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Author Topic: Limited Space Attic Antenna  (Read 974 times)
K4IMM
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Posts: 1




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« on: November 24, 2007, 11:23:31 PM »

I live in a Condo so exterior antennas are a NO-NO. I do have good access to the attic via drop down stairs and it is floored with plywood.  No foil insulation but a gas furnace lives at one end withe foil wrapped duct.  The roof is constructed with the usual trusses and I have a total rectangular area in which to install an antenna that measures 18'-0" by 8'-0". My first thought is to install a closed 52'-0" loop of #12 stranded with an AT-4 automatic tuner at the feed point.  The coaxial feed line from the tuner would run roughly 15 - 20 feet down to my rig which will be located in a second floor room below.  I would like to work 40m CW.  I know that this not the best conditions in which to work but an outside antenna including any on the roof, under the eves, etc. is not an option.  I'm looking for comment on how well or how poorly this installation will work and any suggestion on possible options for my attic space.
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KA5N
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« Reply #1 on: November 25, 2007, 02:11:58 AM »

18 X 8 feet is not big enough for anything close to a half wave on 40 meters.  You could get a 15 meter dipole or maybe a 20 meter dipole into that space.  With loading coils you might get something for 40.  You might look at magnetic loops (MFJ makes a couple)as they are small for the frequency covered.  They are not excessively difficult to build and give good results considering.
Almost any antenna will work better outside than one inside a building.  When the bands are hot (in a couple of years) you will be amazed at what an indoor antenna will accomplish.
Good Luck Allen
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W7AIT
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« Reply #2 on: November 25, 2007, 02:55:53 AM »

ATTIC ANTENNAS QUESTION AND ANSWER, do attic antennas work?

I have successfully put a 40/30/20/17/15/12/10 antenna dipoles fed with a W2AU balun, RG58 coax in the attic.  Although not optimal, they do work.  I never tried 80, too big.

Trick is:

1.   Use a balun.  Required!
2.   Get the wires off the feed as straight as possible for as long as possible before bending the ends.
3.   Ends may be bent all over, even a Z shape, zig zag, just don’t cross over itself.
4.   Use insulated wire, helps shorting out issues.
5.   Keep wire away from wooden trusses, 2X4’s etc by at least a few inches.  Remember wood will detune the antenna / suck up RF, so the better job you do keeping it away from wooden supports the better.
6.   Tune with analyzer.  It may require extra long wire or it may be extra short, depends.  Use cut and try approach so start extra long.  Wire stretchers are very expensive devices and hard to use in the attic!
7.   Watch out for telephone wires, computer wires, heating equipment wires, metal ducts, metal pipes, metal joists, house wiring, romex, etc.  You will couple RF into these things so expect RFI in TV sets, stereo’s, telephones, intercoms, burglar alarms, etc.
8.   Don’t fall through the ceiling and hurt yourself.  Sheetrock can be patched but injury to yourself not!
9.   Performance:  I has a ATTIC 40 for years, worked JA’s, a lot of stateside etc.  They do work.
10.   I measured 20 db loss in attic dipole in my last attic installation.  The tile roof attenuated HF signals by 20 db.  This was a contrast / compare measurement using the Buddipole dipole outside and the fixed attic BW40 in the attic.  20 db is a lot!  So at that attic installation, it was a very poor performer.  Fortunately I  moved to a tract where outside antennas allowed soon after and have been having great success.  Outside antennas the only way to go unless you can’t put up anything else but attic antennas.
11.   Van Gorden engineering makes a line of dipole kits for HF; buy the balun too, they make the antenna work far better than direct feed. Repeat after me “USE A BALUN”!
12.   Be sure to run “RF SAFETY EXPOSURE” before running your attic antennas.  Make sure you meet or exceed requirements for your transmitter power & installation.  Enter results in your log book in case FCC asks to see.
13.   6 meters/ VHF/ UHF – Some of the above applies, though mounting, counterpoise’s, reflectors, are different.  Experiment!
14.   COMPUTER HASH:  Attic or in the room antennas pick up horrible amounts of computer hash.  It can be so bad as to make reception useless.  Also note, SCR controlled light dimmers are awful noise sources too.  Try to position your attic antennas away from computers etc.  Some electronic light controllers known to emit 160 khz RFI & also be KEYED ON BY HAM TRANSMITTERS!

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N4LI
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Posts: 397




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« Reply #3 on: November 25, 2007, 05:30:04 AM »

I spent a few years working with attic antennas, and actually did quite well -- Mixed DXCC/HF, QRP/DXCC, and VUCC/6m done with dipoles and a 3-element 6m yagi.  While this was near the top of Cycle 23, it still shows what can be done, even indoors.

A few thoughts, though, as you go forward:

Avoid traps and coils.  Those appliances can get hot.  You don't want a trap catching fire in the attic; that would just be bad.  Really bad.

Also on the subject of fire -- remember, the voltages on a dipole get very high near the ends.  Again, that can be a fire hazard.

I learned the hard way -- High RF in an attic can trip fire alarms, and summon your local FD late at night.  Your wife won't be happy.

