>RE: Question about end fed dipole Reply
by AE6CP on August 19, 2005 Mail this to a friend!
I would like more info on the coax version if you don't mind. This would be for qrp operation so could I use the mini stuff?
I'm an AIT field service engineer for KT in Albuquerque.
Thanks to everyone for all your info!
::The FCC data base still shows you in Boulder Creek, CA. If you're in Albuquerque, you probably should change that.
Anyway, the "coax version" I discussed can be best described, I think, like this (following). I'll describe a 40 meter dipole version, but the same approach can be used for any band:
Take length of coax long enough to reach from "dipole far end" to "station equipment," and strip the vinyl jacket (only) off the cable at 1/4-wavelength back from the far end. For 40 meters, this would be 33 feet. It can be stripped by making a circular cut through the jacket 33' from the end and bending the cable back and forth at that spot a few times to break the jacket completely clean, then slide off the jacket over the far end and discard it. With some cables, this takes some effort! With RG58C/U (small diameter cable, .195" OD), it's usually pretty easy.
Now, you'll have 33' of braid exposed. Right at the point where that braid meets the jacket on the "balance" of the coax that wasn't stripped -- e.g., right where you just sliced through the jacket to strip it off -- use a soldering iron and some solder to tin the braid for about 1/4", all the way around the cable, at that point. Don't use too much solder; just a little bit does the job perfectly.
Now, using two pairs of long nose pliers, grab that tinned braid with the plier jaws set only 1/4" or so apart at two points on the tinned braid, and wiggle the pliers back and forth to make a clean break of the braid where it's tinned. This will separate the braid from the rest of the cable. Pushing from the end you just cut (using the pliers), push the braid off the end of the cable entirely, exposing 33' of cable dielectric. The 33' of braid should now be laying on the floor, detached from the cable.
Take what was originally the "far end" of that braid (not the tinned end), and push it back over the cable dielectric, to draw it back down over the cable, but 180 degrees reversed from the way it was originally. The goal here is to slide the braid over not only the dielectric material (where it originally was), but beyond that point, and slide it over the vinyl jacket of the coax for the next 33 feet. The desired goal is to have 33' of exposed dielectric (with the center conductor inside!), followed by 33 feet of braid slipped over the jacket of that same coax. Overall length from "end" of the coax to "end" of the braid, once pulled over the jacket, should be 66 feet.
Since you'll be sliding the braid over a cable that's fatter than it originally over, you'll have to make the braid a little bit "fatter" as you use it, by simply bunching it up slightly as you draw it over the vinyl jacket. This sounds hard, but it's not, and works fine.
Once you have the braid slipped over 33' of the vinyl jacket material, stop. Strip about 1/4" or so of vinyl jacket off the coax (right next to where you originally snapped the braid off), to expose a bit of coaxial cable braid right at that point.
Slip the "loose" braid (that you cut off, and then re-slipped over the cable and down the jacket) up until its tinned end is aligned with the 1/4" of braid you just exposed by trimming jacket in the above step.
Wrap some tinned copper busswire (or any sort of copper wire, really) around this joint, to mechanically and electrically bond the two braids together: The 1/4" of exposed braid from the main coax run, and the tinned braid end. Now, using a soldering iron and a bit of solder, solder this area, flowing solder over the wire used to wrap them together. This is the electrical connection for the "center" of the dipole.
Pull the braid that's over the dielectric taut over the dielectric, to stretch it out as long as it can go: It should be just about 33 feet, assuming the coax is "straight" and taut inside the braid. Once it's pulled taut and everything is straight, use some vinyl electrical tape to securely tape the end of the cut braid to the vinyl jacket of the coaxial cable, 33' from the "spliced" center, to hold it in place.
You might go up the length of the braid-over-jacket area and apply a few wraps of tape at other locations, too. You can also use nylon tie-wraps to bind the braid cover to the coax.
Now, install a coax connector on the "station" end of this same coaxial cable.
What you have is essentially a center-fed 1/2-wave dipole with its feedline exiting from one end, routing through the 33' of braid. The radiating element is 33' of center conductor, and 33' of braid material.
This works *exactly* like a center-fed dipole, but the cable is simply routed differently. To help prevent antenna radiated current from coupling to the feedline (undesired), you can make a current balun right at the point where the coax exits the end of the braid material, by making 10-12 turns of the coax cable itself into a tightly wound coil about 3" in diameter, and taped to hold its form.
The nice thing about this approach is the resulting antenna is about 50 Ohms impedance and doesn't require any matching network, so its power handling ability is whatever the coax cable itself can handle. In the case of RG58C/U, that should be at least 500 Watts. If you use RG213/U, it would be at least 1500 Watts.