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Author Topic: Cable Lines in Attic  (Read 2444 times)
KJ4WBT
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« on: November 30, 2014, 11:02:39 PM »

I'm yet another ham about to enter the HF world who has to deal with antenna restrictions in a condo. I currently live on the second floor of a two story condo building. I have attic access and am thinking of putting an Alpha Delta DX-EE up there. Attic is wood construction with no metal fixtures. The antenna would run east to west and would fit perfectly at 40 ft (attic is 45ft). I want something that will give me at least a few bands to work with (20,15,and 10 without tuner) as I learn. My concern is all of my buildings cable lines run across my attic space before being distributed to each unit. I've been operating on 2m and 70cm with a diamond vertical up there at 100w and haven't had any RFI issues thus far other than my pc speakers buzzing on transmit. I can hit just about any repeater in the area and beyond with great clarity Rx and tx.  I've had no complaints from neighbors either. Do any of you guys forsee a major issue with this setup? And is this a decent route to take for an HF beginner? I also started a post earlier in station building a few days ago and those guys have given some great responses that have got me thinking. Figured someone in this thread may be able to comment on expected RFI and attic installations in general. I'm considering the Yaesu FT 857D, as it fits my current price point and offers what I'm looking for.
« Last Edit: November 30, 2014, 11:05:56 PM by KJ4WBT » Logged
K5LXP
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« Reply #1 on: December 01, 2014, 08:04:03 AM »

I'm not so sure I would settle on a particular brand/model of antenna knowing that in the environment you plan on putting it in, it likely will not tune correctly.  Attic antennas can certainly "work" and sometimes work OK compared to outdoor antennas but you can plan on the structure having an influence on it.  In terms of efficiency and pattern your result will be arbitrary, so no point in going through any extra effort to install a specific antenna.  A basic doublet or loop as long as you can get it, fed with balanced line and a tuner will be adjustable to suit the environment and still be an efficient radiator.  I'd start with that and see what you get in terms of RFI ingress/egress and judge overall performance before I'd engineer a specific solution to your exact scenario. 


Mark K5LXP
Albuquerque, NM
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KJ4WBT
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« Reply #2 on: December 01, 2014, 11:19:13 AM »

Until I learn a little more about antennas in general, I'd be more comfortable buying an off the shelf product. Is there something you could recommend as an option? For instance, MFJ has a 20-6 m dipole for $29. Would this do ok to start me out? Due to my work hours I'm not able to contact local hams very often for an elmers help. I consider suggestions from you guys as my mentorship at the moment. Thanks in advance for any help you could give.
« Last Edit: December 01, 2014, 12:01:39 PM by KJ4WBT » Logged
AA4PB
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« Reply #3 on: December 01, 2014, 05:38:12 PM »

The MFJ dipole is a single band standard dipole (exactly like something you would build). It comes with enough wire to cut it for 20M or 15M or 10M or 6M.

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KJ4WBT
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« Reply #4 on: December 01, 2014, 06:22:27 PM »

Ohh gotcha...learning something new with every post. Any ideas on something like a fan dipole? Would be nice to have at least 3 bands on my first antenna to mess around with. I would build but just can't make it happen in the time I have with work schedule right now. I'm the kind of person that will spend weeks just studying up on antennas before I'd attempt it, and that just won't jive with my wife wanting me to enjoy being on the air over my Christmas break. She's even agreed to take her tech test on Dec 13th!  I'm starting to think that she's caught the ham bug from me and is just as interested in long distance contacts as I am.
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N7EKU
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« Reply #5 on: December 02, 2014, 12:04:56 AM »

Hi OM KJ4WBT,

Where to start...

OK, if you buy a commercial antenna, that is no guarantee that it is going to work in your attic.  For sure it will need some tuning.  Also, commercial antennas are usually built for strength in the wind and UV durability -- you don't need that in your attic, so you will be paying for stuff you don't need at this point.  Also, what if you trim your new commercial antenna too short and have to start splicing etc?  That won't feel great.

If I was going to buy anything, it would be a balun.  If you want to coax feed your attic antenna, you will for sure want a 1:1 balun at the center of the antenna in order to keep your coax shield from radiating and causing trouble for other people.  If you want to feed your antenna with balanced line, then you will want it where your feedline goes into your tuner.

Building a dipole is so easy, and when you make a contact on an antenna your made yourself that is really worth it.  Plus, I'm convinced you won't save any time buying one anyway.

How easy?  For a coax fed dipole, take ANY copper wire (for the attic there are no UV or wind/weather worries) and cut two lengths to 234'/band frequency (so e.g. 234'/14MHz or 234'/21MHz).  This will give you excess length for trimming later.  Attach to your center balun, tie up the ends to the rafters with insulators, hook up your coax, run it to your rig or SWR meter/rig and check the SWR.  Trim as needed (or you can just wrap the extra back on itself) to get lowest SWR.  If you get the wire, insulators, and balun, the whole process need not take any longer than an hour (not counting time to route the coax -- don't know how hard that is in your case...)

Get one going, then if you want a fan, add another with the wire stretched at a different angle, or hung at least a foot lower.  Check and trim.

For a balanced line feed, you will need a tuner, but construction of the antenna is even easier as you just make it as long as you can.  You can even bend around 1/4 of their total length around corners zigzag style if you want to get more length.

As a note, 3/8 wavelength is the shortest you typically want to get for a non resonant dipole length on a band (so 40M band need arms at least 234'/7MHz * 0.75 shortening factor = 25').  With this length on a balanced feeder, you can tune it to all bands lower than 40M.  Then only caveat on this type is the feedline length may need to be adjusted so the antenna can be tuned on all the bands you want (there are websites around to check for this like:  http://www.vk5ajl.com/info/G5RV.php).

