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Matching on the MFJ-1800
Hi All,
My original thread seems to have died, still wondering how the folded loop is matched to 50 ohms. (probably isn't? cheap, but works?) I've added a dimensional drawing and some more pics. If more info is needed let me know. Here is a drawing and some more pics. http://i395.photobucket.com/albums/p...intFileJPG.jpg http://i395.photobucket.com/albums/p...connection.jpg http://i395.photobucket.com/albums/p...MFJRuledDE.jpg http://i395.photobucket.com/albums/p...Jruledcoax.jpg Hope I covered everything, Mike |
Matching on the MFJ-1800
On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" wrote:
My original thread seems to have died, still wondering how the folded loop is matched to 50 ohms. (probably isn't? cheap, but works?) I've added a dimensional drawing and some more pics. If more info is needed let me know. See: http://802.11junk.com/jeffl/antennas/mfj1800/ The annotated NEC2 file is attached to the "Main" JPG. Cut-n-paste should get you something to play with. This is NOT an exact representation of the MFJ1800 antenna. The elements are round, not flat. The driven element is a squared off approximation. Some of the dimensions are questionable. Note that the feed impedance is normalized to 288 ohms, the characteristic impedance of a folded dipole, not 50 ohms. Here is a drawing and some more pics. http://i395.photobucket.com/albums/p...intFileJPG.jpg I need an overall length, measured from the CENTER of the driven element (the plastic screw hole) to the CENTER of the last director. That's because a tiny error in the spacing between elements at 1.060" (2.69cm) grows rather rapidly when multiplied by 14 elements. NEC works in wavelengths or meters, not inches. Think metric and forget about using inches. Please check the length of the first and 2nd directors. I don't believe MFJ would make them the same length as the other directors. Also, measure the coax balun cable dimensions. Mostly, I'm interested in the: 1. Shield to shield length. 2. Center pin to start of folded dipole length. (i.e. exposed center wire length). 3. OD of center conductor wire. 4. ID of shield. 5. A good guess as to the type of dielectric (foam, solid, or PTFE). 6. Any markings that might identify the coax. I'll do a better job of modeling the folded dipole driven element later. Modeling all the asymmetrical elements is going to require more time than I want to spend. Maybe if I get inspired to RTFM. It seems easy, but looks tedious. What I've deduced from the model so far is: 1. The characteristic impedance is over 300 ohm without the coax balun/matching/whatever section. 2. A 1/4 wave coaxial matching section isn't going to work as the highest commonly available impedance coax of 93 ohms will only match 50 ohms to about 173 ohms. That thing doesn't look anything like a 4:1 balun (unless there's something we missed under all that shrink tube). 3. With a suitable 4:1 balun, the antenna is slightly shy of the advertised gain spec of 15dBi: http://www.mfjenterprises.com/Product.php?productid=MFJ-1800 with a gain of 14.3dBi. Close enough. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
amdx wrote:
Hi All, My original thread seems to have died, still wondering how the folded loop is matched to 50 ohms. (probably isn't? cheap, but works?) I've added a dimensional drawing and some more pics. If more info is needed let me know. Here is a drawing and some more pics. http://i395.photobucket.com/albums/p...intFileJPG.jpg http://i395.photobucket.com/albums/p...connection.jpg http://i395.photobucket.com/albums/p...MFJRuledDE.jpg http://i395.photobucket.com/albums/p...Jruledcoax.jpg Hope I covered everything, Mike Hi, I used to measure impedance of antenna feed point with home made noise bridge. I know there is a 300 Ohm to 75 Ohm trnasformer balun. |
Matching on the MFJ-1800
On Tue, 17 Nov 2009 18:35:15 -0800, Jeff Liebermann
wrote: http://802.11junk.com/jeffl/antennas/mfj1800/ The annotated NEC2 file is attached to the "Main" JPG. Cut-n-paste should get you something to play with. A few changes. I changed the simulated folded dipole into something more closely resembling the original MFJ folded dipole. I also changed the characteristic impedance to 200 ohms so that more closely resembles a 4:1 match to 50 ohms. Of course, the VSWR went from tolerable to horrible (10:1). I suspect I did something wrong in locating the feed point, but I'll fix that later. When using a geometry editor/generator, I turned my nice neat NEC macros into unreadable gibberish. Sigh... I'll also fix that later. Gain is still 14.1dBi. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
On Tue, 17 Nov 2009 18:35:15 -0800, Jeff Liebermann
wrote: On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" wrote: My original thread seems to have died, still wondering how the folded loop is matched to 50 ohms. (probably isn't? cheap, but works?) I've added a dimensional drawing and some more pics. If more info is needed let me know. See: http://802.11junk.com/jeffl/antennas/mfj1800/ The annotated NEC2 file is attached to the "Main" JPG. Cut-n-paste should get you something to play with. This is NOT an exact representation of the MFJ1800 antenna. The elements are round, not flat. The driven element is a squared off approximation. Some of the dimensions are questionable. Note that the feed impedance is normalized to 288 ohms, the characteristic impedance of a folded dipole, not 50 ohms. My model uses 1/4" elements, and for just the loop, Z = 150 Ohms @ 2250 MHz For the Reflector, driven loop, and director, the Z = 72 Ohms @ 2450 MHz The loop ends are 8 segments(22.5 degrees). EZNEC complains of len/dia ratio being too small, but there's nothing to be done about that. 20 wires total, 62 segments total (for just the first three elements). NEC works in wavelengths or meters, not inches. Think metric and forget about using inches. I went with the dimensions provided - inches no problem. Please check the length of the first and 2nd directors. I don't believe MFJ would make them the same length as the other directors. This is imparting too much engineering for their product. Also, measure the coax balun cable dimensions. Mostly, I'm interested in the: 1. Shield to shield length. 2. Center pin to start of folded dipole length. (i.e. exposed center wire length). 3. OD of center conductor wire. 4. ID of shield. 5. A good guess as to the type of dielectric (foam, solid, or PTFE). 6. Any markings that might identify the coax. THIS is where error is going to intrude, big time. I think there's too much attention to this detail for what it is supposed to do - but I have already written on that subject. To answer the original question, the folded loop Z drops due to the proximity of the other parasitic elements (a normal consequence). However, as to calling it a folded loop seems to be straying from conventional usage as those loops are rather sweeping (large). This may be deliberate if my data conforms to the usage found. As a side note on just three elements, there is a degree of up/down/side look due to the loss of co-planar symmetry of the loop with the other elements. Adding more directors would probably hide this. 73's Richard Clark, KB7QHC |
Matching on the MFJ-1800
On Tue, 17 Nov 2009 20:29:08 -0800, Jeff Liebermann
wrote: On Tue, 17 Nov 2009 18:35:15 -0800, Jeff Liebermann wrote: http://802.11junk.com/jeffl/antennas/mfj1800/ The annotated NEC2 file is attached to the "Main" JPG. Cut-n-paste should get you something to play with. A few changes. I changed the simulated folded dipole into something more closely resembling the original MFJ folded dipole. I also changed the characteristic impedance to 200 ohms so that more closely resembles a 4:1 match to 50 ohms. Of course, the VSWR went from tolerable to horrible (10:1). I suspect I did something wrong in locating the feed point, but I'll fix that later. When using a geometry editor/generator, I turned my nice neat NEC macros into unreadable gibberish. Sigh... I'll also fix that later. Gain is still 14.1dBi. More changes(tm). I fixed the feed point on the flattened folded dipole and the VSWR magically came down to normal (1.47:1 into 200 ohms). Vertical gain dropped to 13.1dBi, but horizontal gain dropped to 6.35dBi. Something (else) is wrong. I'll fix the macros so that the NEC2 file is readable, later. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800 - 3d-01.jpg (0/1)
On Tue, 17 Nov 2009 20:58:00 -0800, Richard Clark
