I don't know who made it, but that comment demonstrates a fundamental
misunderstanding about coax feedline!
When we connect coax feedline to a beam we are also (unwittingly
perhaps) connecting a conductor from one side of the driven element to
ground; that's exactly what the outside surface of the braid is - an
extra, "independent", conductor. Does that sound like a good idea?
What effect it will have depends on how much of the feed current flows
back along the braid, and that depends on the impedance of that path as
"seen" at the feedpoint. It will vary depending on the length of the
coax, its routing, and how it is grounded at the shack end. Routing it
orthogonally to the beam and down the mast does *not* guarantee to
prevent CM current!
Putting a CM choke (1:1 current balun) at the feedpoint is the only
sure-fire way to minimise the CM current, particularly on a multiband
antenna. Assuming the feedpoint impedance is around 50 Ohms and that the
antenna is well balanced, the choking impedance doesn't need to be very
high to minimise CM problems; more importantly, the choking impedance
needs to be high enough to prevent over-dissipation in the balun.
I agree that the cost of a balun doesn't have to be large. Jim has
pointed to some designs in his paper, and there are others on my web site:
http://www.karinya.net/g3txq/chokes/
Finally, the balun loss should not be high. It comprises about 2ft of
coax - how much extra loss does that introduce!
Steve G3TXQ
On 26/11/2014 17:48, Wilson wrote:
Looking a little deeper, they say that if the coax goes down the mast, so it
doesn’t pick up and reradiate a lot of energy (which compromises the pattern),
the balun isn’t really needed.
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