HF/VHF/UHF attic antenna options for 30'x25' attic?

Discussion in 'General Technical Questions and Answers' started by Pushraft, Oct 23, 2008.

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  1. W0IS

    W0IS Ham Member QRZ Page

    You'll actually make very few contacts of 100 miles. The best band for that distance would be 80 meters, and unfortunately, you won't have room for a very good 80 meter antenna.

    Your best bet is 40 and 20 meters. The average distance of your contacts will be about 500 miles. Of course, you'll make some with close-in stations. But between about 30 and 200 miles, you will have very few, because that approximate distance will be within your "skip zone".
     
  2. Pushraft

    Pushraft Banned

    I was back up in the attic checking out where I would put a possible antenna and there are a lot of diagonal wood support beams in the way up there making movement difficult. There is also 10" or so of "snowlike" insulation so I have to be careful where I step. I'm not saying it is impossible to get a half decent antenna working up there I am just saying I have many hurdles to clear.

    The UHF beam antenna I have up there seems to be doing well so that is a good sign. That tells me the wood paneling and the roof shingles are not "killing" the UHF signal so I would suspect they would be even more "transparant" at VHF and especially at HF. I could try a small CB antenna (27 Mhz) up there to test how HF gets out.

    Question: if I just picked a certain band such as 17m or whatever the longest dipole I can fit up there is, will that sucker tune to adjacent bands such as 20m and 15m using the Yaesu's built in 3:1 SWR max automatic antenna tuner? I think you set it once with that model and then when you re-enter that band, it automatically adjusts (from memory) the antenna tuner to lowest SWR for that band.

    Another question: Does the dipole antenna have to be flat on 1 wall of the attic only or can it bend at 90 degrees and continue on another wall if I need more length? My south facing wall is about 30 feet long so I think that will accomodate a 17m dipole which may tune to 20, 17, 15, and maybe even a few more bands.

    If I get my ticket, I would like to start simple and just try a monoband dipole for maybe 17m. Should I use thick wire to get more bandwidth? Starting simple is good because that should minimize problems and give me a base reference to improve from.
     
  3. W0IS

    W0IS Ham Member QRZ Page

    You can absolutely bend it. As I pointed out above, if Pythagorus was right, then if you can get to the peak at one side and the corners on the other side, you can fit a full-size 40 meter dipole.

    The thickness of the wire will have a negligible effect on bandwidth. It certainly won't make the bandwidth wide enough to get 20, 17, and 15 out of the same element.

    As I pointed out above, the 40 meter dipole that will fit will work well on 15 and 6. The SWR will be well above 3:1 on any other band. If you really want to start with a single band, then I would go with 40. My second choice would be 20. But having both would be quite simple.

    With some luck, you could probably install the whole thing from the entry with a long pole. You don't have to worry about it blowing down in the wind, so as long as you get it hanging up in the right spot, you should be good to go.
     
  4. Pushraft

    Pushraft Banned

    I would like to get a cheap rig that can handle at least the bands you are recommending and get at least a simple dipole antenna up there to see if it tunes up on the band it is cut for without too many issues. I cleared out some "snow" insulation so I can now easily see where to step when I walk up there. There are already a few nails sticking out of the woodboard but how would you recommend that I hang the dipole?

    I can actually use the Yaesu FT-767GX for testing until I get my own rig. A friend I am selling it for wants to get the tuning knob fixed at a local amateur radio shop and then resell the rig.

    If they tell him $100 to fix it I may make him an offer for that much less off the normal selling price of those which is about $450 in good working condition unless I can find a similar one cheaper elsewhere. He is concerned because he paid several thousand dollars back when he purchased it which must have been in the late 1980s.

    Another question I had is how do I Earth ground the transceiver properly? The spot where I will be operating the radio is not near any Earth ground.

    If I bend my dipole on my S and W facing walls in a V shape, what will that do to the pattern? If I "cheat" and use both walls, I will get 30+25 feet = 55 feet total so that should be long enough for a 30m dipole.

    My N facing wall is hard to get to with little height clearance so I would rather not hang any part of the antenna there if possible.
     
