Off Center Fed Inverted V Dipole
This was an investigation into whether an inverted V dipole, notionally cut for 20m, could be fed offcenter to work on 20, 17, 15, 12, AND 10. The summary is that you can find a spot where it will be reasonably resonant on 10 and 20 (i.e. two half waves or a single half wave), but the rest is impossible.
A NEC model file was built, and run using 4NEC2 to step through the possible feed points. The dipole is modeled as 2 arms, each with 50 segments (so, about 10 cm/segment) and a bridging segment at the apex. Here's the model and a rendering of the antenna, fed in the 20 m band, showing high current in the middle where you would expect it.
CM Off center fed inverted V CM CM built on a 10m square... CM All segments loaded as copper wire CM iseg is the segment of the leg to feed at CM =1 end fed, =50, basically center fed CM intended to be run with 4nec2 in "evaluate" mode CM CE SY hspace=7.5 SY leg=3.4, apexz=leg/2 SY iseg=1 GW, 101,50, leg,leg,0, .1,.1,apexz, .0005 GW, 102,50, -leg,-leg,0, -.1,-.1,apexz,.0005 GW 301 1 .1 .1 apexz -.1 -.1 apexz .0005 GM 0 0 0 0 0 0 0 10 GE EK GN 2 0 0 0 6 .001 LD 5 0 0 0 5.8E7 EX 0 101 iseg 00 1 0 FR 0 1 0 0 14.3 0.1 XQ EN
I used the "EVALUATE" feature of the 4NEC2 optimizer to change the feedpoint segment number within the wire on one side. 4NEC2 generates a file called "optimizer.log", which you can paste into an Excel spreadsheet for later processing. A separate run is needed for each frequency, so I did 5 runs, at: 14.2, 18.110, 21.3, 24.93, and 28.3 MHz. Here are some results plotted, first, the magnitude of the feed point impedance as a function of the feed point position.
I also used Excel to calculate the SWR, plotting that, then fooled with changing the assumed source impedance. Here's a plot of the VSWR with a 250 ohm (resistive) source. You can see that the match on 10 and 10 isn't all that bad, but you can't even see the match for the other bands.
If you want to fool around yourself, here is the NEC source deck and the Excel Spreadsheet.
radio/antenna/ocfv.htm - 21 March 2003 - Jim
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