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Re: [TowerTalk] [RFI] RFI With Smoke Detectors

To: "towertalk@contesting.com" <towertalk@contesting.com>
Subject: Re: [TowerTalk] [RFI] RFI With Smoke Detectors
From: Robert Harmon <k6uj@pacbell.net>
Date: Wed, 26 Aug 2015 11:15:38 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Roger,

Your experience with open wire line is interesting.  I had a full size delta 
loop strung up for 80 meters off my tower and I fed it with ladder line.
It picked way too much local noise.  I took it down and restrung up an inverted 
v for 80.  The inverted V is  a lot quieter on receive.
I am thinking of stringing up the delta again and this time feed with coax with 
a balun on the ends.  The open wire may have been the 
culprit for picking up all the noise.   


Bob
K6UJ




> On Aug 23, 2015, at 11:49 AM, Roger (K8RI) <k8ri@rogerhalstead.com> wrote:
> 
> 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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>>>> RFI@contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/rfi
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