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Old 02-02-2013, 6:29 PM
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Default My homemade Off Center Dipole Antenna

Following the wiki plans, I made my own off-center dipole antenna for my base scanner. So far, the reception is pretty good on VHF high and UHF. I have yet to hear far away units on VHF low, but I haven't had a lot of time to listen this evening.

I made it out of 1/2" diameter copper pipe, a PVC T fitting, 2 PVC end caps, sheet metal screws, a new 75-300 ohm balun, a piece of PVC pipe to mount the balun, and 12' of RG-6 coax cable. Here are the pics I took while making it.

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Last edited by dmg1969; 02-02-2013 at 6:32 PM..
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Old 02-02-2013, 6:30 PM
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The next 5 photos.
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Old 02-02-2013, 6:31 PM
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Last 2 photos.
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Old 02-02-2013, 6:32 PM
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Helps to attach them.
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Old 02-02-2013, 7:02 PM
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Looks good, very neat. I just build a Yagi but waiting for spring to paint it then I will post pictures.

I might try one of these next. looks like another good project.
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Old 02-02-2013, 8:26 PM
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Perfect! Now if you could just get it up higher in the house. But if it's getting the low band FD signals you're looking for, then no real need.
You've got me thinking about going back into my attic again with a measuring tape to see if I can fit one like yours up there and replace the bell wire version I have up there now. As indoor antennas go the OCFD is hard to beat. But you do need 5 1/2 feet of vertical space... Or a plan to tilt it slightly off vertical.

Last edited by popnokick; 02-02-2013 at 8:27 PM..
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Old 02-02-2013, 8:43 PM
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dmg, perfect job! Looks great. I have the same but mine was made by Grove Enterprises. Works well in my area. I get VHF-UHF fine as can be, but it seems to suffer a bit in the 800Mhz range. But it works lol.
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Old 02-03-2013, 2:37 AM
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Good job; think you'll find the OCFD is naturally resonant for the vhf aero band. Once mounted, slowly turn it until the signal is at it's best. Hears very well off the "back side".

Good Signals!
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Old 02-03-2013, 6:22 AM
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I just heard a police unit in a municipality that I know I could not have heard before, so I am happy with it. The only way to get it higher would be to put it in the attic, but I think it would have to be at a fairly steep angle. I'll have to measure to see exactly how much clearance I have. I would need a longer run of RG-6 and fishing the walls would not be fun. Have to think about it because it is pulling in nicely just here on the main floor.

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Perfect! Now if you could just get it up higher in the house. But if it's getting the low band FD signals you're looking for, then no real need.
You've got me thinking about going back into my attic again with a measuring tape to see if I can fit one like yours up there and replace the bell wire version I have up there now. As indoor antennas go the OCFD is hard to beat. But you do need 5 1/2 feet of vertical space... Or a plan to tilt it slightly off vertical.
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Old 02-03-2013, 8:02 AM
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You did a very nice job there - even better than many that I have built.

Although this antenna is not super-critical, the length dimensions are chosen on purpose, and you may want to put the transformer leads as close to the edge of the pipes as possibe - perhaps under the heads of the screws that go through the pvc and the tubing itself to get a little closer to the edge of the tubing.

The tubing length dimensions are also skewed a little bit by the long length of the transformer leads, which basically act as an extension of the antenna if they are not parallel to each other. My suggestion would be to shorten the leads as much as possible by running the non-parallel part of the leads off at a near a 90 degree angle over to the screw heads in the pvc and cut off any excess lead. (don't stress the bend too much so as not to break the wire leads internally - just shortening things up a little bit will help.)

Again, nice job. Be sure to peruse the 5000 or so threads about the OCFD to see where it's strengths AND weaknesses lie. In the meantime, enjoy!

Last edited by hertzian; 02-03-2013 at 8:07 AM..
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Old 02-03-2013, 8:04 AM
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Good job, the antenna looks great.
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Old 02-03-2013, 12:27 PM
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Well, taking another look in my attic only reaffirmed that I have nowhere near enough room to move the antenna up there. I'm lucky if I have 3' of clearance at the roof peak. I am still not happy with the performance I'm getting in the VHF low range. I can hear the county police dispatch loud and clear because they are in the next town. However, the police units, specifically those over the mountains are still pretty much inaudible. I heard one earlier today, but he must have been in a prime location. As far VHF high and UHF...no complaints.
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Old 02-03-2013, 6:40 PM
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What would be the easiest/best way to weather-proof this so I can temporarily mount it outside to see if it improves my VHF low reception?

