Necessity is the mother of Innovation! I really do not like open wire
feed lines. They allow you to match a high SWR (At the shack). Open
wire to an unbalanced antenna will have a high common mode voltage as
well as signal pickup. I've tried them a couple of times, but always
ended up getting into the house electronics. They also picked up too
much local noise.
The reason for chokes was "it was handy". Being lazy, I decided to try
the second choke where the coax reached the tower as there was already a
connector there. The only extra work that entailed was removing the
weatherproofing from the connectors. It worked and that let me get away
without adding more weight hanging off the center of the dipole. With
the 5 core, 6 turn choke
http://www.rogerhalstead.com/ham_files/AntennaFeed2.htm plus nearly
100 feet of LMR-400 already hanging off the center. That dipole already
has well over 100# of tension to minimize the sag
I am building a single, larger choke, but using the 1/4" coax with a
Teflon dielectric and a double shield of silver plated braid. The cores
are spaced 1/8th inch and epoxied into the Lexan spacers. I've not
settled on the # of cores or turns of coax. The spacers are strips of
1/4" Lexan 3/4" wide by a length to be determined. The slots are milled
1/8th inch deep with 1/8th in spacing. I did one trial using Plexiglass
spacers with 1/4" deep slots, spaced 1/4", but they were too fragile.
The Plexiglass strips were cut from scrap and were free. The Lexan was
cut from a relatively small piece. Still they were less than $2.00
I'll have to shoot some photos of the spacers and the construction.
Hmmm. I'm hearing thunder. Time to check the rigs in the shop.
73
Roger (K8RI)
On 8/22/2015 5:21 PM, Edward McCann wrote:
Roger-
Sounds like you are feeding your fan dipole with coax. If not, and if you are
using open wire or twin-lead, how have you adapted the notion of choke at each
end of the feed line?
73
Ed
AG6CX
Sent from my iPhone
On Aug 22, 2015, at 1:53 AM, Roger (K8RI) <k8ri@rogerhalstead.com> wrote:
Jim is right on!
I have a center fed, sloping fan dipole on 75. People think of dipoles being
balanced, but so many things interact with them a truly balanced dipole would
be a miracle. My sloping, Fan dipole with one end at roughly 90 to 95 feet and
the other at about 15 feet is so far off balance that it took two RF chokes to
keep the RF out of the shack(s) With an AIM that reads to three decimal
places, I can watch the antenna values wander areund as the trees move in the
wind.
My shop is similar to your ham shack. The interior is bonded barn metal on the
walls and ceiling. Doors are metal clad insulated with the man door being steel
with no window. With only one choke designed to give about 5000 ohms of common
mode isolation, it seemed to operate OK, but by 1KW out the LEDs in the station
were glowing.
A second choke, back where the feed line reaches the tower cleaned it up.
Like many I thought The antenna shouldn't be a problem and should be fed with a
balanced, voltage balun. The chokes in Jim's RF tutorial did the trick.
73
Roger (K8RI)
On 8/20/2015 11:42 PM, Jim Brown wrote:
On Wed,8/19/2015 12:29 PM, Stan Zawrotny wrote:
Frank,
Does the interference to the smoke detectors occur at very low RF
output...say 5 or 10 watts? Or does it occur only AT higher output levels.
It increases with power. Gets really bad when I kick in the linear.
What about 5-10 W?
Have you attempted to contact the importer or manufacturer to inquire about
the application of bypass caps to AC power lines or battery leads as well
as other parts of the device to minimize the effect of RF on the chirp
circuit?
An email to the manufacturer, USI Electric Inc., went unanswered.
Have you considered the telephone? Have you contacted the installing
contractor?
My antenna is an 80-6 meter OCF up about 40 feet in the trees.
An OCF antenna is a recipe for LOTS of RF conducted onto the feedline and into
your home.
I don't think it is radiating back into the house.
I'm pretty certain that it IS.
The house is ICF construction with a tin roof. I essentially live in a Faraday
cage. I have to go outside to use my cell phone. The house will withstand a cat
5 hurricane or tornado.
The cell phone is UHF, your ham rig is HF.
That OCF antenna, by virtue of its imbalance, puts a LOT of common mode RF
current on the feedline, which penetrates what you THINK is a Faraday cage (and
it's NOT a Faraday cage unless the metal completely encloses the area you're
trying to shield, AND all the metal parts continuously bonded together). And
when you penetrate that structure with that feedline, it completely defeats any
shielding that you think you have.
73, Jim K9YC
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