wrote: My model uses 1/4" elements, and for just the loop, Z = 150 Ohms @ 2250 MHz The original has an asymmetrical cross section of .330cm x 0.200cm (from the supplied dimensions). I decided that a suitable equivalent would be a round rod, with the same circumferance. of 1.06cm. That's a 0.338cm dia rod. However, I don't know if this is a realistic approximation. Your 0.635cm diameter rods might be a bit wide. For the Reflector, driven loop, and director, the Z = 72 Ohms @ 2450 MHz The loop ends are 8 segments(22.5 degrees). EZNEC complains of len/dia ratio being too small, but there's nothing to be done about that. 20 wires total, 62 segments total (for just the first three elements). My folded dipole ended up with 4 segements (45 deg/seg) at each end and about 42 segments total. 4NEC2 also initially complained about the ratio of the segment length to rod diameter, but I reduced the number of segments and reduced the wire diameter to correspond to the copper tape thickness. 4NEC2 was then happy. http://www.11junk.com/jeffl/antennas/mfj1800/slides/Geometry.html NEC works in wavelengths or meters, not inches. Think metric and forget about using inches. I went with the dimensions provided - inches no problem. I kinda like to do things in wavelengths because invariably I end up recycling (plagerizing) my own designs to operate on different frequencies. With wavelengths, that can be done by just tweaking one or two numbers (GS scale structure card). However, this abomination of an antenna design is unlikely to be useful at other frequencies, so I used meters and not worry about scaling. I find that I make many more errors when using inches than with metric. Please check the length of the first and 2nd directors. I don't believe MFJ would make them the same length as the other directors. This is imparting too much engineering for their product. I'll be generous and not pass judgement on this contrivance until I understand this antenna with its rather odd balun, or whatever it is. It's possible to build an equal space director Yagi-Uda antenna but not with equal length directors. If I feel ambitious, I might feed it to an optimizer and see what optimizing the first two director lengths can offer. Also, measure the coax balun cable dimensions. Mostly, I'm interested in the: 1. Shield to shield length. 2. Center pin to start of folded dipole length. (i.e. exposed center wire length). 3. OD of center conductor wire. 4. ID of shield. 5. A good guess as to the type of dielectric (foam, solid, or PTFE). 6. Any markings that might identify the coax. THIS is where error is going to intrude, big time. I think there's too much attention to this detail for what it is supposed to do - but I have already written on that subject. Me too. However, I was rather guarded in my criticism since I didn't really understand the design of the balun and the purpose of the ferrite beads. My best guess was a 1/4 wave (or 3/4 wave) coax matching contrivance made from RG-62/u (93ohm) to match to 173 ohms. If the feed impedance came anywhere near 173 ohms, and the coax turns out to be 93 ohms, then it has a chance. However, even if it met all the criteria for an effective 1/4 wave match, the sloppy construction and exposed center conductors will certainly have detrimental effects. To answer the original question, the folded loop Z drops due to the proximity of the other parasitic elements (a normal consequence). However, as to calling it a folded loop seems to be straying from conventional usage as those loops are rather sweeping (large). This may be deliberate if my data conforms to the usage found. Well, the impedance is going to be somewhere between a circular full wave loop (about 100 ohms) and that of a folded dipole (about 288 ohms). The problem is that this folded dipole isn't folded very much and more closely resembles a loop. In addition, the reflector and 1st director are not particularly close to the driven element. That makes me suspect (i.e. guess) that their effect on reducing the feed impedance is going to be minimal. As a side note on just three elements, there is a degree of up/down/side look due to the loss of co-planar symmetry of the loop with the other elements. Adding more directors would probably hide this. 73's Richard Clark, KB7QHC -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
"Jeff Liebermann" wrote in message ... On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" wrote: My original thread seems to have died, still wondering how the folded loop is matched to 50 ohms. (probably isn't? cheap, but works?) I've added a dimensional drawing and some more pics. If more info is needed let me know. I need an overall length, measured from the CENTER of the driven element (the plastic screw hole) to the CENTER of the last director. That's because a tiny error in the spacing between elements at 1.060" (2.69cm) grows rather rapidly when multiplied by 14 elements. The CENTER of the driven element (the plastic screw hole) to the CENTER of the last director is 36.7cm (14-7/16") Please check the length of the first and 2nd directors. I don't believe MFJ would make them the same length as the other directors. ALL directors or the same length (within .003") They are 46.6mm in length. Also, measure the coax balun cable dimensions. Mostly, I'm interested in the: 1. Shield to shield length. 54.2mm (2.134") 2. Center pin to start of folded dipole length. (i.e. exposed center wire length). 61.75mm (2.431) 3. OD of center conductor wire. 0.94mm (0.037") 4. ID of shield. 3.3mm (.130) 5. A good guess as to the type of dielectric (foam, solid, or PTFE). My guess is solid poyethylene, if you have a test other than poking it with a pin to get feel of it, which I did. 6. Any markings that might identify the coax. No help there. That thing doesn't look anything like a 4:1 balun (unless there's something we missed under all that shrink tube). Looks like 4 toroids that slide over the coax, you can see the outline of the edges of each toroid. Mike |
Matching on the MFJ-1800
"Richard Clark" wrote in message ... On Tue, 17 Nov 2009 18:35:15 -0800, Jeff Liebermann wrote: On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" wrote: My original thread seems to have died, still wondering how the folded loop is matched to 50 ohms. (probably isn't? cheap, but works?) I've added a dimensional drawing and some more pics. If more info is needed let me know. See: http://802.11junk.com/jeffl/antennas/mfj1800/ The annotated NEC2 file is attached to the "Main" JPG. Cut-n-paste should get you something to play with. This is NOT an exact representation of the MFJ1800 antenna. The elements are round, not flat. The driven element is a squared off approximation. Some of the dimensions are questionable. Note that the feed impedance is normalized to 288 ohms, the characteristic impedance of a folded dipole, not 50 ohms. My model uses 1/4" elements, and for just the loop, Z = 150 Ohms @ 2250 MHz For the Reflector, driven loop, and director, the Z = 72 Ohms @ 2450 MHz The loop ends are 8 segments(22.5 degrees). EZNEC complains of len/dia ratio being too small, but there's nothing to be done about that. 20 wires total, 62 segments total (for just the first three elements). NEC works in wavelengths or meters, not inches. Think metric and forget about using inches. I went with the dimensions provided - inches no problem. Please check the length of the first and 2nd directors. I don't believe MFJ would make them the same length as the other directors. This is imparting too much engineering for their product. Yes it is just a stamped piece of aluminum with rough edges and little nibs here and there that were never cleaned off. Also, measure the coax balun cable dimensions. Mostly, I'm interested in the: 1. Shield to shield length. 2. Center pin to start of folded dipole length. (i.e. exposed center wire length). 3. OD of center conductor wire. 4. ID of shield. 5. A good guess as to the type of dielectric (foam, solid, or PTFE). 6. Any markings that might identify the coax. THIS is where error is going to intrude, big time. I think there's too much attention to this detail for what it is supposed to do - but I have already written on that subject. To answer the original question, the folded loop Z drops due to the proximity of the other parasitic elements (a normal consequence). Understood. However, as to calling it a folded loop seems to be straying from conventional usage as those loops are rather sweeping (large). This may be deliberate if my data conforms to the usage found. By this statement, do you mean the spacing between elements is large so it looses it's folded loop characteristics? Mike |
Matching on the MFJ-1800
