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
>>>> _______________________________________________
>>>> RFI mailing list
>>>> RFI@contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/rfi
>>>
>>> ---
>>> This email has been checked for viruses by Avast antivirus software.
>>> https://www.avast.com/antivirus
>>>
>>>
>>> _______________________________________________
>>> RFI mailing list
>>> RFI@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/rfi
>
>
> ---
> This email has been checked for viruses by Avast antivirus software.
> https://www.avast.com/antivirus
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|