Posted: 8/12/2010 4:10:51 PM EDT
So, I built the double toroid UnUn pictured below...
Used two T130-2's each with nine (9) bifilar turns of 14 ga wire... 
There isn't enough height in this box to allow the two toroids to be rotated 90 degrees to each other, as discussed in another thread... Yesterday, I strung up a 55 ft end fed antenna with a counterpoise on the ground, no ground rod... the antenna was placed around 8 to 10 feet above grade and ran beside a metal downspout and a few other small metal objects. Yesterday, I was able to tune the antenna on 20m and 80m but I could not get a decent SWR on 40m... having the center high SWR had me scratching my head... so, I figured that it was either the low height of the antenna or the proximity of the metal objects or the interaction of the two toroids..... So.. today, I get up on the roof and string the antenna out away from metal objects and get it up closer to 15 ft above grade. As yesterday, I run the counterpoise wire on the ground paralleling the antenna, again no ground rod... so now, I can't tune @#$% I can get 17m and higher, but no acceptable SWR on 20, 40 or 80.... .. now I'm pulling my hair... I'm thinking this has to be the field interaction of the two toroids... I'm going to have to break down and get an antenna analyzer... really need to have more information when playing around with stuff like this... 
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Tried a third location with similar results... far removed from anything metal, within 10 ft of grade...
Could tune 20m and above, nothing of lower freq... So... still had to explain why I could tune on the 80m band on the first set up... Has to be the proximety to the metal downspout... interaction on the freq made the antenna electrically longer, lowered the impedance within the range of my disfuctional transformer/UnUn... UnUn #3 on the table... |
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Why are you worrying about high SWR? Well... that is a good question... pardon my basic understanding, but my thoughts... In this case, it is simply a statement that the UnUn isn't working... or rather a confirmation that the two adjacent toroids do indeed influence each other. Not a concern for the actual function of the antenna as a posting the a failed experiment.... however, First, the FT-857D simply responds differently to a high SWR. Through the FC-30 Tunner, it will retain the settings, for adjacent freq's, if the SWR is 3:1 or below. 3:1 and above and it makes no such effort.... I will have to go back and review the manual, but I have been under the assumption ( dangerous, I know ) that the radio would limit power for high values of SWR... I thought I read somewhere that it starts limiting at 1.7:1... but, I'll have to double check on that.... Secondly, given that I am transmitting through an circuit, the UnUn, a mismatched impedance will result in potentially large reflected power back into the UnUn... granted this one is wound with 14 ga wire, but would still like to keep that in check... shooting for a good match and an understanding of the acceptable boundaries of what I am working with... Third, I do not have the capacity to install a proper antenna. I do not have a tree upon which to place a dipole (blocked by power lines - which are noisy btw). My neighborhood will not allow roof mounted, or tower mounted, antennas. So, I continue to play with and work with antennas within the attic space, along my privacy fence, along my roof line or some other *make the best of it* placement. Minimizing antenna/feed line imbalances, minimizing antenna losses will improve effective radiated power of an already comprimised antenna placement... no? |
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Why are you worrying about high SWR? I'm wondering why he is worrying about an UnUn... 55ft is getting close to a 1/2 wave on 40m, the impedance was just too high for your feed system/tuner to adapt to and/or you're pushing that "unun" too hard at those voltages. Frankly I'm not sure that what you are trying to do is a reasonable application for an unun anyway. To my knowledge they are intended for something like a ground mounted HF vertical, where the feedline tends to act as a ground radial as it runs away from the feedpoint. An unun is used at a point away from the antenna to decouple the currents flowing along the shield and stop it acting like a ground radial all the way back to the shack. |
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Why are you worrying about high SWR? I'm wondering why he is worrying about an UnUn... 55ft is getting close to a 1/2 wave on 40m, the impedance was just too high for your feed system/tuner to adapt to and/or you're pushing that "unun" too hard at those voltages. Frankly I'm not sure that what you are trying to do is a reasonable application for an unun anyway. To my knowledge they are intended for something like a ground mounted HF vertical, where the feedline tends to act as a ground radial as it runs away from the feedpoint. An unun is used at a point away from the antenna to decouple the currents flowing along the shield and stop it acting like a ground radial all the way back to the shack. Trying to build a QRO version of ––> this <–– 
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Why are you worrying about high SWR? I'm wondering why he is worrying about an UnUn... 55ft is getting close to a 1/2 wave on 40m, the impedance was just too high for your feed system/tuner to adapt to and/or you're pushing that "unun" too hard at those voltages. Frankly I'm not sure that what you are trying to do is a reasonable application for an unun anyway. To my knowledge they are intended for something like a ground mounted HF vertical, where the feedline tends to act as a ground radial as it runs away from the feedpoint. An unun is used at a point away from the antenna to decouple the currents flowing along the shield and stop it acting like a ground radial all the way back to the shack. Trying to build a QRO version of ––> this <–– http://www.angelfire.com/electronic2/qrp/MULTIUNUN.jpg I'm not an electrical engineer, but my SWAG is that all that device is doing is putting enough loss into the system that it enables a limited-range tuner to match the random load. With higher power, I'm guessing you'd need much larger toroids to be able to handle the power level. Probably be better off with a terminated antenna, at least then the loss happens after it has a chance to radiate and not before. |
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You should be fine without the unin. If anything coil up about 8 turns of cable to act as a current balun. That's my thinking as well. Coil of coax or some ferrite beads to suppress RF on the shield is a better and lower-loss solution... and a wider-range tuner or some attempt at resonant antennas |
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You should be fine without the unin. If anything coil up about 8 turns of cable to act as a current balun. That's my thinking as well. Coil of coax or some ferrite beads to suppress RF on the shield is a better and lower-loss solution... and a wider-range tuner or some attempt at resonant antennas You will be amazed at where this will work...........YMMV http://www.hamuniverse.com/balun.html Link |
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Thanks for the input... I appreciate any and all thoughts and comments...
