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  #1 (permalink)  
Old 09-23-2008, 9:02 PM
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Default Home made scanner antenna

I'm looking to make something cheap that will boost my signal better than the little telescopic antenna on the back of my uniden scanner. I'm planning on goining out the window of my garage and up about 20 feet. Would this antenna work better than the little telescopic??? Any suggestions are welcome. TJ

Homebrewed Off-Center Fed Dipole (listed under the antenna sedction on homepage) The picture didn't paste here...




Building A Homebrewed Off-Center Fed Dipole Scanner Antenna.

Aluminum/copper tubing construction:

You will need to "press fit" the tubing/"T" connector at the store for proper fit.The tubing/connector is held in place with 2 S/S sheet metal screws for connecting the balun to each element.

Find a "U" bolt to fit your mast.Drill two holes in the support pipe to fit the U bolt.The support pipe is 18" from the "T" to the mast.

Remember,bandwidth increases as diameter of the elements increases. I think,if I remember correctly,at the hardware store,that a few CPVC fittings will fit copper tubing perfectly!

Some say that the 18" element on top mounted works best,Some like the 48" element on top.It does'nt matter,it works the same.

If you use the copper tubing,be sure to paint it with some good,non-conductive paint.I used to paint mine light grey. -Have fun!
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Old 09-24-2008, 12:11 AM
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http://www.radioshack.com/product/in...Id=2103641&cp=
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Old 09-24-2008, 12:49 AM
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Lightbulb Homemade Antenna

http://wiki.radioreference.com/index...ter_Fed_Dipole
I built this, and it works great. I run 3 scanners off it, and am receiving signals as much as 50 miles away.
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Old 09-24-2008, 10:39 AM
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That is the antenna I posted the plans for, but the picture didn't follow.

I'm planning on making it out of some 3/4" copper pipe I have left over from a home project, and ssticking it up in a nearby tree when I get it completed. Most of the listening I do is right around 155MHz so hopefuly it will work out.

I do have a really dumb question though. Does it matter which coax lead goes where? It seems to me like the center wire would go to the short pole, and the ground wire going to the long pipe. I say this only because there seems to be more ground radials on an antenna than the center radial. Does this make sense???
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Old 09-25-2008, 7:17 PM
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Any help on coax lead positions???
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Old 09-25-2008, 7:23 PM
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Quote:
Originally Posted by waterskier View Post
I do have a really dumb question though. Does it matter which coax lead goes where?
The coax doesn't connect directly to the pipe, the wires from the balum connect to the pipe.

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Old 09-25-2008, 11:09 PM
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You are right, the balun connects to the pipe.

The plan for this dipole shows a 48" tube on top, and a 18" tube on the bottom. (seems backwards to me)

Do I need to orient the balun so the center conductor of the coax goes to the short pipe? And the braided portion of the coax going through the balun to the long pipe. (I'd have to use an ohm meter to confirm that I get the correct lead connected to the short or long pipe.)

It Just seems to me the short tube should be on top with the center conductor of the coax connected to it through the balun.
And the bottom has the long tube with ground connected through the balun.

If the 2 tubes were equal length then it wouldn't matter...

I'm reaching out to you designers and folks with experience... am I correct? Or does it make any difference at all? I'm making my comparison to a base station antenna where the ground plane radials are on the bottom and the short radial on top.
Can anyone answer this????? I'm going to build it. I just want to do it right and have it work good for me.

Last edited by waterskier; 09-25-2008 at 11:16 PM..
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Old 09-26-2008, 11:43 PM
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You could always swap the 2 balun leads and see which way it works better as well as trying short end up/down.
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Old 01-03-2014, 11:10 PM
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my experience with a balun was it does not matter witch end is up because the braid and center conductor are connected inside the balun if you attach a piece of coax on it a check for continility you will get a reading so I would put long end up I just built a base scanner antenna last wk with wood dowels some alum foil a little paint and some 75 ohm coax for about $20 works great let me know if your interested and I will try to post a pic and explain
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Old 01-05-2014, 7:13 AM
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It really doesn't matter which lead of the transformer goes to the long or short side.

