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Author Topic: Slim-Jim made from DC cable?  (Read 6102 times)

Posts: 1757

« Reply #15 on: March 19, 2013, 03:40:04 PM »

What I dont understand, is why people come onto a thread that is quite specific about what its asking and what the subject is, just to make bone comments and push their own ideas of what someone should do.

Well, that happens because you didn't tell us what you know, so we fill in parts that you never explained.

If you want better answers, ask better questions.

Now, you have added more information. You've filled in a bunch of gaps and now we know what you know and what you are trying for. Had you started there, you would have gotten better answers.

73 de N4CR, Phil

Never believe an atom. They make up everything.

Posts: 17484

« Reply #16 on: March 20, 2013, 09:25:08 AM »

The velocity factor has only a little impact on the length of the radiator
portion of the antenna, because that is not operating as a feedline.  The
conductor diameter has a little effect:  fatter conductor (larger diameter
wire and/or two wires in parallel) will shorten it slightly.  The Slim-JIM
configuration (two radiator wires in series) may decrease the bandwidth
slightly over using a single wire, or both wires in parallel, but in the
great scheme of things it really doesn't matter a lot:  it's still a half-
wave radiator regardless of the number of wires involved.

The velocity factor does matter, however, in the matching stub, as does
the characteristic impedance of the feedline (and, to some extent, the
feedline losses.)  The best solution is usually to cut the radiator to some
standard length and leave it at that, then experiment with the position
of the shorting bar and the feedpoint for lowest SWR.  (The spacing
between the shorting bar and feedpoint remains relatively constant once
you find a good value, so you usually end up moving both contacts at
the same time.)  I've used pins for both, though sometimes a staple will
work for the short circuit, or you can start with the cable long and
trim it back a bit at a time, shorting the ends as you go.  (Usually I end
up with the matching stub so shredded by the time I get it adjusted
that I build a new antenna using the final dimensions anyway.)

It sounds as though the DC cable is likely to have a lower impedance
due to the conductor size, which will affect the spacing between the
shorting bar and the feedpoint, but I suspect you can make it work
with some experimentation.

There may be some losses in the dielectric at the top of the matching
stub, which is a high impedance point, as PVC isn't the best RF insulator
for that purpose.  But give it a try.

Posts: 10

« Reply #17 on: March 29, 2013, 10:36:42 AM »

I'm new to all of this stuff, but data cables are usually twisted in order to cancel out electromagnetic interference, pretty much the opposite of what we want in an antenna it seems to me. Your not talking about a lot of twist in your cable but it could be enough to kill efficiency of the antenna.

Posts: 17484

« Reply #18 on: March 29, 2013, 01:19:52 PM »

Actually, having twist in the cable isn't a problem.  The currents in the lower
matching section are out of phase (we hope) so it shouldn't radiate anyway.
The currents in the two wires in the upper section are in phase - acting like
a single wire (which works just as well as two wires) so twisting doesn't
eliminate any radiation.
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