"amdx" wrote in message ... "Richard Clark" wrote in message ... On Tue, 17 Nov 2009 18:35:15 -0800, Jeff Liebermann wrote: On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" wrote: My original thread seems to have died, still wondering how the folded loop is matched to 50 ohms. (probably isn't? cheap, but works?) I've added a dimensional drawing and some more pics. If more info is needed let me know. See: http://802.11junk.com/jeffl/antennas/mfj1800/ The annotated NEC2 file is attached to the "Main" JPG. Cut-n-paste should get you something to play with. This is NOT an exact representation of the MFJ1800 antenna. The elements are round, not flat. The driven element is a squared off approximation. Some of the dimensions are questionable. Note that the feed impedance is normalized to 288 ohms, the characteristic impedance of a folded dipole, not 50 ohms. My model uses 1/4" elements, and for just the loop, Z = 150 Ohms @ 2250 MHz For the Reflector, driven loop, and director, the Z = 72 Ohms @ 2450 MHz The loop ends are 8 segments(22.5 degrees). EZNEC complains of len/dia ratio being too small, but there's nothing to be done about that. 20 wires total, 62 segments total (for just the first three elements). NEC works in wavelengths or meters, not inches. Think metric and forget about using inches. I went with the dimensions provided - inches no problem. Please check the length of the first and 2nd directors. I don't believe MFJ would make them the same length as the other directors. This is imparting too much engineering for their product. Yes it is just a stamped piece of aluminum with rough edges and little nibs here and there that were never cleaned off. Also, measure the coax balun cable dimensions. Mostly, I'm interested in the: 1. Shield to shield length. 2. Center pin to start of folded dipole length. (i.e. exposed center wire length). 3. OD of center conductor wire. 4. ID of shield. 5. A good guess as to the type of dielectric (foam, solid, or PTFE). 6. Any markings that might identify the coax. THIS is where error is going to intrude, big time. I think there's too much attention to this detail for what it is supposed to do - but I have already written on that subject. To answer the original question, the folded loop Z drops due to the proximity of the other parasitic elements (a normal consequence). Understood. However, as to calling it a folded loop seems to be straying from conventional usage as those loops are rather sweeping (large). This may be deliberate if my data conforms to the usage found. By this statement, do you mean the spacing between elements is large so it looses it's folded loop characteristics? I meant folded dipole characteristics. (not folded loop) Mike |
Matching on the MFJ-1800
On Wed, 18 Nov 2009 07:07:21 -0600, "amdx" wrote:
However, as to calling it a folded loop seems to be straying from conventional usage as those loops are rather sweeping (large). This may be deliberate if my data conforms to the usage found. By this statement, do you mean the spacing between elements is large so it looses it's folded loop characteristics? Hi Mike, I was a bit obscure on that last point. YOUR antenna's looped dipole has rather large turns for a folded loop. A folded quarterwave (for verticals) or folded halfwaves (for dipoles) generally show 4X impedance boost for same sized wire/element in the loop. Some designs use different sized halves of the loop to change the multiplication factor as this is the primary determinant with closely spaced folds. However, your antenna has rather larger spacing which may lower the multiplication factor IF my model conforms to actual. This choice of larger looping may show MFJ's engineering talent in achieving a natural match. I don't think the rest of the array is going to matter much in getting the "most" gain it could, but it is probably good. 73's Richard Clark, KB7QHC |
Matching on the MFJ-1800
On Wed, 18 Nov 2009 07:00:26 -0600, "amdx" wrote:
The CENTER of the driven element (the plastic screw hole) to the CENTER of the last director is 36.7cm (14-7/16") Thanks. My numbers came out to 36.52 cm which is close enough. ALL directors or the same length (within .003") They are 46.6mm in length. Amazing... Also, measure the coax balun cable dimensions. Mostly, I'm interested in the: Nice. I'll see if I guess(tm) the coax type. It doesn't seem like a good fit for any of the common cables as the center conductor is somewhat larger diameter than any of these listed. See if you can find an exposed center conductor without any tinning or soldering. http://www.epanorama.net/documents/wiring/coaxcable.html One more dimension... the approximate outer jacket diameter of the coax (not including the shrink tube). My guess is solid poyethylene, if you have a test other than poking it with a pin to get feel of it, which I did. Weird(tm). Low loss coax would need to be foam or teflon. Solid polyethylene is easier to work with, cheaper, but not the best. However, a short piece like this balun would not have much loss, so I guess it doesn't matter what flavor is used: http://en.wikipedia.org/wiki/Coaxial_cable Gotta do paying work today. I'll play with this more in the next day or two (so I don't forget what I'm doing). -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
"Jeff Liebermann" wrote in message ... On Wed, 18 Nov 2009 07:00:26 -0600, "amdx" wrote: The CENTER of the driven element (the plastic screw hole) to the CENTER of the last director is 36.7cm (14-7/16") Thanks. My numbers came out to 36.52 cm which is close enough. ALL directors or the same length (within .003") They are 46.6mm in length. Amazing... Also, measure the coax balun cable dimensions. Mostly, I'm interested in the: Nice. I'll see if I guess(tm) the coax type. It doesn't seem like a good fit for any of the common cables as the center conductor is somewhat larger diameter than any of these listed. See if you can find an exposed center conductor without any tinning or soldering. http://www.epanorama.net/documents/wiring/coaxcable.html One more dimension... the approximate outer jacket diameter of the coax (not including the shrink tube). My guess is solid poyethylene, if you have a test other than poking it with a pin to get feel of it, which I did. Weird(tm). Low loss coax would need to be foam or teflon. Solid polyethylene is easier to work with, cheaper, but not the best. However, a short piece like this balun would not have much loss, so I guess it doesn't matter what flavor is used: http://en.wikipedia.org/wiki/Coaxial_cable Gotta do paying work today. I'll play with this more in the next day or two (so I don't forget what I'm doing). Jeff Liebermann I have found the need to work for a living, gets in the way of a lot of fun! I took the coax loose on the MFJ-1800 and and removed the toroids, I found the letters found M1Z/111-RG and then the insulation ended. Argh! Oh, I have a second antenna, so I took that one apart, Eureka! RGS-303 http://wireandcable.thermaxcdt.com/i...d-70-?&seo=110 50 ohm coax. PTFE center insulator, FEP jacket. Mike |
Matching on the MFJ-1800
On Thu, 19 Nov 2009 06:05:20 -0600, "amdx" wrote:
I have found the need to work for a living, gets in the way of a lot of fun! 5 hours on the phone on a slow motion conference call this morning. Absolutely nothing useful accomplished. Such online meetings should be banned, taxed, or both as a threat to national productivity. I took the coax loose on the MFJ-1800 and and removed the toroids, I found the letters found M1Z/111-RG and then the insulation ended. Argh! Oh, I have a second antenna, so I took that one apart, Eureka! RGS-303 http://wireandcable.thermaxcdt.com/i...d-70-?&seo=110 50 ohm coax. PTFE center insulator, FEP jacket. Mike Thanks. 50 ohm coax does not make it a matching section to a 200 ohm folded dipole. However, the ferrite beads are a good way to simply block the reflected power from the folded dipole so that it looks like it's matched. In any case, that reflected power is lost (converted to heat) in the ferrite beads. So much for efficiency. When I change the characteristic impedance of the model from 200 ohms to 50 ohms, the VSWR climbs to 5.5:1. Yech. (Note that the radiation efficiency is 75% with or without the mismatch). I suppose the antenna could be made to function by replacing the coax section with a real 1/4 wave 4:1 balun, but I'll leave that to MFJ to figure out. If you need some more entertainment value, it would be interesting to actually measure the gain of the antenna. Find a known good reputable antenna with similar gain. A panel or patch will work. Find a signal source that isn't infested with reflections (including ground reflections), Fresnel Zone issues, and is fairly stable (i.e. doesn't physically move). Use Netstumbler, WirelessMon, or Kismet to compare the signal strengths. For additional accuracy, use a step attenuator to adjust the signal levels to a common reference level. Better yet, use a spectrum analyzer. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
