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.
See:
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.
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:
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:
with a gain of 14.3dBi. Close enough.
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.
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.
My model uses 1/4" elements, and for just the loop, Z = 150 Ohms @
2250 MHzFor 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).
I went with the dimensions provided - inches no problem.
This is imparting too much engineering for their product.
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
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.
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.
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.
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.
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.
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.
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.
The CENTER of the driven element (the plastic screw hole) to the CENTER of the last director is 36.7cm (14-7/16")
ALL directors or the same length (within .003") They are 46.6mm in length.
54.2mm (2.134")61.75mm (2.431) 0.94mm (0.037")
3.3mm (.130)
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.
No help there.
Looks like 4 toroids that slide over the coax, you can see the outline of the edges of each toroid.
Mike
Yes it is just a stamped piece of aluminum with rough edges and little nibs here and there that were never cleaned off.
Understood.
By this statement, do you mean the spacing between elements is large so it looses it's folded loop characteristics?
Mike
I meant folded dipole characteristics. (not folded loop)
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
Thanks. My numbers came out to 36.52 cm which is close enough.
Amazing...
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.
One more dimension... the approximate outer jacket diameter of the coax (not including the shrink tube).
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:
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).
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
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.
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.
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).
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.
My model shows a more benign mismatch to a 72 Ohm load.
73's Richard Clark, KB7QHC
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).
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.
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.
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
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:
(I'll convert this mess back to a macro form so it's easier to read maybe this weekend).
Oh-oh.
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.
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?
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 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.
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.
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