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Coaxial Dipole?

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sflmonitor

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Does anyone have any experience with these type of antennas? I haven't worked with them but after some quick searching, I guess they are coaxial dipoles? I've seen them on national park office buildings, military bases, etc. Does anyone have any experience with them? Just curious what the benefits are when compared to other commercial base antennas such as fiberglass colinear or folded dipole?
 

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ko6jw_2

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They are vertical half wave antennas. A j-pole is another example of a vertical half wave. Coaxial antennas are difficult to construct for diy. They have the advantage that the sleeve decouples the antenna from the support mast and they do not need a ground plane. Other than that they are not that exotic. While they will exhibit gain over a quarter wave, they don't have any greater gain than any other half wave vertical antenna.
 

mmckenna

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Broad banded, really durable, nice omni-directional radiation pattern.

I used to have one here at work, it had been up for decades and still worked very well.
 

prcguy

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The one in the picture appears to be a Kreco and I have aluminum and brass versions I use on 2m. It is difficult to make one correctly because you need a dielectric "plug" inserted between the skirt and mast or skirt and feedline to make the inside surface of the skirt appear longer than the outside and size/placement of the plug can be critical. Kreco uses a large thick Plexiglass doughnut about an inch up inside the skirt for this.

As mentioned, they are just a half wave center fed dipole, nothing special but they are rugged and have reasonable band width. There is a version with a folded/grounded vertical element and that should have more band width.

I also have a Kreco 3 element Yagi that has a coaxial dipole as the driven element, very unusual.
prcguy
 

NC1

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If you want to build something similar, try the link below. I built one tuned to the 2m Amateur Radio band and it works great for both RX and TX.

All the formulas and tables are there to make what you want, along with instructions for the PVC enclosure and mounting instructions.

My cost was $0 because I had all the parts laying around. Give it a try, you might be surprised with how well it works.

The Vertical Bazooka Antenna - Ham Radio Library
 

prcguy

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These types of coaxial dipoles have some loss due to the folded back braid laying against the plastic outer covering of the coax and most outer coverings are a lousy dialectic with loss. If you use Teflon insulated coax it will work better but you really need to keep the feedline away and isolated from the inside skirt of the coaxial dipole and keep it centered as it exits the skirt.
prcguy

If you want to build something similar, try the link below. I built one tuned to the 2m Amateur Radio band and it works great for both RX and TX.

All the formulas and tables are there to make what you want, along with instructions for the PVC enclosure and mounting instructions.

My cost was $0 because I had all the parts laying around. Give it a try, you might be surprised with how well it works.

The Vertical Bazooka Antenna - Ham Radio Library
 

NC1

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These types of coaxial dipoles have some loss due to the folded back braid laying against the plastic outer covering of the coax and most outer coverings are a lousy dialectic with loss. If you use Teflon insulated coax it will work better but you really need to keep the feedline away and isolated from the inside skirt of the coaxial dipole and keep it centered as it exits the skirt.
prcguy
Agreed, but didn't want to get too technical.

I figure start with the basics and adjust from there. It's all about experimentation and having fun with it.
 

sflmonitor

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Great info guys. Just when you think you know about antennas, there is always something new. thanks for the great detailed information.
 

K9DAK

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I've been making these antennae out of leftover coax for quite a while. Made a 154 MHz version for the scanner that supplies my Broadcastify feed, and a 162 MHz version for the weather radio at my work. Didn't pay much attention to the shield length for receive only, and both work great... for zero cost.
 

NC1

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I've been making these antennae out of leftover coax for quite a while. Made a 154 MHz version for the scanner that supplies my Broadcastify feed, and a 162 MHz version for the weather radio at my work. Didn't pay much attention to the shield length for receive only, and both work great... for zero cost.
You got that right. These make an excellent receive antenna for scanning, and you can make them to target the frequency or band you desire. For the price it really can't be beat. Of course the antenna manufacturers will disagree ;)

For transmit, just use a meter to get the lowest SWR and it is just fine without getting into the murky details. I have one for use indoor when there are storms in the area that make me run around disconnecting cables going into the house.

If anything, they sure do beat a rubber duck antenna.
 

phask

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Anyone want to build me one for 800mhz? If so PM me. My skills are just not as good as in the old days.
 

Golay

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The Vertical Bazooka link pretty much shows the antenna I have made at work. Have put dozens of these at 460 for forklifts and at 153 for remote control locomotives that want to go thru a roll up door that isn't quite all the way up. Make the antenna as the link says, cover it with heat shrink, and it's basically a rubber duckie. I myself make the shield about 10% longer than the center conductor. Then put the heat shrink over everything. The resonant will always be lower than what I want. Then I nip the end about a 1/4" at a time until I get where I want, fold a piece of 3M Super 88 over the top and heat shrink in place.
 
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