But, I think we took a left turn on the thread somewhere.... probably when I was asked "why be concerned about SWR..." While I am constantly working with and playing with antennas... and do have an antenna challenged residence... that really wasn't the intent of this post/thread.. It was more a discussion on the function of an UnUn made of two adjacent toroids, and the apparent influence they have on each other, as opposed to a single One of the *leasons learned* from this little test, is that without the proper testing equipment, you really are left with a lot of speculation about what is actually happening and nothing more than a *diagnosis by exclusion* for trying to come to any meaningful conclusion... But again... I do appreciate the thoughts and comments... I learn something everyday here... even when a thread takes a left turn  
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Thanks for the input... I appreciate any and all thoughts and comments... But, I think we took a left turn on the thread somewhere.... probably when I was asked "why be concerned about SWR..." While I am constantly working with and playing with antennas... and do have an antenna challenged residence... that really wasn't the intent of this post/thread.. It was more a discussion on the function of an UnUn made of two adjacent toroids, and the apparent influence they have on each other, as opposed to a single One of the *leasons learned* from this little test, is that without the proper testing equipment, you really are left with a lot of speculation about what is actually happening and nothing more than a *diagnosis by exclusion* for trying to come to any meaningful conclusion... But again... I do appreciate the thoughts and comments... I learn something everyday here... even when a thread takes a left turn
Left turns seem to happen a lot on the internet...  Back on subject...
Well you are correct, without good test equipment, you are really in the dark. That's why some folks scrimp and save for good T&M equipment at a young age, and then use that capability to lever up their knowledge and level of success. It works... As far as the coupling between the two torroids, [torroids inherently have small external fields compared to plain 'coils'] why not try a few things: First, unsolder one of the torroids so you can turn it at rt angle to the other, reconnect and see it you note a difference. I doubt you will... Second, since you used two torroids because you didn't have a larger one or a larger one wouldn't fit in your box, run lower power and wind a single torroid unun with smaller wire and see if you note a difference. [Maybe find some enameled wire from R-S or a transformer or something.] You can also stack the torroids if you can use smaller wire for your experiment. |
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Left turns seem to happen a lot on the internet...
Yep... and, always something interesting to unfold.... That's why some folks scrimp and save for good T&M equipment at a young age, and then use that capability to lever up their knowledge and level of success. It works...
Within my long list of character traits - that agrivate my wife - is my inability to let something go until I understand it.... so, I can't put this asside unresolved. I may have to set it on a shelf until such time as I have more toys/equipment... just not there yet... Second, since you used two torroids because you didn't have a larger one or a larger one wouldn't fit in your box, run lower power and wind a single torroid unun with smaller wire and see if you note a difference.
From the current ARRL Extra Class Manual, page 4-36 "To reduce unwanted coupling, the donut-shaped toroid core is used.... Nearly all of the toroidal inductor's magnetic field is contained within the toroid core. Toroidal inductors are one of the most popular inductor type in RF curcuits because they can be located close to each other on a circuit board with almost no interaction. " So, as you noted and it described above... in theory... two toroids in close proximity should work... but as of yet in my setup, they are not... and I am lacking the skills and tools to sort through this... There is a lot of learning, in this one simple little project, for me.... Why equal turns of the bifilar winding actually performs the impedance matching... on the surface, not a lot of transformer action with two equal turn wires... I assume the direct short to ground is the functional culprit, but, don't yet completely understand it... Why the counterpoise wire vs a direct short wire to a ground rod will perform the same with this arrangement... Like I said... stuff I still have a lot of reading, studying and playing with - once I have a few more toys...... I have a couple more things I want to do with this set up... unlikely to resolve and answer everything... then, I'll set it asside until I am better prepared to tackle this one again... Thanks again for the input!! |