On Thu, 19 Nov 2009 11:48:12 -0800, Jeff Liebermann
wrote: Thanks. 50 ohm coax does not make it a matching section to a 200 ohm folded dipole. Perhaps not an optimal one (which would be the geometric mean of the source/load) but an effective one - if it were designed so (I don't think it was ever intended to be one). However, the ferrite beads are a good way to simply block the reflected power from the folded dipole so that it looks like it's matched. In any case, that reflected power is lost (converted to heat) in the ferrite beads. So much for efficiency. In fact the beads won't do that at all. They see only the common mode circuit. Reflected power is going to reside in the transverse mode circuit where the beads are invisible. When I change the characteristic impedance of the model from 200 ohms to 50 ohms, the VSWR climbs to 5.5:1. Yech. (Note that the radiation efficiency is 75% with or without the mismatch). My model shows a more benign mismatch to a 72 Ohm load. 73's Richard Clark, KB7QHC |
Matching on the MFJ-1800
In article ,
Jeff Liebermann wrote: Thanks. 50 ohm coax does not make it a matching section to a 200 ohm folded dipole. However, the ferrite beads are a good way to simply block the reflected power from the folded dipole so that it looks like it's matched. In any case, that reflected power is lost (converted to heat) in the ferrite beads. Ummm... I'd disagree (or at least quibble) on at least two grounds. Ground #1: the ferrite beads would only block non-balanced current flow back down the outside of the coax. They will have no effect at all on power which is reflected back down the inside of the coax (the center conductor and the inside of the shield) from any impedance mismatch where the coax meets the folded dipole. In effect, the presence of the beads (if they're choking the coax properly) actually ensures that the transmitter *does* see the true effect of any impedance mismatch. The transmitter is just as likely to see a higher SWR than a lower one, when the beads are added. Ground #2: the beads do not necessarily result in a significant loss of power. If their RF impedance is high enough at the frequency of use, then RF current flow through them will be negligible, and there won't be a loss of power. Power loss in choke-ferrites tends to be worst when the RF impedance is both resistive, and too low for the application (i.e. still allows substantial current flow, which then results in dissipation of power inside the choke). -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Matching on the MFJ-1800
Jeff Liebermann wrote:
50 ohm coax does not make it a matching section to a 200 ohm folded dipole. However, a 1/4WL side-by-side balanced run of 50 ohm coax with a Z0=100 ohms makes an excellent match from a 200 ohm load to a 50 ohm source. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
Matching on the MFJ-1800
"Jeff Liebermann" wrote in message ... On Thu, 19 Nov 2009 06:05:20 -0600, "amdx" wrote: I have found the need to work for a living, gets in the way of a lot of fun! 5 hours on the phone on a slow motion conference call this morning. Absolutely nothing useful accomplished. Such online meetings should be banned, taxed, or both as a threat to national productivity. I took the coax loose on the MFJ-1800 and and removed the toroids, I found the letters found M1Z/111-RG and then the insulation ended. Argh! Oh, I have a second antenna, so I took that one apart, Eureka! RGS-303 http://wireandcable.thermaxcdt.com/i...d-70-?&seo=110 50 ohm coax. PTFE center insulator, FEP jacket. Mike Thanks. 50 ohm coax does not make it a matching section to a 200 ohm folded dipole. However, the ferrite beads are a good way to simply block the reflected power from the folded dipole so that it looks like it's matched. In any case, that reflected power is lost (converted to heat) in the ferrite beads. So much for efficiency. When I change the characteristic impedance of the model from 200 ohms to 50 ohms, the VSWR climbs to 5.5:1. Yech. (Note that the radiation efficiency is 75% with or without the mismatch). I suppose the antenna could be made to function by replacing the coax section with a real 1/4 wave 4:1 balun, but I'll leave that to MFJ to figure out. If you need some more entertainment value, it would be interesting to actually measure the gain of the antenna. Find a known good reputable antenna with similar gain. A panel or patch will work. Find a signal source that isn't infested with reflections (including ground reflections), Fresnel Zone issues, and is fairly stable (i.e. doesn't physically move). Use Netstumbler, WirelessMon, or Kismet to compare the signal strengths. For additional accuracy, use a step attenuator to adjust the signal levels to a common reference level. Better yet, use a spectrum analyzer. Jeff Liebermann Sure, as soon as I try to make you King you find work for me :-) Ya, I have quad panel on the boat, when I get some time I'll try the comparision. BTW, I had a friend purchase an Alfa-Awus-036 Wifi Adapter. He said it worked very well! So I ordered one, I replaced a TP-Link TL-WN321G with the Alfa-Awus-036. I had 23 signals received with only about 4 usable signals on the TL-WN321G. After I installed the Alfa I received 36 signals and all of them have a signal strength that would make them usable. (Of course some are encrypted) This thing screams! I had used the TL-WN321G for several years and thought it was good until I tried the Alfa. http://www.amazon.com/Adapter-Wardri...7004388&sr=8-4 I'm using it with a 19dbi* panel antenna aimed a a 7 story condo. http://www.fab-corp.com/product.php?...cat=255&page=1 Thanks, for all the input guys. * advertised |
Matching on the MFJ-1800
On Thu, 19 Nov 2009 13:20:01 -0800, Richard Clark
wrote: My model shows a more benign mismatch to a 72 Ohm load. Richard Clark, KB7QHC I built several models of the antenna folded dipole assembly. The simple rectangular rod folded dipole yielded about 300 ohms. A slightly better simulation of the rounded ends, but still using a round rod, was about 260 ohms. Converting it to a flat wire ended up about 280 ohms. I never got anything even close to 72 ohms. It's my http://802.11junk.com/jeffl/antennas/mfj1800/ Show my your NEC2 deck and tell me what I did wrong, and maybe I'll believe that it's 72 ohms. Incidentally, the possibility that I screwed up somewhere in the model is quite real: http://802.11junk.com/jeffl/antennas/mfj1800/mfj1800.txt (I'll convert this mess back to a macro form so it's easier to read maybe this weekend). -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
On Thu, 19 Nov 2009 18:46:59 -0600, "amdx" wrote:
Sure, as soon as I try to make you King you find work for me :-) I hate to tell you this, but that's what kings are paid to do. They give orders. Do you require a public proclamation, executive order, or pontifical bull (in Latin), in order to be properly inspired? Now, pleae do some testing. After all this is your antenna, your question and your problem. Ya, I have quad panel on the boat, when I get some time I'll try the comparision. Doing it on a boat might be a problem. You're too close to the water which will probably be inside the Fresnel Zone. The water also acts as a great reflector. Thinks also move on a vessel, making stable readings difficult. I do my testing across a valley, where there's little chance of ground reflections (and there's a convenient 2.4GHz RF source from the local mountain top WISP). The path also has a wall of 40 meter high trees on both side to attenuate any interference. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
On Fri, 20 Nov 2009 08:52:25 -0800, Jeff Liebermann
wrote: On Thu, 19 Nov 2009 13:20:01 -0800, Richard Clark wrote: My model shows a more benign mismatch to a 72 Ohm load. Richard Clark, KB7QHC I built several models of the antenna folded dipole assembly. The simple rectangular rod folded dipole yielded about 300 ohms. A slightly better simulation of the rounded ends, but still using a round rod, was about 260 ohms. Converting it to a flat wire ended up about 280 ohms. I never got anything even close to 72 ohms. It's my http://802.11junk.com/jeffl/antennas/mfj1800/ EZNEC shows: Impedance = 73.13 + J 18.78 ohms which, I suppose, could have the reactance driven out if I shift frequency. Show my your NEC2 deck and tell me what I did wrong, and maybe I'll believe that it's 72 ohms. Incidentally, the possibility that I screwed up somewhere in the model is quite real: http://802.11junk.com/jeffl/antennas/mfj1800/mfj1800.txt (I'll convert this mess back to a macro form so it's easier to read maybe this weekend). I don't have an option of NEC2 deck. One thing you might check, and is something I reported about, is does your model have the loop symmetrical to the plane of the directors/reflector? I followed all of Mike's dimensions and I note that your lobe characteristics don't show his lack of driven element symmetry - mine do. Again, I have modeled only the three elements (Ref/Dr/Dir) as the additional directors are unlikely to perturb drive point Z as much as to push it from 73 Ohms up to your high 200s (triple?). Another point, as I have described, I used 1/4 inch diameter wire in place of larger flat sheet metal elements (which I note you try to replecate, but only once). True, 1/4 inch is not as big as any flat dimension, but as Roy reports on equivalence, flat is not the same as diameter, but flat performance is closer to a smaller diameter round wire. Hence the 1/4 inch. 73's Richard Clark, KB7QHC |
