interesting coax vertical antenna...

[mr_fx]

http://www.cbradioclub.com/media/kunena/attachments/975/coaxantenna.jpg

It seems like you could build this as a 1/2 wave or even a 5/8 wave if you built a matching section

also I guess you could even use ferrite beads instead of the ugly balun


what issues do you fellas see with this?

 

[vkrules]

I've made a few of these for portable use.Two things I found . If you make the radiator out of coax ( ie just strip the shield off) the length is much shorter , than the length posted ,due to velocity factor .And you may have to adjust the choke for best isolation.

 

[mr_fx]

basically the shield of the coax is doubling as the other side of the dipole, but it is still wrapped around the center conductor... that is what is confusing me

BUT I guess it's basically a 1/4 wave version of the Imax 2000, or the A99

 

[WA8ZYT]

To be a true coaxial antenna the shield that is peeled back from the center conductor should be doubled back on the shield of the lower part of the antenna, making a quarter wave stub or counterpoise. Then the choke would not be needed.

 

[W5LZ]

Think of it as a 'Zepp' antenna using coax instead of parallel feed line. That choke is one way of 'stopping' the outer shield from radiating (not really, but sort of).
If you doubled the shield back over the outer insulation as was suggested, that choke would definitely be needed because of the effect of high voltage at the ends of a dipole inducing unwanted currents on the outer shield of the feed line. Sorry 'bout that, but that's how it is...
- 'Doc

 

[WA8ZYT]

A lot of companies make those coaxial antennas for low/high band commercial two way radio and no chokes were ever used or needed.

I have also made antennas in a pinch for 2 meter FM portable operation, and never had a problem with common mode currents.

The LFA antennas use a quarter wave stub off the loop to prevent common mode currents, as well.

 

[northern35s]

No need to peel the coax shield back, remember coax has three conductors, the centre conductor, the coax shield inner and the shield outer, that's why it is called an unbalanced feedline. Make your lower coax section half the length of your dipole and use single PVC coated wire for the other half, soldered, crimped or whatever, to the centre conductor of your coax

I'd also suggest using a torroid to choke the coax, that way you can have a simple, lightweight, choke with much higher impedance than the ugly balun design. You could even build the choke into a simple box with SO239 connectors either end, terminate the dipole in a PL259 plug and your have an interchangeable design with any length of feedline you choose / need

 

[Captain Kilowatt]

No need to peel the coax shield back, remember coax has three conductors, the centre conductor, the coax shield inner and the shield outer, that's why it is called an unbalanced feedline. Make your lower coax section half the length of your dipole and use single PVC coated wire for the other half, soldered, crimped or whatever, to the centre conductor of your coax

I'd also suggest using a torroid to choke the coax, that way you can have a simple, lightweight, choke with much higher impedance than the ugly balun design. You could even build the choke into a simple box with SO239 connectors either end, terminate the dipole in a PL259 plug and your have an interchangeable design with any length of feedline you choose / need

Actually it has two conductors but three paths for RF to follow:blink: That is not the reason it is an unbalanced feedline.Coax is an unbalanced feedline because one conductor is ground referenced. That ground may be real earth ground or simply chassis ground as in an HT. Balanced feedline has no ground return path.

 

[northern35s]

Actually it has two conductors but three paths for RF to follow:blink: That is not the reason it is an unbalanced feedline.Coax is an unbalanced feedline because one conductor is ground referenced. That ground may be real earth ground or simply chassis ground as in an HT. Balanced feedline has no ground return path.

I disagree, coax is unbalanced due to it's construction, not how we use it, if for some reason I attach coax to say a dipole, and then rather than put a PL259 on the other end, I attach the screen and centre conductor to the posts of a balanced tuner, does the coax then become balanced?

 

[WA8ZYT]

No, it does not become balanced. The center conductor and shield are two different sizes. Ladder line has two conductors the same size.

 

[HomerBB]

No, it does not become balanced. The center conductor and shield are two different sizes. Ladder line has two conductors the same size.

This answer also obviously references the construction of the feedline similarly to 35's statement.
So, isn't everyone agreeing?
It seems to me that CK's post speaks to the consequence of the coax construction whereas 35's to the nature of its construction, neither, in the case of coax, occur without the other.

So when is there an occurance of unbalanced feed line systems other than coax? direct antenna wire to the back of transmitter terminal with earth ground to txmtr chassis? others?

 

[Captain Kilowatt]

I disagree, coax is unbalanced due to it's construction, not how we use it, if for some reason I attach coax to say a dipole, and then rather than put a PL259 on the other end, I attach the screen and centre conductor to the posts of a balanced tuner, does the coax then become balanced?

I missed this response. We are actually both correct depending on how you look at it. I was looking at the normal use of coax. I cannot imagine why anybody would ever connect coax as if it were balanced feedline as in your example above. HOWEVER if one were to use two pieces of identical coax one could indeed create a shielded BALANCED feedline with a characteristic impedance twice that of a single piece of coax ie two parallel pieces of 75 ohm coax will yield a 150 ohm balanced feedline that is shielded. In this case it is indeed HOW we use it that determines if it is a balanced feedline or not.

 

[W5LZ]

Or another way of saying the same thing is that just because a 'balanced' tuner is 'balanced' doesn't mean that the system is then balanced. Doing it that way is the same as using a balanced feed line with an unbalanced tuner. You wind up with an imbalance.
- 'Doc