Choke position on a HexBeam
I just put up a K4KIO HexBeam and placed the ferrite choke close to the feed point as recommended by Leo Shoemaker.
We were discussing this online tonight and some feel it should be placed below the base plate as to not interfere with the radiation pattern.
What was the concern about interfering with the radiation pattern? The choke, by itself, is so small that it won't scatter any appreciable waves.
If you put the choke away from the feedpoint, current will flow on the coax shield between the feedpoint and the choke which WILL distort the radiation pattern. Probably not a LOT because it's not a long length, but if you choke at the baseplate there's this random length of conductor that's basically hanging off HALF of the antenna in terms of radiation.
If the choke is at the feedpoint instead and the feedline is symmetrically placed with respect to the elements, current won't flow on it and the pattern will not be distorted.
The feedpoint is the right place for the choke.
I'm curious what people were worried about putting the choke at the feedpoint tho?
Have you looked at this page:
As Dan says, very close to the feedpoint is the best position for conducted CM current; but of course it's almost impossible do that on a hexbeam because you have a different feed position for each band. The obvious feedpoint to choose is the top-most: 20m on a five band version.
If you have balanced feedline (rather than coax) between the different band feed positions, then again the top-most feedpoint is the correct position.
The recommendation to place the choke below the baseplate is another "ham myth"; I've even see folk mount the choke at the baseplate in such a way that the metal baseplate and mast completely bypassed the choke function!
Last edited by G3TXQ; 06-11-2012 at 06:57 AM.
I just installed a K4KIO hexbeam, I am thrilled by the low SWR. I did not choose to use the ferrite bead balun sold by K4KIO, but rather used one of my own designs useing a t-130-2 torroid with 6 turns of coax opposed by 6 turns of coax to form a choke balun. This I mounted in a plastic box just below the mounting plate, making sure the feed coax was wire tied up against the center feed post. Since the shield of the coax is at the same potential as the center post I figured any disturbances from revirse current should be a minimum if the coax was tied up against the post.
So far the preformance seems quite good.
If you look at how DX Engineering does it, they put theirs right on top of the center mast (on top of the antenna). So maybe tape the coil to the side of the mast at the top. my 2 cents from a hexbeam owner.
It's advisable to give some thought to how you dress input and output leads either side of a choke. For example, on the K4KIO hexbeam the centre-post forms one leg of the transmission line on the balanced side of the choke; if you now take the feedline coax on the unbalanced side of the choke, and neatly tape it to the centre-post along its whole length, you will have created a capacitor which bypasses the choke.