Attic Dipole

[KA4DPO]

UPDATE:

I'm making progress. I constructed a centerfed dipole 51" long fed with RG-58 for my SW *reciever* (Yaesu VR-5000).

Mounted my MSquared 6m HO Loop horizontal and ran RG-58 coax to the radio room.

http://www.m2inc.com/pdf_manuals/6M%20HO%20Loop.pdf


Decided to mount my 2m vertical to the back of my house and ran RG-8 to the radio room. Hope the HOA guys don't see it.

Ordered the Hy Power 2B2040LQ dipole; it's on back order


I started building an 80m loop antenna in the attic. I'm about half done. Pulling 254 feet of #14 insulated wire through the attic without stepping through the ceilings is hard work!! I have blown insulation and finding rafters to stand on is a challenge. I'm going to tune it to 3.985. I also have an extra loop of 31' of wire to drop the freq down to 3.560 for CW QRP use. So far I've run the RG-8 to the radio room. Distance of RG-8 run is about 20 feet.

Question: Will insulated wire require a 4% reduction in length to tune to freq (the 80m loop)?


None of the antennas have been connected to my coax switch yet. I bought an MFJ 259B antenna analizer to tune the loop with. Hope to have them done this weekend.

Because your antenna is in the attic tuning it is going to be difficult to tune using standard formulas because of the proximity to so much stuff. Your antenna will likely be resonant at a lower frequency than predicted because of loading by all of the stuff in the attic. I have found that a center fed flat top will almost always out perform a loop for multi-band use when placed in an attic but try the loop first and see what you think.

20 feet of coax is not too bad so if you must use it use LM-400. You can also use 300 ohm ladder line, it is much smaller than the 450 ohm line and will have a lot less loss than even 20 feet of coax.

[K8JD]

Skip the loop/tuner and go with the trap dipole. If you try to run ladderline or twinlead down inside waLLS YOU MAY BE RUNNING NEXT TO WIRING FOR YOUR POWER AND HAVE PROBLEMS, COAX CAN RUN ANYWHERE.
The trap dipole, if trimmed properly, requires no tuner and is a good match to coax line and there will be minimal losses or interaction.
Just keep as much distance forom metal HVAC equiment as possible.

[KA4DPO]

Why in the world would you want to run a trap dipole? Traps are lossy and the bandwith on each band is very narrow. A flat top and tuner will give you all band capability depending on the length of your antenna but at least 40 through 10. You can use coax if you need to, it will have more loss than twinlead but it will work. In fact, at 20 feet and at 100 watts you can use regular TV twinlead just fine.

[N4UP]

I have a similar problem/challenge though my 3rd Floor attic is smaller ( floored space is 30 L x 18 W x 11 H ). I just installed the MFJ-1775 rotatable mini dipole ( 40-20-15-10 meters ) on a 6' tripod ( no traps; has loading coils for each band ) . Thanks to insights gleaned from this thread I was not surprised that the resonant frequency is quite a bit lower than advertized. I am now in the process of trimming the elements to resonate near the bottom of each band. With my attic dimensions I can only rotate the dipole around 45 degrees. However, I plan to install the MFJ-1775W for WARC coverage, and both will fit in my attic, and both will swing up to 45 degrees without being too close to either the roof or each other. I am using 25 feet of LMR-400 with a line isolator on both ends. The coax runs down the stairs and into the radio room. Of course with the antennas at 6' above the floor one must have eye-protection, as the 24 elements of the MFJ-1775 are quite pokey ( and I'm 6'3" tall so I am a prime target ).

I don't know if it will make any difference or not, but I covered the entire floored space with two layers of carpet. There is an air handling unit, some duct-work, and some wiring up there, but most of it is under the floor. I would anticipate some ( hopefully ) modest RFI problems with any attic installation.