73 and good antenna building!


Mark/n7eku



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WX7G
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« Reply #6 on: December 02, 2014, 05:10:17 AM »

You want an off-the-shelf antenna that covers 20-10 meters and will work the first time in an attic. The MFJ-1777 ($60) is a dipole fed with ladder line that can be shortened to fit in your attic. An antenna tuner is required and MFJ has many that will work. The MFJ-901B at $99 is suitable.

This antenna trimmer to 40' will work well on 30-10 meters and will tune and work, but not as well, on 40 meters. If you can angle the ends at a 90 deg angle (up, down, to the side) and make the antenna 50' long it should work well on 40-10 meters.

Ham Radio Outlet has both items in stock.
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AA4PB
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« Reply #7 on: December 02, 2014, 06:35:14 AM »

Ladder line feed is probably not very good for an attic antenna because of the difficulty of running the line into the apartment and the likelihood of the ladder line radiating RF if it is not kept perfectly balanced. Everything has its trade-offs. Personally, I'd stick with coax and a balun at the antenna feed point for most attic installations.
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W8JX
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« Reply #8 on: December 02, 2014, 10:45:36 AM »

About 20 years ago a friend of mine started into ham radio as a novice and QRP when he lived in a apartment with attic access. He only did CW and he used a G5RV routed dog legged around attic and a low power tuner. It worked out quite well for him and low power meant no TVI complaints. A year later he got his 20 wpm extra. He has long since bought a house but still prefers CW when he does get on.
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You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
N7EKU
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« Reply #9 on: December 02, 2014, 11:51:00 AM »

For sure AA4PB,

If you want to run balanced feedline, you need to think about it radiating.  A 1:1 current balun at the entrance to your rig or antenna tuner should take care of that though.

However also of importance is that you don't want to run the line near any metal conductor -- it needs to stay at least a couple of inches away.  Like you don't want your feedline resting on someone else's coax line!  So KJ4WBT, you will want to check out the route to the attic if you want to think about running a balanced feedline.

The last MFJ antenna mentioned would be a good one, except you will have to chop 1/2 of it off and you also have 100' of feedline (which may or may not be useful).  But you still need to purchase or make a 1:1 current choke balun if you want to do that.

If I had a clear shot to the attic for balanced feeder, I would just get some 300 ohm TV twin feed since that works great indoors and is smaller and easier to deal with.

No balanced feedline dipole is certain to "work first time" on a particular band unless you know that you won't end up with a current node there at your rig or antenna tuner.  This presents a very high impedance that tuners cannot work with.  This is easily calculated though (check the reference I gave above).

73 and cheers everyone!



Mark.





« Last Edit: December 02, 2014, 12:24:34 PM by N7EKU » Logged
KJ4WBT
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« Reply #10 on: December 02, 2014, 03:55:05 PM »

Thanks for the input regarding the cable lines. They appear to only be near where the antenna will go at one end. They run diagonal through the attic coming in through the attic vent at the peak (have no idea why they installed this way). If I'm not having trouble with 100w on 2m, is there potential that the cable lines being near the end of the antenna causing a problem? (Not just for the radio but for neighbors cable feed). Also, how far away should I make sure I try and keep the antenna away from the lines at a minimum?
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WB2LQF
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« Reply #11 on: December 07, 2014, 09:22:58 PM »

Let me just tell you what works for me.  I put a 44 foot end fed Zepp in my attic, fed with about 14 feet of 450 ohm window line to an SGC237 auto-coupler.  From there, it's a clean 50 ohm coax run to my rig. This antenna is also known as a "Cebik 44' doublet."  With this antenna I can work 40 meters through 10 meters, including the WARC bands.  In a "pinch" I can work 80 meters where the efficiency is much less.  I live in a two story townhouse.  My antenna wire is #14 copper, 22' on each side of the center insulator.  The window line comes straight down through a slit in the ceiling and into a linen closet where it's attached to the auto-coupler which sits on a shelf near the ceiling. The coax cable runs to my second floor shack on the other side of the wall. With this setup I have instant QSY and automatic RF-initiated tuning. I have no RFI issues.  I can run up to 100 watts but I generally operate QRP or QRPp.  This is a balanced antenna so a "ground" is not at issue.  The feed line probably radiates but who cares? Radiation is good in this case and it's vertical to boot.  I work all over the world with 5 watts on this antenna. I have used resonant dipoles and I once owned the DX-EE forty footer.  But this current antenna has served me reliably since 2011 and gives me the one thing I value the most:  the ability to instantly QSY.  So this is what works for me and I'm sure you will eventually arrive at your own best solution to an increasing challenge for hams today.
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AI8O
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« Reply #12 on: December 08, 2014, 12:44:59 PM »

 I put my HF antenna in the attic under my roof.

One problem I ran into was sudden very deep reductions in the antennas performance.

It took several "repair" attempts before I realized that the reductions in performance occurred during and right after rainstorms.

Evidently the rain water was mixing with dust, covering the shingles, and shielding the antenna.
As soon as the roof dried out; the antenna was performing as before.
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Semper Ubi, Sub Ubi!
KJ4WBT
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« Reply #13 on: December 08, 2014, 01:18:41 PM »

Glad you mentioned that as I'm sure there is a good chance I may encounter it. That will save me a lot of head scratching. So far have selected the 857D with a YT-100 so I can setup a backpacking station with a wire antenna as well as use it in the shack. I'll be installing a dipole for 20 and 10m with a 1:1 balun to prevent RFI from the feedline since I have neighbors real close. Yalls comments have really helped me wrap my head around getting started in HF. It's a decent leap from just 2m 70cm.
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