't necessarily track each other. *My point was about environment, not instrumentation anyway. *You don't need $1000 meters, but you might need $1,000,000 environment. The description of Apple's echoless environmental chamber used to test their iPod is a marvel of engineering, and cost a stack of dollars. His work was published in QST and I didn't find any authoritative repudiation, or even strong criticism, of his design. The same could be said of publishing it in Playboy. *QST needs content to offer subscribers. *Vanity articles satisfy that need. *Think of these articles as the introduction to a topic, not the final word. Why would the presence of the trap in the lower half wave of the two meter collinear half wave J-pole wreck the tuning on two meters. The trap's tuning is heavily influenced by the geometry of the elements because they are also part of the tuning. *The trap disturbs that symmetry. *You have also introduced new L and C components that further upsets the total circuit. *These things are not super-critical when you consider that moving a resonance (say) 145KHz out of 145MHz is only a shift of 0.1%. *That isn't super-critical either. Or maybe it is. I'm only trying to learn here. Let me know if this helps. 73's Richard Clark, KB7QHC Richard Ed Fong's article can be found on the ARRL website at http://www.arrl.org/files/file/Technology/tis/info/pdf/ 0302038.pdf. I was able to access the above link without any password or other challenge. I just reread it and found that he does address the changes in the two meter element brought on by the inclusion of the seventy centimeter trap. For one thing it shortens the two meter element somewhat. So let me propose a modification process for my collinear dual half wave two meter j-pole and you can tell me if you can think of ways to rearrange or modify the steps so as to move towards by hoped for results. What I hope to achieve is to maintain the gain of the stacked, out of phase, dual half wave antenna on two meters AND trap the lower have wave to a single half wave on seventy centimeters. Here are the steps I was thinking of: Fabricate a trap from 1/2" copper pipe coming out of a tee that has been cut in half through a plane perpendicular to the foot of the tee. Remove the top half wave and the phasing loop from the existing collinear dual half wave antenna Attach the stub to the lower half wave so that the open bottom end of the stub is one half wave on UHF from the top of the matching stub. Measure the SWR on 446 MHz. of the newly trapped two meter radiator. Adjust stub location for minimum SWR at 446 MHz. Measure SWR at 146 MHz. Shorten that radiator to lowest SWR on two meters or until the two SWR readings for VHF & UHF are equal. Reattach the upper two meter half wave and phasing stub. Measure antenna for SWR on both VHF & UHF again Adjust length of of upper two meter half wave if necessary. The antenna analyzer that I will use will be an AEA Technology SWR Meter: 140-525. It happens to be what I have available to me. Does my process seem to be along the right line or would you suggest a different order of operations? I used to use a 48 element Collinear antenna on 439.25 MHz for Amateur TV in Ohio. It was designed by WA8RMC and constructed by WB8LGA. The procedure to verify that each element was contributing something to the array was to power it with a small signal then watch a field strength meter as far away as you could see it and touch each element to see if it would produce a wiggle on the meter. No wiggle means that particular element was not doing anything. I think you could do something similar to verify things are coupling. That particular antenna was a collection of 24 dipole/reflectors in harness. I have been thinking about building another. It was only 4 feet by 6 feet but a whole lot of wind resistance. I intentionally did not clip this post to keep things in context... John Ferrell W8CCW |
Antenna Modification Advice
On Tue, 05 Apr 2011 10:58:25 -0400, John Ferrell
wrote: Another possible obvious symptom is you are moving a transmission line while keyed down, and you notice your SWR shifts at the same time. Is it because of a faulty connection? Sometimes yes, but we are talking about CM; and when it is CM, then that shift in SWR is because a tuned circuit (the line is now part of a dynamic system) has been changed. I had not thought of that. I will have to check that out. Hi John, Another method (that I have not tried, and may be apocryphal) is to take large piece of tinfoil in your hand (the tin foil is gripped around the line) and move it along the line while watching the SWR meter. The tin foil is used as a means to increase YOUR surface area to the line because you are the major conductor that is moving. You have diverted the Common Mode path away from where it was going, by moving into your hand, through your body, to your feet, and ground. One very sure method is to insert an additional length of transmission line in series (will require a barrel connector). Some lengths are more dramatic in the shift of SWR than others (again, this is about wavelength). This method has been reported to us here more than several times - unfortunately the reporters hadn't connected the dots to recognize this was a confirmation of CM. It seems to me that I might get an indication of current on the outside using a Grid dip meter as a detector at various points on the transmission coax. Get a split core ferrite. Snap it around the coax. Add a short length of wire through the core. Connect the ends of the wire to an LED (now a complete circuit). Move the core/LED along the line during key-down. Did the LED glow? Make sure to test the LED first to see if it glows (I can't imagine why it would not). Test the LED after this survey to see if it glows (I can imagine that CM current could have fried it). Play with adding more turns of the wire through the split core. You have a simple portable RF transformer/indicator. 73's Richard Clark, KB7QHC |
Antenna Modification Advice
On Tue, 05 Apr 2011 11:00:18 -0700, Richard Clark
wrote: On Tue, 05 Apr 2011 10:58:25 -0400, John Ferrell wrote: Another possible obvious symptom is you are moving a transmission line while keyed down, and you notice your SWR shifts at the same time. Is it because of a faulty connection? Sometimes yes, but we are talking about CM; and when it is CM, then that shift in SWR is because a tuned circuit (the line is now part of a dynamic system) has been changed. I had not thought of that. I will have to check that out. Hi John, Another method (that I have not tried, and may be apocryphal) is to take large piece of tinfoil in your hand (the tin foil is gripped around the line) and move it along the line while watching the SWR meter. The tin foil is used as a means to increase YOUR surface area to the line because you are the major conductor that is moving. You have diverted the Common Mode path away from where it was going, by moving into your hand, through your body, to your feet, and ground. One very sure method is to insert an additional length of transmission line in series (will require a barrel connector). Some lengths are more dramatic in the shift of SWR than others (again, this is about wavelength). This method has been reported to us here more than several times - unfortunately the reporters hadn't connected the dots to recognize this was a confirmation of CM. It seems to me that I might get an indication of current on the outside using a Grid dip meter as a detector at various points on the transmission coax. Get a split core ferrite. Snap it around the coax. Add a short length of wire through the core. Connect the ends of the wire to an LED (now a complete circuit). Move the core/LED along the line during key-down. Did the LED glow? Make sure to test the LED first to see if it glows (I can't imagine why it would not). Test the LED after this survey to see if it glows (I can imagine that CM current could have fried it). Play with adding more turns of the wire through the split core. You have a simple portable RF transformer/indicator. 73's Richard Clark, KB7QHC When TV was in in its infancy and in Ham TV we frequently used 300 ohm twin lead transmission lines. The effect was dramatic when you wrapped foil around the twin lead and moved it up an down the feedline. If a half wave of line was accessible, You had the full range of tuning. I also have a meter stick with a pair of wires on which I can slide a shorting bar. The usual indicator is a 50 micro amp meter with a 1n34 germanium diode and either a dipole or sense loop attached. Nearly all of that work was done at 440 mhz. I will think about that LED indicator. Usually I keep the power low when doing this sort of thing. The threshold required to trigger the led. Also, once it triggers the abrupt transistion would probably affect the operating conditions. Perhaps it is time to increase the power a bit for such experiments... John Ferrell W8CCW |
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