Note that for best results, use an *isolated* transformer such as the suggested Radio Shack ones #15-1140 mentioned in the article. This antenna already has major common-mode issues due to the offset feed, and transformers that share a common ground only make it worse. The 15-1140 is isolated.

Since your listening is mostly in the 155mhz region, an even better antenna (don't get me wrong, I think the OCFD is fine antenna IF you understand it's compromises) would be to merely make a simple dipole.

For 155mhz, two equal 18-inch pieces of tubing would suffice. One side to the shield and the other to the center conduction with NO transformer.

Since you have to run the feedline away from then antenna horizontally for at least a few feet before dropping it down vertically in either case, if all you want is 155mhz coverage, then the center-fed dipole with equal-length tubing and no transformer would be ideal.

Last edited by hertzian; 01-05-2014 at 7:17 AM..
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Old 01-21-2014, 7:01 AM
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Would like to hear more about your wood dowels and alum foil antenna.
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Old 01-23-2014, 3:01 PM
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I purchased my Bearcat BC75XLT 01/11/2014 any trying to learn how to use it.
I built the "Off-Center Fed Dipole" RadioReference.com - Scanner Frequencies and Radio Frequency Reference using 48" & 18" PCV pipe wrapped with Aluminum Foil and Radio Shack #15-1140 reception was good.
I then switched out the 48" with another 18" reception was not as good.
I would like to build 48" & 18" using copper pipe.
Can I use 1/2" pipe or do I need to use 3/4" pipe?
Do I need to paint it?
I just want a good antenna to scan the following:
123.05000,151.43000,152.99000,153.53000,154.17500, 154.20500,154.31000,154.78500,154.99500,155.05500, 155.13000,155.14500,155.24250,155.34000,155.89500, 155.94000,158.13000,159.85500,160.02000.
Any help and suggestion will be greatly appreciated.
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Old 03-28-2014, 3:53 PM
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So,

center-fed- no balun,

off center- balun.

What about coax? 75 ohm, 50, does it matter?
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Old 03-28-2014, 4:12 PM
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I'm not a fan of the RR offset dipole thing and someone here did a nice study on it but I forget the results. The way I see it is the offset dipole fundamental frequency where it would have a good pattern at the horizon when used vertically is around 85Mhz, but there is nothing to receive there. At other higher frequencies it will have a null at the horizon or the lobes will point up and down and anywhere but the horizon where its needed.

I think a Discone and probably the cheapest version like a Workman T601 (around $33) would probably do you very well. Most tuned antennas drop off drastically in performance when you get away from their sweet spot and it looks like you need to cover quite a wide range in the VHF band but nowhere else. The only real problem with the typical scanner Discone besides not having any gain to speak of, is the pattern shifts upward after about 500MHz causing some serious degradation at 800MHz and that is the usual complaint.

Since your reception is within the best range of most Discones sold for scanner use, you will get very consistent performance across all the freqs mentioned.
prcguy

Quote:
Originally Posted by Jimwheel View Post
I purchased my Bearcat BC75XLT 01/11/2014 any trying to learn how to use it.
I built the "Off-Center Fed Dipole" RadioReference.com - Scanner Frequencies and Radio Frequency Reference using 48" & 18" PCV pipe wrapped with Aluminum Foil and Radio Shack #15-1140 reception was good.
I then switched out the 48" with another 18" reception was not as good.
I would like to build 48" & 18" using copper pipe.
Can I use 1/2" pipe or do I need to use 3/4" pipe?
Do I need to paint it?
I just want a good antenna to scan the following:
123.05000,151.43000,152.99000,153.53000,154.17500, 154.20500,154.31000,154.78500,154.99500,155.05500, 155.13000,155.14500,155.24250,155.34000,155.89500, 155.94000,158.13000,159.85500,160.02000.
Any help and suggestion will be greatly appreciated.

Last edited by prcguy; 03-28-2014 at 4:25 PM..
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