"Best" Multiband Wire for Flat Roof (51'x24')

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WZ6K, Aug 3, 2017.

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  1. WZ6K

    WZ6K Premium Subscriber QRZ Page

    I'm a newly relicensed General class operator (after almost 40 years away), now living in the Sunset District of San Fransisco. I have a flat roo with general dimensions of 51' x 24'). Almost no obstructions if I go long corner to long corner, with a mostl perpendicular feed line. Very wIlling to do a ladder feed, (if I can understand why and how to terminate it to my 2nd story "shack" Prefer coaxial cable termination for convenience. How long a run? Maybe 35-45'.
    The roof construction is basic stud-tar-bitumen comp-gravel. I'm considering a G5RV Jr. (pre-made) or one of the Doublets. Obviously I'd like as much wire/band-bandwidth as possible, but I'll get one day on the roof with help, and I'd like the best outcome. NEVER more than 100W, 756PRO, Tuner. I'd like 20M/40 M and whaterver more "I can get "easily." BPSK-31 and SSB.
    Can I lay the wire directly on the roof? If I raised it 6-12" would if be worth it?
    Simple low cost "tweaks" suggestions are welcome. No, I'm not likely to make it myself.
    I do get that almost every antenna has "compromises", I'd be greatly appreciative of kind/pleasant suggestions. I firmly acknowledge in advance that I'm a technical "doofus" semi-old, grumpy and sometimes lazy. Simplicity, cost, ease of placement with best hopes that I won't need to go back on my roof frequently are also major considerations.
    Your kind guidance, suggestions and assistance are welcome and appreciated! Kind 73, Rich
     
    Last edited: Aug 3, 2017
  2. KK4YWN

    KK4YWN Ham Member QRZ Page

    how high up? i think i'd consider a pole on every corner and one big giant loop fed with ladder line.
     
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  3. KC8VWM

    KC8VWM Ham Member QRZ Page

    Laying a wire flat on the roof means there may be interaction with 1,000's of roofing nails and sheet metal work as used along the roof edges .

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    It also means your antenna will pick up all kinds of electronics noise operating from within you house including the can opener.
     
    Last edited: Aug 4, 2017
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  4. N4UM

    N4UM Ham Member QRZ Page

     
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  5. N4UM

    N4UM Ham Member QRZ Page

    I'd go with a Hustler vertical (a 6-BTV) (without the 80 meter resonator) mounted in the exact center of the roof . Each of four 40 meter radials would run to a corner and bend along the edges for about 6 feet. The other sets of radials for 30, 20, 15 and 10 meters would fit easily without requiring bends. Use 4 radials per band. You can use #14 THNN house wire for radials from Lowes or Home Depot. You can hold the radials in position with bricks at their ends while waiting for a few dabs of roofing tar to set up and keep them positioned. Guy the antenna to the 4 corners of the roof using 100 KG. test braided dacron fishing line from Bass Pro Shops. Get yourself a piece of pressure treated 2x12 lumber about a foot long and mount it on the roof with a bed of tar. Then get a hinged bracket to fasten the antenna to the lumber so that you can easily lay it over for adjustment. The Hustler vertical provides a lot of band per buck and can easily be fed with coax. It'll get you on 5 bands with a decent low angle of radiation for DXing.
     
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  6. AK5B

    AK5B Ham Member QRZ Page

    Good, sound suggestions so far and here's one more: IF you can put a couple of masts on either long corner you might be able to make or install a 40-20M fan dipole with the far ends of the 66' portion (40M) hanging down on either side. The main current flowers closer to the center so the ends won't matter as much if they'e hanging down some (think stretched-out inverted U). The hanging ends should clear the ends of the 20M dipole with enough room to avoid much, if any interaction, too. Those two bands are a good way to go at this stage in Cycle 24, too. No need for balanced line/tuner with a fan unless you want to use it on other bands---coax will be fine and easier to route among the roof metal, ducting and so on.

    Side note; I once lived at 1418 La Playa out there across from Ocean Beach----way back in 1970. If you have to live in San Francisco that's a good place to be as any. Used to try to work VK portable on 6M parked out there decades later when I was first licensed. Riding the Judah streetcar all the way down to the downtown terminal was fun, too. How time flies.

    Get whatever you put up as high as you can away from the building; even an elevated vertical will be much better a foot or three above the roof.

    Flat roofs are a godsend for us hams that live in the city, too.

    73 and aloha from the South Coast,


    Jeff
     
    Last edited: Aug 4, 2017
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  7. KW4TI

    KW4TI Ham Member QRZ Page

    The usual advice, get it higher and longer, is useful. A vertical is probably ok but may not be accepted by the neighbors. The other disadvantage of the vertical is that it may be more susceptible to noise in a very high noise environment. A full wave loop would probably work well, would be about 135 feet long at 40 meters, and might be less susceptible to domestic noise, which there is going to be plenty of, and doesn't require radials or a ground. This possibility is worth investigating.

    If this doesn't work, you can consider a 40 m off-center fed dipole. This should cover 40 m, 20 m, 15 m. and 10 m bands, and is 66 feet long, so it should fit diagonally on your roof. If you buy one, make sure it comes with a good dual-core 4:1 Guanella balun. Don't accept air core, buy one with two 43 or 61 ferrite cores in it if you can, because a good balun is a must for an off-center fed-dipole to work. The balun makes or breaks this design, so get one from Balun Designs or DX Engineering. For example, here's a OCFD that is 21 m long, with 4.2 m from one end and 16.8 m from the other end:

    upload_2017-8-4_10-31-51.png

    It's also certain that because of your complicated environment, you will have to do some tweaking to account for the roof and whatever else is up there. So see if you can borrow an antenna analyzer when you go up on the roof.

    73,
    Dan
    KW4TI
     
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  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I definitely would not lay it on the roof. 6"-12" won't gain you much, either.

    You really need a 10' (preferably taller) mast at each end to support it as high above the roof as possible, whatever it is!
     
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  9. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    If you can put a mast in the center, you could do an inverted V fan dipole, fed with coax.

    N8CMQ Jeff
     
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  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Good idea, and often one mast can be "taller" than two masts would be.

    Taller is better, when it gets you higher above the roof and all the noise generated in the house.
     
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