Off Center Dipole

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oneworld

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Few questions regarding homebrew off center dipole. I have one made of ½” copper pipe with pvc T, balun and end caps. I also have made one with just wire and balun. Question is ½” copper ok? Or should it be ¾”. Would there be any difference? And for the wire dipole stranded wire or sold wire 12 gauge. I used solid. Just looking for some input and experiences form users. Thanks
 

ampulman

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I seem to recall reading that the larger diameter tubing has a greater bandwitdth. But, being that the antenna covers multiple segments of the spectrum, does it really matter?

In my case, I made one from a pair of TV rabbit ears, splayed out 180 degrees. It's a little short on the long leg, but works like a champ from my 2nd story radio room. I can get 2 or 3 extra signal bars in some cases.

Amp
 

hertzian

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You should be fine with either wire or tubing, I've built many out of each depending on what I had on hand.

Yes, the tubing will provide a wider bandwidth as the tubing gets larger, but with the OCFD you'll find plenty of impedance dips and nulls, which mitigate this somewhat. For wire, I'd stick to solid because in some cases as stranded wire oxidizes, it can create rectifiers which can cause noise eventually.

Much of the decision comes down to how much copper can you afford, and how much weight you want to hang which could be an issue indoors.

When you spread the transformer legs apart to attach to the tubing, they become part of the antenna elements, so you may want to take that into account when you cut the wire or tubing elements.

Canonically, an OCFD is a half-wave dipole fed about 1/3 of the way in, instead of in the center. This means that the antenna will be resonant on the EVEN harmonics, however there is the case where it will NOT perform well on the 6th due to the current and voltage distribution at the 6th harmonic. The actual feedpoint impedance varies from approximately 200 - 400 ohms as you sweep across the bands, hence the need for a 4:1 transformer (the typical tv-type 300:75 ohm transformer) to attach to coax. Either 50 or 75 ohms is fine from a rx-only stanpoint.

Since it is fed asymetrically at the 1/3 point, the transformer / balun is simply a transformer - the tv-type voltage balun is overwhelmed by the asymetrical nature guaranteeing that the shield of the coax becomes part of the antenna. This may or may not be an issue until you put it up. It does help on low-band VHF, but then ideally one should run the feedline horizontally away from the dipole for at least a few feet or more, but now that is primarily on the wrong polarization.

As you go higher in frequency, the elevation angle starts to look nearly straight up, especially at 400 mhz and above, although the pattern is distorted somewhat by the coax braid.

Soo.... while it may be electrically efficient, from an antenna standpoint, the OCFD is not usually ideal for UHF - unless signals in your area are so strong that the elevation angle is not an issue.

So I wouldn't really sweat the differences between 1/2 and 3/4 inch tubing, unless you just like to play with antennas like I do. You could choke the OCFD with ferrites up to about 200 mhz or so, but you'd need about 5 to 10 chokes of the RS 273-105 (type 43) to accomplish anything. That could be expensive.

You might not find any difference with the "short end up" or the "long end up". My preference is for the short end up, with the center conductor to the short end, since it gets the feedpoint higher off the ground indoors, and is easier to hang from a balance standpoint.

The OCFD is a fun antenna, but I wouldn't go bananas trying to perfect it if it performs ok for you "out of the box". :) There are just too many compromising variables to deal with. If it plays ok, it stays.
 
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oneworld

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Thanks for all the help and information. I made one for my work 24" top 9" lower works great long range. I have my made one 48" top 18" lower and have used it inside leaning in corner until I decide where outside to mount. I'm sure I will see great results. For now I'm using solid wire 48" top 18" lower and results are fair.
I do have a question. Once mine is mounted outside can I feed it into my house cable system that goes to rooms throughout the house? I have Dish Network and don't use cable in walls. Any thoughts? Thanks.
 

prcguy

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You should be able to diplex the antenna with the Dish, you would need a diplexer at the point where you connect the two systems and one at every point you want to bring out the antenna signals. Once you have diplexed the two systems you could just connect the Dish cable to the scanner and it will probably work fine but there will be lots of signals between 950 and 1450MHz mixed in that might cause some intermod on the scanner and the diplexer keeps this out of the scanner.
prcguy

Thanks for all the help and information. I made one for my work 24" top 9" lower works great long range. I have my made one 48" top 18" lower and have used it inside leaning in corner until I decide where outside to mount. I'm sure I will see great results. For now I'm using solid wire 48" top 18" lower and results are fair.
I do have a question. Once mine is mounted outside can I feed it into my house cable system that goes to rooms throughout the house? I have Dish Network and don't use cable in walls. Any thoughts? Thanks.
 

n4yek

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I do have a question. Once mine is mounted outside can I feed it into my house cable system that goes to rooms throughout the house? I have Dish Network and don't use cable in walls. Any thoughts? Thanks.
I don't see big problem, it just depends on how many connectors your cable has between the antenna and the scanner. Each connector will add loss to your system.
 
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