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Author Topic: 40 m Loop antenna broke - can I make an off center fed 40 meter dipole?  (Read 1551 times)
AG5T
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« on: April 28, 2012, 11:35:50 AM »

I had a 40 meter loop in the air, but apparently the wind and tree branches snapped part of it. I have reconfigured it as an off-center fed type dipole. The short end of the wire is about 35 feet or so. I could go up and clip the other part of the wire and make a true 40-meter dipole, but before I do so I wanted to ask if an off-center fed dipole could work 40 plus some other bands. It wouldn't be an exact 40 meter dipole, but with a tuner, I'm guessing I might be able to tune several bands, including 30 meters, which is one band where I don't have any antennas up for that band. Any thoughts? Thanks. (Also with the loop I had a section of 72 ohm coax between the 50 ohm coax and the balun. Since it's no longer a loop, I am curious how that will affect everything).
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WA4FNG
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« Reply #1 on: April 28, 2012, 02:25:20 PM »

Check this out. Google is your friend. Search 40m OCF dipole for other references.

http://va3stl.wordpress.com/2010/07/26/200-ohm-feed-point-off-centre-fed-dipole/
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KC9Q
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« Reply #2 on: April 28, 2012, 04:23:50 PM »

The "classic" OCF for 40 is 22.5 ft for the short side, and 45 ft for the long side.  Use a 4:1 CURRENT Balun and 50 ohm cable.  Using 75 ohm cable will change the VSWR somewhat - it would have to be measured.
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W1JKA
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« Reply #3 on: April 29, 2012, 12:52:32 AM »

    KC9Q is right.You may want to check out( Len Carlson-K4IWl)New Carolina windom.I built one,works as stated.As I only work 40/30/20 meters cw and only need the tuner for 30 meters.I have tried it with and without the the 1:1 line isolator at bottom of the 10 ft. vertical Q section and makes no difference up to 100 watts anyway and with both 50/75 ohm feed line ,again no difference.A/B with my my three 40/30/20 m dipoles at same height/direction/feedline.Dipoles signals more broadsided and a bit noiser,Windom Lobes more off the ends a little quieter and better for DX,I think this may be due to the 10 ft. vertical Q section.Bottom line:I use my dipoles for N/S stations and my windom for E/W stations.I am very happy with both set ups.Good luck.   
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AG5T
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« Reply #4 on: April 29, 2012, 03:16:35 PM »

Just a follow up to my earlier question. Last night I checked into the 3905 Century Club net on 7.2335 and used my tuner to get a 1-1 swr (at least the tuner thought so). I worked several contacts and then for some reason instead of my needle jumping back and forth as it should for talking in ssb, it was pegged all the way to the right. At that point, nobody could hear me. I had to go off frequency, retune and then get back to the net. I was fine after that. Is it possible that after several transmissions, the hidden swr was actually getting so bad it pegged that needle to the right and shut me down? I plan on doing some more tests on 40 meters to see what happens - I may try some digital. I thought about cutting the antenna down to the right size for a regular 40 meter dipole, but I would still have that 72 ohm section. I used to be able to reach the balun I was using and disconnect that piece, but due to Hurricane Ike and other factors I can't reach up there to the pole to take out that section. My other option is to cut all the wires that I can reach and go up and put in a 40 meter dipole (I actually built one today). The only thing is that one leg of the dipole would run north and south and the other east and west, due to me being on a small city lot in Houston. If I had the finances, I would think about some type of vertical without radials and with counterpoise, again due to lack of space. Right now what I have 1005/7 Mhz = 143 feet which was my loop, but it broke so that one section of the ocf dipole is about 40 feet long and the rest is aprox. 103 feet, so if there is a length I could trim it to that might work. Just exploring options. I hope my radio shutting down (needle pegged right) was just a fluke thing. Smiley
« Last Edit: April 29, 2012, 03:18:12 PM by AG5T » Logged
WB6BYU
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« Reply #5 on: April 29, 2012, 06:15:12 PM »

Actually, the Off-Center Fed Loop should have the same pattern as a regular
loop antenna (assuming that there is simply a break in one side with no wire
missing.)  A regular full-wave loop has two high voltage points, one quarter
wave either side of the feedpoint.  Cutting the loop simply rotates the
current distribution around to where the high voltage point is at the break.

In this specific case, there is only a small change in impedance and current
distribution when the break is 40' from the feedpoint.  On 20m the impedance
is considerably higher (perhaps 600 ohms) because the feedpoint isn't at the
optimum point along the loop.  But again, the radiation pattern is about the
same, but with the current distribution rotated a bit around the loop.


The sudden change in SWR sounds more like an intermittent connection somewhere
along the loop (or on the feedline.)  Are you using insulated wire?  There might be
a break somewhere inside the insulation.  Otherwise try to check the various coax
connectors, especially for a bad connection to the shield.

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KB4QAA
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« Reply #6 on: April 29, 2012, 06:16:56 PM »

i'm not sure why this situation is causing so much agonizing....

You could have just tied the broken wires of the loop in a square knot and twisted the bare ends together and been back on the air!  Smiley
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