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Re: [RFI] Coaxial Choke

To: dgsvetan@rockwellcollins.com,Andy <ingraham.ma.ultranet@rcn.com>
Subject: Re: [RFI] Coaxial Choke
From: Pete Smith <n4zr@contesting.com>
Date: Wed, 15 Apr 2009 17:14:33 -0400
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
I suspect, but don't know, that your results would be substantially 
the same with no balun at all.  This whole current balun thing is a 
relatively recent phenomenon, and for 40-odd years most of us never 
did anything but connect our coax to the center of our dipoles and go to it.

73, Pete N4ZR

At 04:51 PM 4/15/2009, dgsvetan@rockwellcollins.com wrote:
>Andy and All,
>
>No "science" as such, but I am a big believer in APPLIED technology (which
>may or may not have "real" science asociated with it.
>
>Several years ago, I decided to make a quick and dirty dual band
>inverted-V a permanent part of my antenna farm.  I say "quick and dirty"
>because this antenna was constructed more than 15 years ago as a fan
>dipole with 2 pairs of wires attached to a common center insulator, with
>one pair tuned to around 3850 kHz and the other pair tuned to around 7250
>kHz.  The antenna had NO balun whatsoever, but had performed well at many
>Field Day and similar events.
>
>When I decided to make this my "NVIS" special by hanging it at 32 feet
>above the ground off a side arm on my 48 foot tower, I realized that the
>lack of a balun would produce not only "weird" radiation patterns, but
>likely also result in oddball VSWR readings due to cable sway in the wind
>and so forth.  After all, both sides of the feeder coax shield would be
>hot with RF.  Further, the antenna would be connected to my Ameritron
>5-way remote switch that allows me to select any one of several antennas
>on my HF tower and I suspected that a "hot" balanced antenna would likely
>interact in nasty ways with non-hot antennas..
>
>I was not about to rebuild the antenna, so I added a balun to the feed
>line.  That balun consists of about 8 or 9 turns of RG-58A/U wound into a
>coil of about 7 to 8 inches diameter and secured with UV-resistant cable
>ties.  The end of the coil that goes to the center insulator of the
>inverted-V has 2 long ferrite beads (unknown maker and mix) slipped over
>the cable.  (These beads are about 2 inches long each, roughly 5/8 inch
>OD, and have an ID of about 0.35 inch, perfect for slipping over RG-58
>cable.)  They were "liberated" years ago from a filter assembly that was
>supposed to be effective down to 100 kHz, which is why I selected those
>beads.  I protected the ferrite beads with WX-proof shrink tubing.
>
>This "system" has been in continuous use at my station for more than 6
>years and works perfectly.  What's the Z of the balun?  Beats me.  Don't
>know, don't care.  VSWR is stable under all wind and water conditions, I
>do not detect the presence of lobes on either band.  In fact, this antenna
>is my #! "killer" for working counties and states within 300 miles in
>daytime, and for covering the lower 48 and Canada on those 2 bands at
>night.
>
>73, Dale
>WA9ENA
>
>
>
>
>
>"Andy" <ingraham.ma.ultranet@rcn.com>
>Sent by: rfi-bounces@contesting.com
>04/15/2009 10:08 AM
>Please respond to
>Andy <ingraham.ma.ultranet@rcn.com>
>
>
>To
>"RFI List" <rfi@contesting.com>
>cc
>
>Subject
>Re: [RFI] Coaxial Choke
>
>
>
>
>
>
> > The material in the ARRL Handbook is woefully out of date and quite
> > inadequate.
>
>I'm not sure if "out of date" is exactly right in this context (neither
>the
>coax nor the theory changed, did they?), but I believe you are right that
>air would coax chokes are inadequate for HF.  OK for VHF, maybe for the
>upper HF bands too, but not as a general HF band choke.  Not nearly enough
>impedance to choke off common-mode currents.
>
>There was an email thread about this on one of the many other email lists
>I
>subscribe to, saying pretty much the same thing.  To be effective at HF,
>one
>must use some ferrite core(s) to make an effective choke.
>
>Andy
>
>
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