Matching on the MFJ-1800
On Fri, 20 Nov 2009 09:10:27 -0800, Jeff Liebermann
wrote: You lost me here. Just what are the beads suppose to do if not block the reflected signal? Presumably, they do serve some useful purpose. They block common mode current. That is their purpose as a CHOKE. A good BalUn contains choking, but a choke is not always a BalUn. For a quarterwave BalUn, the beads serve to isolate each end which is both a choking AND a BalUn necessity. As the loop is isolated from the sheet metal, isolation is preserved (allowing the section to "possibly" serve as a quarter wave choke - neglecting the geometric mean of Zs). Now, you're suggesting that they could make the VSWR worse? That is only a possibility, not a necessity. I've never seen a Yagi-Uda antenna with a 50 ohm coax hung directly onto a folded dipole because there are usually easy ways to do the matching and balanced to unbalanced conversion. Yagis are often balanced by design. You should have seen 50 Ohm coax coiled nearby the feedpoint as a choke (an alternative method to using beads). If you have not, then you have probably seen an installation that suffered degraded performance (and either to the operator's unending grief, or to their total unawareness). You give some sense of already knowing this with your comment that follows: Digging a deeper hole, I've been assuming that if the ferrite beads were not there, the coax cable will radiate. After all, that's one purpose of a balun, to prevent coax radiation from mangling the pattern. That's still a dubious proposition due to the large length of exposed center conductor at both ends of the coax piece, which certainly will radiate some. I can add that to the model, but I don't know how to model the ferrite beads. I've written to that where I took exception to you modeling the Z as series, and I pointed out it was shunt. Apparently you didn't follow that comment. You model the beads as either creating an open circuit, or adding a huge resistance in-line. The net result to current is the same. The long and short of it is that to model the exterior of the coax, you add a wire of same length back to the choke section and call it a day. 73's Richard Clark, KB7QHC |
Matching on the MFJ-1800
"Jeff Liebermann" wrote in message ... On Thu, 19 Nov 2009 18:46:59 -0600, "amdx" wrote: Sure, as soon as I try to make you King you find work for me :-) I hate to tell you this, but that's what kings are paid to do. They give orders. Do you require a public proclamation, executive order, or pontifical bull (in Latin), in order to be properly inspired? Now, pleae do some testing. After all this is your antenna, your question and your problem. Ya, I have quad panel on the boat, when I get some time I'll try the comparision. Doing it on a boat might be a problem. You're too close to the water which will probably be inside the Fresnel Zone. The water also acts as a great reflector. Thinks also move on a vessel, making stable readings difficult. I do my testing across a valley, where there's little chance of ground reflections (and there's a convenient 2.4GHz RF source from the local mountain top WISP). The path also has a wall of 40 meter high trees on both side to attenuate any interference. Jeff Liebermann All that may be true, sorry King, is true. My antennas are 14ft above the water and the signal only goes over water for about 30ft before a concrete parking lot fills the rest of the distance to my targets. Total target distance is about 600ft. The antennas are tipped upward about 10 degrees. Here's the test I performed, I aligned the panel and the yagi on a pole with about the 2ft between the yagi and the center of the panel. I aimed at the target as accurately a I could see. (Center of condo building) During testing I did not terminate the unused antenna. I used net stumbler for signal strength numbers, I received 42 signals with the Panel, and 44 signals with the yagi. The one signal I usually use went from -48db (panel) to -50db (yagi), That's +2db for the Panel. Then I did a Netstumbler screen print of the first 35 signals using the yagi and then the panel. I added all 35 signal strengths and divided by the 35, to get an average signal. (35 is all that fit the screen without scrolling) The Panel AVE = -58.11db, the Yagi AVE = 59.9db that is + 1.79db for the Panel. So if you have any faith in my method, it looks like the yagi is down about 2db from the Panel. I'm impressed, the MFJ-1800 yagi is advertised as a 15dbi antenna, the panel as 19dbi antenna. Here's the Panel seller for the 19dbi panel antenna. http://www.fab-corp.com/product.php?...cat=255&page=1 At one time I found antenna plots for it, but can't locate them now. Here's the yagi seller. http://www.mfjenterprises.com/Produc...uctid=MFJ-1800 Thanks, Mike PS. Previously a saw 33 signals, I had never tried to optimize by adjusting the antenna, 33 was good enough. While mounting the yagi I moved the position of the panel, looks like I found a better position. :-) |
Matching on the MFJ-1800
In article ,
Jeff Liebermann wrote: Oh-oh. *grin* Ground #1: the ferrite beads would only block non-balanced current flow back down the outside of the coax. They will have no effect at all on power which is reflected back down the inside of the coax (the center conductor and the inside of the shield) from any impedance mismatch where the coax meets the folded dipole. Agreed, if there were a balun present. Without a balun, there's going to be some radiation from the coax cable, presumably from the shield. Disclaimer: I don't fully understand how baluns work, and certainly don't understand what the piece of coax with ferrite beads on the MFJ-1800 is suppose to do. A piece of coax, with several ferrite beads or cores around it, *IS* a balun. It's a 1:1 "current balun". It has the effect of ensuring that current flow into the radiator (the folded dipole, in this case) is balanced - that is, the current flowing into each of the FD's two feedpoint ends is equal. In effect, the presence of the beads (if they're choking the coax properly) actually ensures that the transmitter *does* see the true effect of any impedance mismatch. The transmitter is just as likely to see a higher SWR than a lower one, when the beads are added. You lost me here. Just what are the beads suppose to do if not block the reflected signal? Presumably, they do serve some useful purpose. Now, you're suggesting that they could make the VSWR worse? Well, let's see. From your questions, and from your comment that you don't really understand how baluns work, I suspect that there are several aspects of what's going on here which are being "mushed together". Let's tease them apart, with an example, and see if this clarifies thing. We're dealing with the following inter-related things: characteristic impedance, VSWR, reflections, balance, RF flowing where you don't want it, and feedline radiation. Let's start with a simple example: a folded dipole (characteristic Z of 300 ohms), being fed by a twin-wire feedline (characteristic Z also 300 ohms), from a balanced RF source. RF is going to flow up the feedline... and the current at a point on each conductor will be equal-and-opposite that of the point immediately opposite it on the other conductor. The feedline is "balanced". At each point on the feedline, the RF voltage and the RF current will be in a specific relationship, as defined by Ohm's Law (E=IR). Rewrite this as R=E/I, divide the voltage you measure by the current you measure, and you'll get 300 ohms as the result... this ratio is the characteristic impedance of the line. When the RF reaches the folded dipole, it will "see" the same characteristic impedance and will flow into the dipole without any sort of disruption. Because the dipole is physically symmetrical, and (just as important) because there's nowhere else for the current flowing up either side of the feedline to go, the antenna is also