[RFI] RFI With Smoke Detectors

[Roger (K8RI)]

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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