Antennas in the attic can cause RFI in the house, particularly if your cable TV lines route through it.  Expect disagreements with the kids.  "Dad, you're messing up SpongeBob again."  "I don't care, son.  I have a nice 6m opening to the Caribbean.  Go away."

Watch the power.  I would never exceed 100W indoors.  The fire danger increases exponentially, and the RF exposure starts getting problematic on higher bands.  I know a guy running 1 kW on an attic dipole; I am glad I am not his insurance carrier!  I ran mostly QRP in my attic; it worked fine, and caused fewer problems.  SpongeBob even was left unmolested.

Have fun with it.  But, be safe.

Peter, N4LI
Near Memphis
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AD5VM
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Posts: 26




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« Reply #4 on: November 25, 2007, 07:52:49 AM »

I lived in a third floor apartment for two years and had a great antenna farm. The attic was 20x20 with 15' high peak so it was above average as far as attic spaces go. I had a two element 20 meter hexbeam pointed North East. A loaded 40 meter dipole and an 80 meter isotron.

But... my first attic antenna was just a loop going all the way around the attic. The cool thing about loops and dipoles is that the wire can be bent in all directions. The gain of the loop depends on the size of the aperture (the area of the inside of the loop) the radiation pattern of the loop depends on where it's fed and it's shape.

So, you can shove a 40 meter loop in there and just zig zag the wire back and fourth in the rafters to make a loop (remember the shape of those POW!! BAM!! ZOOM!! graphics from Batman comic books!) use insulated wire and string keep the wire from touching the wood. Mount your antenna coupler right on a rafter and attach the loop to it. Another apartment antenna trick is to use an artificial ground from MFJ. Get yourself a couple hundred feet of 20 gauge insulated wire and run it under the baseboards from your shack desk ALL THE WAY around the parameter of your condo interior. This will act as a tuned counterpoise for the entire station. It not only got rid of RF in my shack, but it improved my TX signal, slightly lowered RX noise and lowered the SWR on all my antennas. Make sure you double insulate the tip of that wire wherever it is because there will be high voltage present. If you have a high noise level from everyone’s plasma TV's and microwaves like I did, an MFJ-1026 helped a great deal.

Good luck!
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AA4PB
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« Reply #5 on: November 25, 2007, 05:01:55 PM »

Avoid traps and coils. Those appliances can get hot. You don't want a trap catching fire in the attic
------------------------------------------------------
Humm, in 40+ years of hamming I've never had a trap or loading coil get hot or catch fire with 100W.
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AC5UP
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Posts: 3956




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« Reply #6 on: November 25, 2007, 06:06:56 PM »

I've been thinking about putting a terminated loop in my attic as an SWL item that might throw a signal well enough for domestic contacts on the upper HF bands... Consider a loop with an 800 ohm non-inductive resistor rated at half your expected TX power equidistant from the feed point on the 18' span.

Feed it with a 4:1 dual core Guanella style current balun into a parallel run of RG-59 or RG-62 arranged like shielded twin lead. Tie the shields together at both ends and wire the center conductors like a balanced feed line. Hang another 4:1 current balun at the radio end and see how it plays. The terminating resistor can be a group of surplus 2 watt carbon items wired in series / parallel (depending on what value(s) you can score on the cheap) into something that an Ohm meter sees as 800, +/- 10% or so. Ten 2 watters will dissipate 20 watts (nominal), a little more with an intermittent duty cycle like CW or SSB --- And I wouldn't recommend running more than 40 - 50 watts into * any * indoor antenna.

You want something balanced that's ground independent like a loop, setting up a terminated loop might give you several bands to play with. Also, don't be afraid to test with a 1:1 balun at the radio end. Could give you another band, could suck boulders. If you're home brewing the baluns this won't cost more than a little time and wire. At the power you'll be running you can probably get away with 1" cores and wire salvaged from CAT-5 Ethernet cable scraps. While you're on that thought, you could build the loop from CAT-5 as well with all conductors in parallel. Think of it as Compu-Geek Litz Wire.

18' x 8' isn't much to work with, and there are two schools of thought on this... One is to arrange the loop with the largest possible enclosed area, the other is to get as much wire in the loop as you can. Visualize bow-ties and tortured rectangles.

If all else fails, think "Kinky Fan Dipole" for 10m, 12m, and maybe 15m.

(?)

An ugly antenna makes more Q's than none at all... Right?
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AC5UP
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« Reply #7 on: November 25, 2007, 06:15:14 PM »

One Other Thot: Since we're talking loop, I don't see a screaming need to limit yourself with a two dimensional concept... If you have some height in the attic, why not use it? Like... Four V's arranged into a rectangle, or maybe a V on the short sides with a W on the long sides.

Do not park a VW in the attic, just keep those two letters in mind the next time you eyeball the space.
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WB6SSW
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« Reply #8 on: November 26, 2007, 04:36:12 PM »

I'm in a similar situation and use a Buddipole.  Works better than anything else I've tried over the years.  

If you do go this route or any other, expect that the surrounding framing, wiring duct work, etc. will affect the tuning of the antenna substantially.  I've got my Buddipole resonant at 7.035 right now, but it isn't setup anywhere near where the tuning chart would have it adjusted for that frequency.  It will be a lot of "cut and try" or you'll need an antenna analyzer to bring the antenna into tune.
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