balanced, and equal-and-opposite currents will flow into the two sides of the antenna. This is a nice setup, electrically - everything behaves as if we would like. Now, let's change the FD to a loop, with a feedlpoint impedance of around 100 ohms. That is to say, when current flows into the feedpoint, it must do so where the ratio of voltage-over-current is lower. When RF flowing up the (balanced, 300-ohm) feedline hits the loop's feedpoint... well, we have a mismatch. The voltage-to-current ratios in the feedline and the loop will differ. The physical effect of this is that only part of the RF power successfully enters the loop. The rest is reflected back down the feedline, as a wave travelling in the opposite of the original direction. [The same thing happens when a radar wave in space hits an object - the impedance difference between free space and the material of the object reflects much of the power back towards its origin.] The forward (original) and reverse (reflection-generated) waves will reinforce (constructively) and interfere (destructively) as they travel along the lines. At points where their voltages are in phase, the voltages will add and the voltage across the line will be higher. At points where they interfere (phases are opposed) the voltages will be lower. The ratio between these two (higher-voltage and lower-voltage) points defines the VSWR, and the transmitter will "see" this. In this case, we have reflected power... and we have a VSWR of higher than 1:1. However, the whole system is still balanced - it's physically and electrically symmetrical. Now, let's set that example aside for a moment, and look at another one. Let's consider a simple dipole (center-fed, non-folded), with an impedance of 72 ohms. This is a physically symmetrical (balanced) antenna. Let's feed this with coax, having a characteristic of 72 ohms. Is the coax balanced? Yes and no. In one sense, it's balanced... if you look at the current flow on the inside of the coax, you'll find that the flows on the center conductor, and the inside of the shield, are equal-and-opposite. Also, the EM fields created by the RF flow are entirely within the coax (if the shield has full coverage, as in a hardline), which means that RF doesn't "leak out" and inteference has a hard time "leaking in". This is why you can run coax through conduit or next to metal objects without causing problems. In another sense, though, the coax is not balanced, because it consists of *three* conductive surfaces... the center conductor, the inside of the shield, and the outside of the shield. When current flowing up the coax reaches the end (e.g. the antenna feedpoint), the symmetry can be broken: - Current flowing up the center conductor has only one place it can go - to one side of the antenna feedpoint. - Current flowing up the inside of the shield has *two* paths to which it can flow. It can flow into the other side of the antenna feedpoint, *and* it can "wrap around" the end of the shield and flow back down the outside of the coax. It will tend to do both, in a complex ratio which depends on the impedances that it "sees" for each of these two paths. RF flowing back down the outside of the coax will generate radiation, which will add to / interfere with the RF radiatiating from the dipole. This will tend to alter the antenna's pattern somewhat. In this example (72-ohm coax connected to 72-ohm dipole), we won't necessarily see an even current flow into the dipole, with a nice clean 1:1 VSWR on the line. We can still see some reflected power. Why? Because of the "third wire" - the outside of the coax. Its impedance will appear in parallel with that of one side of the dipole... it will 'accept' some RF current (in effect, diverting it from its side of the dipole) and this will alter the feedpoint impedance of the antenna system. The imbalance of the coax (the presence of the "third wire") has disrupted our nice pretty theoretical picture. We've got more current flowing into the center-conductor side of the antenna (where there's only one path) than we have flowing into the other side (since part of the current has been diverted to flow back down the outside of the coax), and the antenna is no longer behaving in a balanced fashion. How do we prevent this from happening? By using a balun... a device which has an unbalanced (usually coaxial) feed on one side, and which *forces* an electrical balance on the other side. Some baluns force equal-and-opposite voltages on the two terminals on the balanced side, while others force equal-and-opposite currents on either side. Some baluns are designed to present the same impedance on each side (1:1 baluns) while others are designed to have different impedances connected to either side (the 4:1 balun is the best-known). A series of ferrites, clamped around a coax, are a simple (and often very effective) form of current balun. There's no impedance transformation built into this design - it's a 1:1 type. How does this "choke balun" create balance? Quite simply: it blocks current flow on the "third wire" (the outside of the coax) by placing a high RF impedance in series with any current flow on this path. As an example: consider the 72-ohm coax-and-dipole example. Let's assume, for the sake of argument, that the RF impedance seen along the outside of the coax (the "third wire") is on the order of 100 ohms or so (not unreasonable if it's a couple of wavelengths long). Without a balun of some sort, there's going to be a fair amount of current flow back along this "third wire", because the impedance isn't all that much higher than the 36 ohms (that of one-half of the dipole) in which it is in parallel with. Now, let's clamp a few nice ferrites onto the coax. These add *lots* of inductance to the "third wire"... for the sake of argument let's say it's on the order of 1000j ohms. All of a sudden, the third wire presents a *much* higher impedance at the feedpoint than it used to... it's 30 times as high as that of the dipole segment to which it's connected. Only a tiny amount of current can flow into the "third wire". Since the current flowing up the inside of the feedline is balanced, and since the current reaching the feedpoint now has only one place it can go effectively (into the antenna), the current flow into the antenna is symmetrical - balance has been restored. Because the feedpoint impedance is no longer being significantly affected by the "third wire", we see almost exactly a 1:1 VSWR. If we've added enough inductance (or enough RF resistance) to the "third wire", and choked off the current flow to essentially nothing, then the power loss in the choke is negligible... because there isn't enough current flowing to generate power to dissipate. This is beginning to sound like the discussion about whether it's acceptable to use a high VSWR antenna. The standard answer is that if the transmitter can take it without going into protection or oscillation, then it's acceptable. I think that's oversimplifed a bit. I think there are really three factors to consider: - Will the transmitter be stable and happy (not self-protect, not oscillate, not burn up)? - Will the transmitter actually deliver full rated power (or close to it) into the impedance that it "sees" as a result of the higher SWR? This impedance may be lower or higher than its rated load, and may have significant reactance in it. - Will the additional ("excess") losses in the transmission line, as a result of the higher current at some points, be a problem? I don't have a problem with that, although I've never seen a Yagi-Uda antenna with a 50 ohm coax hung directly onto a folded dipole because there are usually easy ways to do the matching and balanced to unbalanced conversion. The "Cheap Yagi" design is the closest to that... a half-folded dipole, with the directors and reflectors positioned so that the HFD has a 50-ohm feedpoint impedance. Direct coax connection (ferrite bead is optional). Digging a deeper hole, I've been assuming that if the ferrite beads were not there, the coax cable will radiate. After all, that's one purpose of a balun, to prevent coax radiation from mangling the pattern. It could radiate, depending on the length of the coax. That's still a dubious proposition due to the large length of exposed center conductor at both ends of the coax piece, which certainly will radiate some. I can add that to the model, but I don't know how to model the ferrite beads. I believe you'd want to model the coax balun as a combination of two elements: a pure transmission line, and a "third wire" connected at the antenna feedpoint and running back parallel to the transmission line (this being the effect of the outside of the coax). To model the ferrite beads, add a lumped impedance in series with the third wire, at the point at which it connects to the antenna. As a wild-and-almost-certainly-wrong SWAG, try adding 100+100j (to mimic the purely inductive, and the lossy-resistive aspects of the ferrite). -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Matching on the MFJ-1800
