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#8
November 18th 09, 03:42 PM
posted to rec.radio.amateur.antenna,alt.internet.wireless
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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 |
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