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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