  5. W0IS

    W0IS Ham Member QRZ Page

    Since you don't have to worry about the wind blowing it down, it's not critical in the least. I would just tie the center insulator to a convenient nail using string or rope, and then run the wires from there. Tie them down at the end with more string or rope. In fact, you really have no need for a center insulator. Tie the coax to the string or rope, and just attach the wires to the two wires in the coax.

    With a dipole, no RF ground is necessary. All you will need is the ordinary ground provided by a 3-prong plug. If an RF ground is necessary for some other type of antenna, this can often be accomplished with quarter-wave wire(s) for the bands you will be using.


    The exact pattern of a dipole is unimportant, and is difficult to predict in cases like this. If you need to bend the antenna to fit it in the available space, this will have little effect.

    The dipole will have some directional characteristics, but they are not very pronounced. Chances are, you won't even notice them.
     
  6. Pushraft

    Pushraft Banned

    I read up on the Yaesu FT-767GX all mode transceiver and although it got great reviews mostly, one warning was HF RF hitting the chassis could possibly whack out the proper functioning of the rig (control logic). I was concerned because my "could be" ham shack will be below the attic dipole and dont want any issues like that. There is a ground screw on the back of the transceiver and I thought that was supposed to be connected to Earth ground to help that situation.

    Unfortunately when the rig is also up high away from Earth ground, I suspect it is difficult to get a good Earth ground without that wire acting as an antenna itself.

    If I can erect a simple dipole for the lowest band that will work with my attic size constraints and I could use the built in tuner for a few adjacent bands and all of them not adversely affecting the radio due to RF hitting the chassis in the shack, then I might have something reliable worth playing around with.

    I was just curious if I could use the ground pin on the 117V wall outlet as a chassis ground for HF to bleed off static buildup or any stray RF hitting the chassis.

    I dont want problems like this and even worse, a possibility I will get shocked. At 100W, that is enough voltage (71V I think) to get a jolt.
     
  7. WA4ILH

    WA4ILH Subscriber QRZ Page

    An RF ground is certainly desirable but not necessarily required if you are feeding a resonant antenna with a low SWR. Especially if your antenna is inside the attic. A Safety ground (for AC power), on the other hand is required and yes, you can use the eletcrical (3rd wire pin) for your ground. Doesn't your power cord have a ground wire in it?
    Tom WA4ILH
     
  8. W0IS

    W0IS Ham Member QRZ Page

    Since you do insist on using a single dipole on adjacent bands, you will need a better tuner and a better ground than you have. The SWR will be considerably higher than 3:1.
     
  9. Pushraft

    Pushraft Banned

    Correct. I have nothing to add to the hobby. It is way too technical and to be really even remotely good at it, one needs a degree in Electrical Engineering or the equivalent knowledge of electronics. Most "normal" people dont even know what a capacitor is and have probably never heard of the electrical term reactance. Most people probably dont know how an antenna works and probably dont want to know because many people think they are "ugly" and obstrusive.

    CB, FRS, Wifi 2.4, and other license free bands give me enough to play around with without having to know how to work 50 buttons on a transceiver. There is still lots I have to do with those yet.

    Also, since I have an attic antenna constraint, the performance will probably be bad compard to people with giant beam antennas. Even if I made a few contacts it would be mostly because of the big antenna on the OTHER end, not on my end. I would probably get laughed at when I told them I had an attic dipole. They would probably send me a card and some food stamps too.

    It seems like there are too many obstacles for me to do this now. Oh well, there are always models trains.. oh but wait, dont those use electricity too? Damn!
     
  10. KA9VQF

    KA9VQF Ham Member QRZ Page

    Man, just do it.

    Take the test,

    Pass it,

    Put some wire somewhere, preferable in the air,

    Get on the air,

    Make contacts,

    Or just sit there asking advice that you have no intention of following with your over analysis of things.

    Your call.

    If you ‘get off’ with your limited, license free, operating fine, no one on the ham bands will really miss you.

    If however you do as I suggest above you might be able to actually find someone, somewhere in the world, with similar interests as you have and be able to talk to them about it.
     
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