Coaxial Cable Antenna Elements??

[blue runner]

Anyone have any experience using just the coax shield for antenna elements?

I have lots of 3/4" dia cable TV SEMI-RIDGED coax that would physically make good antenna elements for 10 & 11 meters.

I don't intend to use the center conductor connected to anything (unlike in the "velocity factor shortened" antennas - lots of loss) and just use the aluminum tubing shield for the active element.

It is not practical to remove the center conductor and doing so would probably reduce the rigidity of the coax too.

I just wonder if the unconnected center conductor would significantly load the signal?

Anyone have any info on that?

 

[Marconi]

Anyone have any experience using just the coax shield for antenna elements?

I have lots of 3/4" dia cable TV SEMI-RIDGED coax that would physically make good antenna elements for 10 & 11 meters.

I don't intend to use the center conductor connected to anything (unlike in the "velocity factor shortened" antennas - lots of loss) and just use the aluminum tubing shield for the active element.

It is not practical to remove the center conductor and doing so would probably reduce the rigidity of the coax too.

I just wonder if the unconnected center conductor would significantly load the signal?

Anyone have any info on that?

I've used regular RG8 coax for a dipole, but I used the center conductor as the support and the shield as the radiator. I didn't think about any conquences of attaching the two together, and soldered the shield and center together.

 

[Captain Kilowatt]

Anyone have any experience using just the coax shield for antenna elements?

I have lots of 3/4" dia cable TV SEMI-RIDGED coax that would physically make good antenna elements for 10 & 11 meters.

I don't intend to use the center conductor connected to anything (unlike in the "velocity factor shortened" antennas - lots of loss) and just use the aluminum tubing shield for the active element.

It is not practical to remove the center conductor and doing so would probably reduce the rigidity of the coax too.

I just wonder if the unconnected center conductor would significantly load the signal?

Anyone have any info on that?

I have some of the same stuff and I highly doubt it is strong or rigid enough to be used as elements on 10/11m. Take a piece of it about 9 feet long and hold it at one end or clamp one end in a vice. See how much downward force it takes to bend it. Unlike tubing it will not spring back into place after taking much bending force. The weight is a factor in that as well as the fact the aluminum used in it is paper thin and not hardened like in tubing. It is made to flex a bit and be rolled on a spool etc.

 

[Robb]

Use 1/2" EMT tubing if you want to make a dipole antenna.
Very, very cheap and rigid; about $3 for each 10 ft piece.

If you want use that coax; then make a loop antenna (very cool) out of scrap wire and then use that coax for the needed matching section. However, you don't need to use more than 5' 6" of it to do that. Or you can make a lot of loop antennas and use it all up - lol!

 

[blue runner]

I'm thinking about using it in different shapes like maybe a 1/2 wave loop, spiral etc, supported by thin cypress or cedar strips.

I just don't know it the center conductor will interfere.

I guess the thing to do is just make a vertical dipole with it and compare its performance to my 1/2 wave vertical.

I'm hoping someone may have had some experience with it and save me some trial & error.

Thanks for the replies.

 

[Captain Kilowatt]

If the center conductor is not connected to anything it will not effect anything. I do know that it has been used as a vertical antenna by several folks but usually it was strapped to a tree trunk or hung from a high branch. If you were into amateur radio I would suggest it be used for 2m or 70cm helix antennas. It works great in that applications.

 

[Beetle]

Easiest way to find out is to build a prototype and test it under real world conditions. Then make changes (one at a time) to see what comes up different from the time before. Take voluminous notes and keep them where you can find them ten years from now when you're remembering how well "x" antenna worked.

 

[blue runner]

Here is a thought.. a RFD (Resonant Feed-line Dipole) works because RF travels on the inside of the coax shield in one direction and the outside of the coax shield in the other direction.

What do you suppose is going on inside aluminum tubing elements. Seems like RF should be flowing on the inside surface of the tubing as well as the outside surface of the tubing. Does the RF on the inside go to waste?

Think there would be a difference in performance of a solid conductor element as opposed to a hollow tubing element?

 

[W5LZ]

"...because RF travels on the inside of the coax shield in one direction and the outside of the coax shield in the other direction..."
Nope, sorry. The desired RF flows in both directions inside that shield.
- 'Doc

 

[blue runner]

I thought the phases were in opposite polarity. Thanks for the enlightenment. After a moment of thought I realized that if they were opposite polarity then the currents on the surface of the shield of the lower half of a RFD 1/2 wave antenna would be messed up and of incorrect phase to make a half wave radiator.

I think that I understand that at the lower bands the RF current skin effect is not happening or is less significant, only as the frequency increases does the skin effect come into play.

If HF 10 meter currents are flowing on the inside surface and the outside surface then what is happening inside a hollow aluminum tubing element as opposed to a solid element? Do the inside currents somehow radiate too?

This thought is involved in using coax for a radiator element with only the shields connected to the transmission line. I expect that there will be currents flowing on the outer shield surface and inside shield surface of the coax element and that the center conductor of the coax element will also become energized by proximity and capacitive coupling through the dielectric material of the coax element even if it is not otherwise physically connected to anything.

This is getting too complicated and making my head hurt.