Edit: I think I thought of a way. Loosen the two screws attaching the balun contacts to the both copper elements and remove the balun. Remove the screws securing the PVC side arm and remove the 12" PVC pipe. Slide a larger PVC T fitting over the antenna. Drill 2 holes to allow access of a screwdriver to re-attach the balun contacts to the copper elements. Drill 2 holes to allow access of a screwdriver to re-attach the 12" PVC side arm. Attach the balun back to the PVC side arm with wire ties. Slide a larger diameter piece of PVC over the one holding the balun. Drill holes in end PVC caps large enough to allow the copper elements to pass through and attach them to the top and bottom of the larger PVC T fitting. Silicone the end caps o the T fitting and also any gap around the copper elements. Drill a hole in another PVC end cap to allow the coax to pass through. Secure to the larger diameter PVC side arm with silicone and also silicone around the coax. That's my vision to weatherproof it.


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Last edited by dmg1969; 02-03-2013 at 7:39 PM..
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Old 02-03-2013, 7:00 PM
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If "temporarily" means a day or two in dry weather (no rain, ice, sleet, or snow) as I suggested for testing effectiveness on the roof (tripod/clamps etc.) Then I wouldn't sweat making it weatherproof. Otherwise, you may want to spray all with DuPont Krylon KleerCoat, and make liberal use of Coax-Seal tape on connections, exposed areas, and encapsulating balun and coax connector. Remember though you're going to need a good earth ground, coaxial grounding block, etc. if you decide to permanently install. The copper tubing OCFD is very inviting to nearby lightning, not to mention the need to bleed off static buildup that occurs on any outdoor antenna.
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Old 02-03-2013, 7:09 PM
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Also.. This may seem obvious but in case you've never seen an eave mount, here is one. Very simple, inexpensive, readily available, and sturdy permanent mount: http://www.dennysantennaservice.com/image/21022193.jpg
However, you prob want to use PVC for the main mast then do a standoff mount of the OCFD so that there is not a metal mast running close to and parallel with the OCFD elements. That would likely have very bad effects.
But unless you are willing to take on this permanent mount project knowing that it may not produce the full result you're looking for, do the temporary test first.
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Old 02-03-2013, 7:54 PM
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I won't put forth any great effort for the trial. But, yes...if I do the permanent mount, I will weatherproof the antenna, ground it and add the coax grounding block. I would still probably go with a deck mount but I have a lot to consider.
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Old 02-04-2013, 5:25 AM
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Quote:
Originally Posted by Fast1eddie View Post
Good job; think you'll find the OCFD is naturally resonant for the vhf aero band. Once mounted, slowly turn it until the signal is at it's best. Hears very well off the "back side".
Note that the OCFD length is pretty much an offset dipole resonant down in the lower part of the FM broadcast band! If you live in an area with strong FM broadcasters, and a weak scanner front-end, the OCFD can be an invitation to FM overload / desense for airband enthusiasts. A radio shack or better PAR filter can cure it, but the point is that the ocfd is very broadband, and sometimes that is not a good thing depending on your environment.
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Old 02-09-2013, 7:06 PM
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Well, I decided to give the antenna a whirl up in the attic before mounting it outside. It is not vertical, but angled at about 45 degrees, but is definitely picking up farther. I am picking up police mobile units 2 counties away and they sound crystal clear. I have not heard much low band radio traffic since I have been able to listen. I am curious to see how much that has improved.

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Last edited by dmg1969; 02-09-2013 at 7:25 PM..
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Old 02-09-2013, 7:13 PM
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It appears the antenna is pretty close to some electrical wiring. That's probably not the best place to have it, but if it doesn't seem to be having any 'noise' problems I don't think I'd waste a lot of worry on it.
Hope it works out well for you.
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Old 02-10-2013, 6:28 AM
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I see you added the PVC weatherproofing.
The fact that going up to the attic... even at an angle... improved distant, weak mobile signals is a very good sign that your plan for a roof mount is going to help a lot.
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