On Fri, 20 Nov 2009 10:46:40 -0800, Richard Clark
wrote: Show my your NEC2 deck and tell me what I did wrong, and maybe I'll believe that it's 72 ohms. Incidentally, the possibility that I screwed up somewhere in the model is quite real: http://802.11junk.com/jeffl/antennas/mfj1800/mfj1800.txt (I'll convert this mess back to a macro form so it's easier to read maybe this weekend). I don't have an option of NEC2 deck. No problem. Just post or email me the .EZ file. 4NEC2 will import it or I'll just switch to EZNEC. (I promise not to complain about your using inches). One thing you might check, and is something I reported about, is does your model have the loop symmetrical to the plane of the directors/reflector? Unfortunately, yes. I got lazy and planted the folded dipole centered about the axis of the other elements. I don't think (guess) it would make much difference, but I'll move it to the correct position this weekend. Got any theory as to why the vertical and horizontal patterns are so different? That only appeared when I switched to the flat wire folded dipole. They were symmetrical with the wire rod driven element. I followed all of Mike's dimensions and I note that your lobe characteristics don't show his lack of driven element symmetry - mine do. Again, I have modeled only the three elements (Ref/Dr/Dir) as the additional directors are unlikely to perturb drive point Z as much as to push it from 73 Ohms up to your high 200s (triple?). Yes, something is different. In past tinkering, I've found that 3 elements is sufficient to characterize the feed impedance, where the additional elements just improve the gain and pattern. Another point, as I have described, I used 1/4 inch diameter wire in place of larger flat sheet metal elements (which I note you try to replecate, but only once). True, 1/4 inch is not as big as any flat dimension, but as Roy reports on equivalence, flat is not the same as diameter, but flat performance is closer to a smaller diameter round wire. Hence the 1/4 inch. Well, I used a rod with the same circumference as the flat (asymetrical) elements. Methinks the element diameter would have an effect on the bandwidth of the antenna, but not on its characteristic impedance. 73's Richard Clark, KB7QHC thanks much... -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
Dave Platt wrote:
A piece of coax, with several ferrite beads or cores around it, *IS* a balun. It's a 1:1 "current balun". According to The ARRL Handbook, a "w2du balun", popularized by Walter Maxwell. -- 73, Cecil, IEEE, OOTC, http://www.w5dxp.com |
Matching on the MFJ-1800
"Richard Clark" wrote in message ... On Fri, 20 Nov 2009 08:52:25 -0800, Jeff Liebermann wrote: On Thu, 19 Nov 2009 13:20:01 -0800, Richard Clark wrote: My model shows a more benign mismatch to a 72 Ohm load. Richard Clark, KB7QHC I built several models of the antenna folded dipole assembly. The simple rectangular rod folded dipole yielded about 300 ohms. A slightly better simulation of the rounded ends, but still using a round rod, was about 260 ohms. Converting it to a flat wire ended up about 280 ohms. I never got anything even close to 72 ohms. It's my http://802.11junk.com/jeffl/antennas/mfj1800/ EZNEC shows: Impedance = 73.13 + J 18.78 ohms which, I suppose, could have the reactance driven out if I shift frequency. Show my your NEC2 deck and tell me what I did wrong, and maybe I'll believe that it's 72 ohms. Incidentally, the possibility that I screwed up somewhere in the model is quite real: http://802.11junk.com/jeffl/antennas/mfj1800/mfj1800.txt (I'll convert this mess back to a macro form so it's easier to read maybe this weekend). I don't have an option of NEC2 deck. One thing you might check, and is something I reported about, is does your model have the loop symmetrical to the plane of the directors/reflector? I followed all of Mike's dimensions and I note that your lobe characteristics don't show his - mine do. 73's Richard Clark, KB7QHC Hi Richard, What do you mean by "lack of driven element symmetry" ? Mike |
Matching on the MFJ-1800
On Nov 13, 8:28*am, "amdx" wrote:
*Hi All, *My original thread seems to have died, still wondering how the folded loop is matched to 50 ohms. (probably isn't? cheap, but works?) *I've added a dimensional drawing and some more pics. If more info is needed let me know. Here is a drawing and some more pics.http://i395.photobucket.com/albums/p...Jruledcoax.jpg * Hope I covered everything, * * * * * * * * * * * * * * * * *Mike The folded dipole is going to have about 4 times the impedance of a plain old dipole.(POD) In this case if you used a POD with your yagi antenna the feedpoint impedance would be in the neighborhood of 10 ohms with a folded dipole it will be more like 40. Not a bad match for 50 ohm coax Jimmie |
Matching on the MFJ-1800
On Fri, 20 Nov 2009 18:26:17 -0600, "amdx" wrote:
Hi Richard, What do you mean by "lack of driven element symmetry" ? Mike Hi Mike, The driven loop is not symettrical to the plane of the directors and reflector, thus it peers down (or up, or to the side - depending on deployment). Look boresight down the boom. The loop is off-center. 73's Richard Clark, KB7QHC |
Matching on the MFJ-1800
"Richard Clark" wrote in message ... On Fri, 20 Nov 2009 18:26:17 -0600, "amdx" wrote: Hi Richard, What do you mean by "lack of driven element symmetry" ? Mike Hi Mike, The driven loop is not symettrical to the plane of the directors and reflector, thus it peers down (or up, or to the side - depending on deployment). Look boresight down the boom. The loop is off-center. 73's Richard Clark, KB7QHC Good, I thought that might be the answer but wanted to check. Glad I added that detail to the drawing, do you think that is engineered to control output impedance? What happens to the impedance if you center it? Could you post your model or send it to me, I have a fellow ham that tried to model the FD and could not get it to work. He would like to see one that does work and find out what he did wrong. Eznec if you have it. Thanks, Mike |
Matching on the MFJ-1800
On Sat, 21 Nov 2009 06:24:49 -0600, "amdx" wrote:
Could you post your model or send it to me, I have a fellow ham that tried to model the FD and could not get it to work. He would like to see one that does work and find out what he did wrong. Eznec if you have it. Thanks, Mike Hi Mike, It has been posted to you as you asked. Jeff also has a copy. Can you tell us what you mean about your buddy's model and how he "could not get it to work?" Did he follow your dimensions? 73's Richard Clark, KB7QHC |
Matching on the MFJ-1800
"Richard Clark" wrote in message ... On Sat, 21 Nov 2009 06:24:49 -0600, "amdx" wrote: Could you post your model or send it to me, I have a fellow ham that tried to model the FD and could not get it to work. He would like to see one that does work and find out what he did wrong. Eznec if you have it. Thanks, Mike Hi Mike, It has been posted to you as you asked. Jeff also has a copy. Can you tell us what you mean about your buddy's model and how he "could not get it to work?" Did he follow your dimensions? 73's Richard Clark, KB7QHC He's out of town for a couple of days, when he returns I'll quiz him. Mike |
Matching on the MFJ-1800
On Fri, 20 Nov 2009 11:54:10 -0800, (Dave Platt)
wrote: A piece of coax, with several ferrite beads or cores around it, *IS* a balun. It's a 1:1 "current balun". (...) Wow. Now, the light comes on. I haven't had any experience with current baluns (except with consumer grade cheap TV/FM tuner front ends) and didn't recognize the name or understand how they worked. Thanks for the very nice, clear, and detailed explanation. (I can't even find anything in your explanation to complain about). -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
"Jeff Liebermann" wrote in message ... On Fri, 20 Nov 2009 11:54:10 -0800, (Dave Platt) wrote: A piece of coax, with several ferrite beads or cores around it, *IS* a balun. It's a 1:1 "current balun". (...) Wow. Now, the light comes on. I haven't had any experience with current baluns (except with consumer grade cheap TV/FM tuner front ends) and didn't recognize the name or understand how they worked. Thanks for the very nice, clear, and detailed explanation. (I can't even find anything in your explanation to complain about). -- Jeff Liebermann For those of us that need more help. http://vk1od.net/balun/W2DU/index.htm |
Comparision NO Feedback??
