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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