The Off Kilter Dipole

I couldn’t help it. A few days ago, I was looking for something else (a common pass time for me) and in the cabinet I opened, I saw the off center fed dipole that I built when I first got back into the hobby. I had it up at the old house. It was an attempt to have a physically smallish antenna that could operate on several bands. I used it for at least on ARRL Field Day. There were problems, most of which are not expected to have been with the design of the antenna 🙂

While I have not found a specific online source of the original instructions, I have found several essentially identical sources, so I will refer the reader to Google on the matter.

Basically, cut a wire to the full length of a half wave dipole for the lowest frequency desired, in my case 40 meters so a bit short of 66 feet, plus a foot or so working and tuning room. Instead of cutting it in the center, the design I chose had me cut it 14% off center, 42′ 2-1/4″ and 23′ 8-3/4″. You connect the legs to a 4:1 balun and voilà, OCF dipole. Note the wording “14% from the center”.

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Here’s where the online information and what I was holding in my hand depart. As I write this, I have not carefully measured the legs of the antenna, but I strongly suspect that I may have cut the short leg not 14% from the center but 14% from the end. That leg of the antenna is significantly shorter than 23 feet and I really think it’s more likely about 9 feet, which would indeed be 14% from the end. Definitely a newbie sort of error. Turns out that 14% from the center is the same as 36% from one end, which most directions specify, rather than 14% from the center. Had I originally found instructions written that way, I may not have made that particular mistake. 🙂

Now it gets maybe kinda weirder.

I strung that antenna up Saturday morning. Well, very temporarily “up”, with one end anchored to the workshop door frame and the other to the Mule. The short end is nearest the camera.

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I connected the RigExpert to it with an 18″ jumper and did several SWR scans using it as a standalone unit.

Turns out, the antenna appears to be tuned a little low, but it’s not so far off that it would be unusable, particularly with a tuner. On the low end of 20 meters, might not even need a tuner.

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Those low SWR dips are just below 40, 20 and 15 meters.

I think it’s funny that, at the time I was testing it on Saturday, I did not yet know about the 14%-from-the-wrong-point error I apparently made when I built the thing, so this didn’t seem to be off base at all.

Scanning the whole range that the RigExpert can do reveals an even more surprising bit. From a vector analysis point of view, the antenna appears to work at 2:1 or better from about 70MHz up. I suspect real world results might not bear that out.

ocf-all

This scan is a little rough because there are only 100 points on a plot covering a full 170MHz. Armed with the information I now have about this antenna, I think I want to set it up and scan it at higher resolution and see if I can learn enough to understand why it works at all. My intuition suggests that the high frequency response might be due to the one really short leg. I want to see exactly where that lowest SWR dip around 150MHz is and what relationship that has to the actual length of the short leg. I think the still reasonable response at lower frequencies may be related the antenna’s overall length and maybe harmonics of 7, 14 and 21 MHz.

I do not yet fully understand the Smith chart, but this doesn’t look like one to learn on, at least not at 170MHz bandwidth.

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Unrelated to that, I also rearranged the equipment rack to accommodate a Mirage 160W VHF amplifier. The power supply I have for it is only 20 amperes, so it may not be up to the task. It is also missing the binding post nut.

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At first glance, I thought this was a 1/4″ x 28 tpi stud, but it turns out to be 6mm x 0.75 pitch. Once I have the power supply connected, I will try out the amp.

Interestingly enough, the thing pulls about 8 amps just sitting there. I am mildly suspicious.

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