I'll try the comparision. Doing it on a boat might be a problem. You're too close to the water which will probably be inside the Fresnel Zone. The water also acts as a great reflector. Thinks also move on a vessel, making stable readings difficult. I do my testing across a valley, where there's little chance of ground reflections (and there's a convenient 2.4GHz RF source from the local mountain top WISP). The path also has a wall of 40 meter high trees on both side to attenuate any interference. Jeff Liebermann All that may be true, sorry King, is true. My antennas are 14ft above the water and the signal only goes over water for about 30ft before a concrete parking lot fills the rest of the distance to my targets. Total target distance is about 600ft. The antennas are tipped upward about 10 degrees. Here's the test I performed, I aligned the panel and the yagi on a pole with about the 2ft between the yagi and the center of the panel. I aimed at the target as accurately a I could see. (Center of condo building) During testing I did not terminate the unused antenna. I used net stumbler for signal strength numbers, I received 42 signals with the Panel, and 44 signals with the yagi. The one signal I usually use went from -48db (panel) to -50db (yagi), That's +2db for the Panel. Then I did a Netstumbler screen print of the first 35 signals using the yagi and then the panel. I added all 35 signal strengths and divided by the 35, to get an average signal. (35 is all that fit the screen without scrolling) The Panel AVE = -58.11db, the Yagi AVE = 59.9db that is + 1.79db for the Panel. So if you have any faith in my method, it looks like the yagi is down about 2db from the Panel. I'm impressed, the MFJ-1800 yagi is advertised as a 15dbi antenna, the panel as 19dbi antenna. Here's the Panel seller for the 19dbi panel antenna. http://www.fab-corp.com/product.php?...cat=255&page=1 At one time I found antenna plots for it, but can't locate them now. Here's the yagi seller. http://www.mfjenterprises.com/Produc...uctid=MFJ-1800 Thanks, Mike PS. Previously a saw 33 signals, I had never tried to optimize by adjusting the antenna, 33 was good enough. While mounting the yagi I moved the position of the panel, looks like I found a better position. :-) |
Matching on the MFJ-1800
On Fri, 20 Nov 2009 13:18:15 -0600, "amdx" wrote:
"Jeff Liebermann" wrote in message .. . On Thu, 19 Nov 2009 18:46:59 -0600, "amdx" wrote: Sure, as soon as I try to make you King you find work for me :-) I hate to tell you this, but that's what kings are paid to do. They give orders. Do you require a public proclamation, executive order, or pontifical bull (in Latin), in order to be properly inspired? Now, pleae do some testing. After all this is your antenna, your question and your problem. Ya, I have quad panel on the boat, when I get some time I'll try the comparision. Doing it on a boat might be a problem. You're too close to the water which will probably be inside the Fresnel Zone. The water also acts as a great reflector. Thinks also move on a vessel, making stable readings difficult. I do my testing across a valley, where there's little chance of ground reflections (and there's a convenient 2.4GHz RF source from the local mountain top WISP). The path also has a wall of 40 meter high trees on both side to attenuate any interference. Jeff Liebermann All that may be true, sorry King, is true. What a king says is by definition true, even if it's wrong. My antennas are 14ft above the water and the signal only goes over water for about 30ft before a concrete parking lot fills the rest of the distance to my targets. Total target distance is about 600ft. The antennas are tipped upward about 10 degrees. Sounds good. You're out of the Fresnel Zone and the parking lot isn't going to contribute any reflections. Here's the test I performed, I aligned the panel and the yagi on a pole with about the 2ft between the yagi and the center of the panel. Not so good. You can't leave a bunch of potentially resonant metal hanging around that close to the antenna. The unsused antenna could easily be re-radiating anything it pickups up from the other antenna. One antenna at a time please. I aimed at the target as accurately a I could see. (Center of condo building) During testing I did not terminate the unused antenna. Ugh. If you're shooting into what's essentially and indoor wireless router, you're going to get reflections from the building and in particular, from the room where the AP is located. Also, not so good. I used net stumbler for signal strength numbers, I received 42 signals with the Panel, and 44 signals with the yagi. How steady were the readings? With my experience with Netstumbler, if I wait a few minutes, the readings will move around perhaps 2-5dB. Are you doing this on Channel 6 (middle of the band) or near the band edges? The one signal I usually use went from -48db (panel) to -50db (yagi), That's +2db for the Panel. Then I did a Netstumbler screen print of the first 35 signals using the yagi and then the panel. I added all 35 signal strengths and divided by the 35, to get an average signal. (35 is all that fit the screen without scrolling) The Panel AVE = -58.11db, the Yagi AVE = 59.9db that is + 1.79db for the Panel. Good idea but not really valid as you're probably measauring the side lobes of the antennas. An antenna with many side lobes is going to hear more stations than one with just a single major lobe. So if you have any faith in my method, it looks like the yagi is down about 2db from the Panel. I'm impressed, the MFJ-1800 yagi is advertised as a 15dbi antenna, the panel as 19dbi antenna. I have less faith in the RSSI linearity of thatever you're using for a wireless device. -50dBm isn't particularly saturated, so presumably, the comparison is valid. There might be some gain compression at high signal levels, which explains why you're seeing 2dB difference instead of 4dB. I would have predicted a larger difference as my NEC model of the MFJ-1800 predicts a gain of about 13dBi. I would have expected an even larger difference. Here's the Panel seller for the 19dbi panel antenna. http://www.fab-corp.com/product.php?...cat=255&page=1 At one time I found antenna plots for it, but can't locate them now. Here's the yagi seller. http://www.mfjenterprises.com/Produc...uctid=MFJ-1800 -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Matching on the MFJ-1800
"Jeff Liebermann" wrote in message ... On Fri, 20 Nov 2009 13:18:15 -0600, "amdx" wrote: "Jeff Liebermann" wrote in message . .. On Thu, 19 Nov 2009 18:46:59 -0600, "amdx" wrote: Sure, as soon as I try to make you King you find work for me :-) I hate to tell you this, but that's what kings are paid to do. They give orders. Do you require a public proclamation, executive order, or pontifical bull (in Latin), in order to be properly inspired? Now, pleae do some testing. After all this is your antenna, your question and your problem. Ya, I have quad panel on the boat, when I get some time I'll try the comparision. Doing it on a boat might be a problem. You're too close to the water which will probably be inside the Fresnel Zone. The water also acts as a great reflector. Thinks also move on a vessel, making stable readings difficult. I do my testing across a valley, where there's little chance of ground reflections (and there's a convenient 2.4GHz RF source from the local mountain top WISP). The path also has a wall of 40 meter high trees on both side to attenuate any interference. Jeff Liebermann All that may be true, sorry King, is true. What a king says is by definition true, even if it's wrong. My antennas are 14ft above the water and the signal only goes over water for about 30ft before a concrete parking lot fills the rest of the distance to my targets. Total target distance is about 600ft. The antennas are tipped upward about 10 degrees. Sounds good. You're out of the Fresnel Zone and the parking lot isn't going to contribute any reflections. Here's the test I performed, I aligned the panel and the yagi on a pole with about the 2ft between the yagi and the center of the panel. Not so good. You can't leave a bunch of potentially resonant metal hanging around that close to the antenna. The unsused antenna could easily be re-radiating anything it pickups up from the other antenna. One antenna at a time please. Ok, measuring antennas is never easy, I'll hopefully get some time next week. I won't bore you with the details, but the campaign I'm running is eventually going to have the party overthrow a king. :-) I'll install one antenna at a time. I aimed at the target as accurately a I could see. (Center of condo building) During testing I did not terminate the unused antenna. Ugh. If you're shooting into what's essentially and indoor wireless router, you're going to get reflections from the building and in particular, from the room where the AP is located. Also, not so good. Well, I guess I'm $#%^ed. Measuring antennas is never easy. I used net stumbler for signal strength numbers, I received 42 signals with the Panel, and 44 signals with the yagi. How steady were the readings? With my experience with Netstumbler, if I wait a few minutes, the readings will move around perhaps 2-5dB. Are you doing this on Channel 6 (middle of the band) or near the band edges? I didn't see any changes but I only paid attention to the one signal I normally use. I received on the channel the source transmited on. Here's a file of the two screenprints with channel and signal strength. http://i395.photobucket.com/albums/p...fiantennas.jpg For what its worth. The one signal I usually use went from -48db (panel) to -50db (yagi), That's +2db for the Panel. Then I did a Netstumbler screen print of the first 35 signals using the yagi and then the panel. I added all 35 signal strengths and divided by the 35, to get an average signal. (35 is all that fit the screen without scrolling) The Panel AVE = -58.11db, the Yagi AVE = 59.9db that is + 1.79db for the Panel. Good idea but not really valid as you're probably measauring the side lobes of the antennas. An antenna with many side lobes is going to hear more stations than one with just a single major lobe. Who knows, I think the whole condo is in the major lobe and nothing but water on one side and on the other side I doubt there are any signals. I do recall one -80 db signal so maybe something in the side lobe. So if you have any faith in my method, it looks like the yagi is down about 2db from the Panel. I'm impressed, the MFJ-1800 yagi is advertised as a 15dbi antenna, the panel as 19dbi antenna. I have less faith in the RSSI linearity of thatever you're using for a wireless device. -50dBm isn't particularly saturated, so presumably, the comparison is valid. There might be some gain compression at high signal levels, which explains why you're seeing 2dB difference instead of 4dB. I would have predicted a larger difference as my NEC model of the MFJ-1800 predicts a gain of about 13dBi. I would have expected an even larger difference. Measuring antennas is never easy. I note my laptop within 20ft of the router has a -36db signal using netstumbler. Thanks for your help Jeff, Mike Thanks to Richard too. |
Matching on the MFJ-1800
On Sat, 21 Nov 2009 18:14:42 -0600, "amdx" wrote:
Here's a file of the two screenprints with channel and signal strength. http://i395.photobucket.com/albums/p...fiantennas.jpg For what its worth. Hi Mike, Seems like hardly a difference (within half a dB) of each other. 73's Richard Clark, KB7QHC |
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