[KB5UBI]

Because you are dead set on a coax feed, I'm with NY3V's suggestion: "You can add and fold back about 7' to both ends of a 52 foot dipole to give you the required length for 40 meters, and add a 20 meter dipole to the same feed point". You should end up with a good 40/20/15 antenna. The radiation pattern on 15 meters may have additional lobes.

[W1AJO]

OK, the 254 feet of #14 wire for the 80m loop is done. Whew! It was a royal pain. Knowing what I do now, I don't think I'd build the loop if I had to do it over. Way too hot and really hard not stepping through the ceiling (thank God I didn't). The way I designed the loop I can trim back quite a bit to bring it on freq.

My plan is to try the loop and see how it does. I'm also going to use the Hy Power 20/40 dipole and see how it does as well. I've always wanted an antenna analizer so this gave me the excuse to buy the MFJ 259B. I'm also going to see how the 51' dipole I made works.

My coax switch can handle 6 antennas - so far I have 4. Might have to build something for 17, 15, 12 & 10m bands.

OK, I'm off to solder on some PL-259 connectors!

[WA4OTD]

I had a 270 foot loop in my attic and it worked well for stateside. It would load on 160M with some effort but was very narrow bandwidth.

I cut it and had 160M dipole loop and was good on 160 stateside but no DX. I had 46 states confirmed on 160 with that antenna in short order.

OK, the 254 feet of #14 wire for the 80m loop is done. Whew! It was a royal pain. Knowing what I do now, I don't think I'd build the loop if I had to do it over. Way too hot and really hard not stepping through the ceiling (thank God I didn't). The way I designed the loop I can trim back quite a bit to bring it on freq.

My plan is to try the loop and see how it does. I'm also going to use the Hy Power 20/40 dipole and see how it does as well. I've always wanted an antenna analizer so this gave me the excuse to buy the MFJ 259B. I'm also going to see how the 51' dipole I made works.

My coax switch can handle 6 antennas - so far I have 4. Might have to build something for 17, 15, 12 & 10m bands.

OK, I'm off to solder on some PL-259 connectors!

[WB2WIK]

OK, the 254 feet of #14 wire for the 80m loop is done. Whew! It was a royal pain. Knowing what I do now, I don't think I'd build the loop if I had to do it over. Way too hot and really hard not stepping through the ceiling (thank God I didn't). The way I designed the loop I can trim back quite a bit to bring it on freq.

My plan is to try the loop and see how it does. I'm also going to use the Hy Power 20/40 dipole and see how it does as well. I've always wanted an antenna analizer so this gave me the excuse to buy the MFJ 259B. I'm also going to see how the 51' dipole I made works.

My coax switch can handle 6 antennas - so far I have 4. Might have to build something for 17, 15, 12 & 10m bands.

OK, I'm off to solder on some PL-259 connectors!

Hmmm.

The first thing I do, anywhere, is install a "floor" in the attic if I ever want to use it for anything at all, even storing luggage or holiday decorations.

Takes no time and costs very little. Home Depot will cut plywood sheets per your specification for usually $1 per cut on their huge saws. I just tell them what I want, and lay down a 3/4" plywood floor all over my attic for very little cost, using 6d nails. Takes maybe 30 minutes and then I can walk around, or at least crawl around.

Next thing I do is install lighting so I can see what I'm doing.

Then, antennas.

I've installed attic floors everywhere I've ever lived (a lot of places), including rentals where I knew I wouldn't be there more than six months.

[W1AJO]

I installed 'walkways' down the middle of the attic but to get to the edges was the issue. Climbing to the top of the rafters was a chore as well. I have a light in the attic but it's in the center. I used a drop cord with a light to illuminate the area I was working in.

So what do you do with all the blown insulation?

[AE5JU]

Wire is insulated stranded 14 ga THHN.

Leave off the beads and the Jetstream center insulator, and instead use a 1:1 current balun. I like the Buxcomm B15C11.

http://www.packetradio.com/catalog/i...huvlib3fng09q3







Paul - AE5JU