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Author Topic: Attic Dipole RFI  (Read 4450 times)

Posts: 9

« on: May 20, 2005, 08:36:47 PM »

I just moved into a house that does not let me put up antennas outside,  so I am stuck with putting the antenna in the attic.  First of all,  I am using a 1:1 Balun and running bare copper wire run along the wood beams.  My question is,  I know that a antenna inside will get into stuff,  can this be put to a minimum by using a low pass filter?

Posts: 229

« Reply #1 on: May 21, 2005, 11:01:44 PM »

It probably won't help. A low pass filter reduces the level of television and FM broadcast frequency harmonics and spurious RF energy a rig might send to an antenna, but most problems with this kind of setup are caused by the on-frequency energy you WANT to radiate.

What helps is keeping your antenna wires as far as possible from and at right angles to nearby wiring and pipes, reducing power, and making sure you keep RF off the coax shield with a balun, common-mode coil-of-coax choke, ferrites or the like.

When you've reduced antenna coupling to the wiring, appliances and home electronics as much as you can, then start working on decoupling the victim devices with filters, ferrites, different outlets, better shielded or shorter connecting cables and the like and (if necessary and possible) modifying or replacing those that are either oversensitive to interference or interfere with your operation themselves.

About antennas: *Be creative!*

I've used a balanced, open-wire-fed Delta antenna made of hard-to-see blue/blue-white telephone twisted pair that started in the attic, ran from the attic vent to and along a back-yard fence and back. A tuner provided an acceptable match on 80-10 meters, some bands of course working better than others.

You may be able to use as an antenna some object whose presence outdoors -- lawn chairs, barbecue, bird feeder, training wire for vines or roses, large window screens -- is tolerated. Coax might be concealed in or disguised as a piece of hose or drip tubing. Some folks have used a wooden patio umbrella as a top-hat-loaded vertical.  Metal window frames may if large enough (I had one 12 feet long by six feet tall once) be tuned as shunt fed loops.

Everything is an antenna if you use your imagination.

Good luck!


Posts: 21764

« Reply #2 on: May 23, 2005, 09:35:07 AM »

Previous response was great, and I wouldn't argue with any of it.

Low pass filter extremely unlikely to help anything, since what causes RFI is the intended transmitting frequency and power, not unintended (harmonics), which is all a filter can help with.

Instead of bare copper wire, I'd use insulated wire, and use end insulators or a length of string at each end of any attic-installed wire antenna to help assure no high voltage points on the antenna come in contact with potential flammables, like wood.  I've burned wood easily with 100W when an arc formed between antenna wire and a nail in the wood.  This usually happens at the very ends (tips) of a wire doublet, but can occur every 1/2-wavelength back towards the feedpoint from those ends as well, if you're using a single doublet on multiple bands.

*Proximity* is what creates RFI problems, so the farther you can get your antenna(s) from "stuff," the better off you are.  A doublet outside, laying on top of the roof (assuming the roofing material is an insulator), is better than one inside the attic.  One suspended slightly above the roof is even better.  And one not near the house at all is best.

If possible, consider an "invisible" doublet that's all "outside," but installed in such a way it cannot be easily seen.  Using thin, dark-colored wire often makes this possible.



Posts: 555

« Reply #3 on: May 27, 2005, 11:04:53 PM »

In older houses, electrical wiring was run in the basement and fed up to outlets, switches, etc.  In newer houses, the opposite is often true, the wiring runs in the attic.  While an attic dipole may indeed work well in an older home, in the newer home interference can be coupled in both directions, from the antenna into the house wiring and from wiring to antenna.

Another problem with attic antennas is the close proximity of the antenna to the living space.  High RF fields can exist even with moderate power levels.  I would not consider running QRO into an attic antenna.

While federal preemption allows TV antennas (with a few very specific exceptions), TV antennas can be used as part of an effective stealth amateur antenna system.  Consider exercising your right under federal laqw to have an outdoor TV antenna at the maximum height allowed (up to 12' over the roof line) you would bbe amazed how good a medium sized TV antenna works as a capacative hat for a vertical antenna.

Check to see if your homeowners association allows you to erect a flag pole.  Flag pole antennas are a mainstay in stealth antenna technology.

Is there a local professional or college team?  A small helium filled blimp with the team logo on the side can be tethered by a nice metal wire rope...which can be loaded up with a tuner, then winched down and stored in a small storage shed (blimp barn?).  Since it is not a perminent structure, and simply a decorative device, your HOA may turn a blind eye.

Copper rain gutters last longer and add to the value of a home.  They also can be soldered to and will load up nicely.  Check with a continuous gutter company about having the gutters formed out of copper.

Lastly, if you have a standard 8' fence around your back yard, consider a Hustler mobile mount with ground radials run from the base. it might be visible from the air, but at ground levely nobody will see it sticking out over the fence.  Hustler has resonators which are rated for legal limit power.  There are several multiple resonator devices available which would allow you some measure of bandswitching.  If you have an autotuner, placing it in a suitable weatherproof box at the base of the antenna will help you with additional bandwidth.

Good luck and 73


Posts: 132


« Reply #4 on: June 01, 2005, 11:47:41 PM »

I'm going with the same route as the author of the original post. I too am in an apartment and I have access to the attic where I've mounted a G5RV Jr. If I do have RFI, I'll probably resort to an outdoor magnet wire. That may be a challenge to pull off as I'm on the 2nd (and top) story of my apt. building. I'll have height but no ground. A counterpoise for each band will then be necessary. I guess we'll jsut have to wait and see what happens.

Posts: 6

« Reply #5 on: June 06, 2005, 10:59:27 AM »

What about a bird feeder?

Posts: 410


« Reply #6 on: June 03, 2005, 03:27:00 PM »

Take a look at what Jack, WM0G has done

C Ya

Posts: 44

« Reply #7 on: June 14, 2005, 12:19:58 AM »

You are going to have swr issues with an attic antenna. Trying to trim it to fit its un-modelable environment is a pain. If you go that route, make sure you have a well grounded trans-match.

Your RFI issues will come from Front End Overload.
You will likely get into the telephone, and the touch lamps in the house for sure. You may also find problems with Broadcast recievers.

A better option for you may be a DOTR or Dipole ON the roof. If you absolutly HAVE to go this route - try laying a 40 meter dipole on your roof. Use insulated wire that is the same color as your roof. You will still have SWR issues, but your RFI will be a lot lower.

A better option is an out door antenna made of magnet wire. Try a getting as much magnet wire as high as you can and tuning it as an end fed random wire.
That will out perform your dipole on the roof.

Another option is a mobile mag mount antenna. You can magnet it to an outdoor metal table or something like that.  I would prefer that over an indoor antenna myself. Gud luk es 73 Carl W8WZ

Posts: 2808

« Reply #8 on: June 16, 2005, 10:37:53 PM »

A G5RG Jr is (I think) an unbalanced dipole.  It doesn't need a ground wire or counterpoise.

Put a balun at the antenna feedpoint to avoid common-mode currents if you use coax feedline.  [If you're using ladder-line feed, the balun will be at the transceiver- or tuner-end of the ladder line.]

I bet that with 100 watts radiating in the attic, you _will_ have RFI problems, whatever antenna you use.  

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