70cm Coax Collinear - need Balun help

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NYCone

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I'm very new to ham radio, licensed in the last few months. I'm trying to build a coax collinear for 70cm following designs like:


I've modified the materials a bit, I'm using LMR400 for the elements. My question comes from the balun aspect. I don't understand how baluns are designed and the methods I'm using call for toroids and a copper tube to be placed on the feedline.

My questions are as follows:
  1. FT50-43 toroids (called for in one design) won't fit over LMR400, so I ordered some FT83-43s instead. Will these work properly?
  2. Are there other easier options for the balun than the copper tube, toroid method?
  3. Some folks have commented that these designs don't really work. Are there folks who have built these who know how well they work?
Any help is appreciated.

Thanks
 

prcguy

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The FT-83-43 ferrite will work fine as a replacement. The "balun" shown in the link is a two part thing. The first is a 1/4 wave "bazooka", which is a 1/4 wavelength resonant tube over the coax starting at the feedpoint and running down the feelline, insulated at the feedpoint and grounded 1/4 wave down from the feedpoint. That will encourage RF not to travel (much) beyond the feedpoint and down the coax. The second part is the ferrite which will absorb some RF on the shield and reduce feedline radiation. I would not use LMR400 as its difficult to solder and the dialectic will melt if you look at it too long.

I'm very new to ham radio, licensed in the last few months. I'm trying to build a coax collinear for 70cm following designs like:


I've modified the materials a bit, I'm using LMR400 for the elements. My question comes from the balun aspect. I don't understand how baluns are designed and the methods I'm using call for toroids and a copper tube to be placed on the feedline.

My questions are as follows:
  1. FT50-43 toroids (called for in one design) won't fit over LMR400, so I ordered some FT83-43s instead. Will these work properly?
  2. Are there other easier options for the balun than the copper tube, toroid method?
  3. Some folks have commented that these designs don't really work. Are there folks who have built these who know how well they work?
Any help is appreciated.

Thanks
 

Ubbe

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I did a coax vertical from RG6, that's a very easy coax to work with and there are lots of instruction for it. It was for 1090MHz and was checked with a VNA. It didn't work that good. It had to be 3 times longer than a "normal" 1090 antenna with some 6dBi gain to get the same performance but it did work at a lot of other frequency bands as well, probably because of its size. I didn't bother with any ferrites or metal tube baluns or had any ground radials, that might have made it less effecient. I tilted it to see if the direction loob where pointing up in the sky but that didn't help as it still had the same omni coverage.

/Ubbe
 

prcguy

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The elements of a coax colinear come out shorter than 1/2 wave due to the velocity factor of the coax. Compared to an exposed dipole array it takes twice as many coax colinear sections to equal about the same gain. That's partially because the element spacing is not optimal and ends up whatever the resonant lengths of coax are with velocity factor.

I did a coax vertical from RG6, that's a very easy coax to work with and there are lots of instruction for it. It was for 1090MHz and was checked with a VNA. It didn't work that good. It had to be 3 times longer than a "normal" 1090 antenna with some 6dBi gain to get the same performance but it did work at a lot of other frequency bands as well, probably because of its size. I didn't bother with any ferrites or metal tube baluns or had any ground radials, that might have made it less effecient. I tilted it to see if the direction loob where pointing up in the sky but that didn't help as it still had the same omni coverage.

/Ubbe
 

NYCone

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I've been lucky with the LMR400 and soldered the 1/2 wave segments. I just need to do the 1/4 wave and balun portions before I can test it.My questions were geared towards the balun options and folks real life experiences.
 

NYCone

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The other question that arises ios the two refewrences I outlined show a differnet terminal end of the antenna.

One shows putting a 1/4 wire on top of the last 1/2 wave element and soldering to the center wire only. The other shows using a 1/4 coax AND 1/4 wave wire to the SHORTED top element. Is one correct?
 

ko6jw_2

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As far as I can recall the top section is just a 1/4 whip connected to both the shield and the center conductor of the last piece of coax. The balun is simply a connection between the shields of the second coax section and the bottom section (at the bottom of both sections).

I have looked at these antennas for almost 50 years. Whenever I feel like building one I lie down until it passes. By the way using solid core dielectric coax which has a lower velocity factor makes the antenna shorter and saves coax. For a variety of reasons I would not use LMR400.
 

NYCone

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Thanks.

The issue is the last two sections on one construct is 1/4 wave coax shorted to 1/4 solid. The other is 1/2 wave coax NOT shorted to 1/4 wave solid. I wasn't sure which was a better design. Thoughts?
 

ko6jw_2

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The issue is the last two sections on one construct is 1/4 wave coax shorted to 1/4 solid. The other is 1/2 wave coax NOT shorted to 1/4 wave solid. I wasn't sure which was a better design. Thoughts?
OK, there is no better design. This is one of those projects that you just have to build and experiment with. Make a choice and build the antenna. I've never known anyone who actually built one. Granted, commercially made vertical collinear antennas may use something similar, but as a one off you will have to experiment.

As an example, I set off to build an extended double Zepp for 6 meters. None of the designs I looked at mentioned one key point. That is, they use a tuned feed line. Once I finally found that out I was one the air and making contacts. It is still around and I may put it back up when six opens up more.

The coax collinear is the same thing. Build it, get it working and then tell us how you did it. Have fun. This is what it's all about.
 

NYCone

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OK, there is no better design. This is one of those projects that you just have to build and experiment with. Make a choice and build the antenna. I've never known anyone who actually built one. Granted, commercially made vertical collinear antennas may use something similar, but as a one off you will have to experiment.

As an example, I set off to build an extended double Zepp for 6 meters. None of the designs I looked at mentioned one key point. That is, they use a tuned feed line. Once I finally found that out I was one the air and making contacts. It is still around and I may put it back up when six opens up more.

The coax collinear is the same thing. Build it, get it working and then tell us how you did it. Have fun. This is what it's all about.
Good point. This is my first antenna, so I'm just trying to get opinions. I've never even tested an antenna before.
 

bharvey2

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Whenever I feel like building one I lie down until it passes. By the way using solid core dielectric coax which has a lower velocity factor makes the antenna shorter and saves coax. For a variety of reasons I would not use LMR400.
I thought about building one of these once but as you say, the desire soon passes. That being said, I wonder if hardline like 1/2" Heliax might be a better choice for no other reason than it might be easier to strip and solder (at least with the proper tools)
 

merlin

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Anyone ever dismantle a commercial Phelps Dodge (now cellwave) vertical antenna, this is the very same thing.
Difference is the coax in the Phelps Dodge is semi rigid coax. the end element locks into the lightning spike at the tip.
DC grounded which helps reduce noise.
 

merlin

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I thought about building one of these once but as you say, the desire soon passes. That being said, I wonder if hardline like 1/2" Heliax might be a better choice for no other reason than it might be easier to strip and solder (at least with the proper tools)
For their ease of construction and RG6 fits nicely in a length of PVC pipe. they are worth the couple hours to build.
 

bharvey2

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Who knows, maybe the "because I can" factor that I mentioned in another thread may come in to play some time. I'm sure I have enough old hardline and aptly sized PVC laying around to give it a try.
 
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