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Dipole on roof

Sep 23rd 2015, 16:47

AG5AL

Joined: Sep 16th 2015, 14:25
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I bought a 100 ft. diplole and my intention is to put it on the roof of the house and attach the wires to trees. I was going to run the ladder line down the roof and enter a balun under one of the eves and then take coax into the house. I was told that the ladder line could not lay directly on the roof due to fire hazard. Can it just lay on a board or something or does it need to be offset by a foot or so? Also one of the wires will go past my chimney which has metal flashing. Can it be insulated from the metal or does it have to stand off a fot or so?
Thanks, John Lewis AG5AL
Sep 23rd 2015, 16:59

W1VT

Joined: Apr 4th 1998, 00:00
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Ladder line is low loss when it is surrounded by air. Water or a lossy dielectric will compromise the efficiency of a ladder line feeder. In theory, you could evaluate your roof material by making a capacitor, using the roof material as a dielectric, and measuring the Q of the capacitor.
You may be able to install a mast to support the feedline.

Zack Lau W1VT
ARRL Senior Lab Engineer
Sep 23rd 2015, 21:10

WA0CBW

Joined: Apr 4th 1998, 00:00
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I have found that keeping the ladder line at least 4 times the width of the ladder from all materials (especially metal sheathing) is usually sufficient to keep them from interacting with each other. Stand-offs are available or you can make them out of plastic pipe sold in most hardware stores.
Bill
Sep 23rd 2015, 22:32

AG5AL

Joined: Sep 16th 2015, 14:25
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Thank you very much. That is what I need to know.
John
Oct 20th 2015, 20:03

AG5AL

Joined: Sep 16th 2015, 14:25
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Total Posts: 0
Have another question about my antenna. Brought the ladder line to a balun placed under the eaves, then out of the balun to the radio. I have not grounded the balun yet, if that makes any difference. I am using a Yeasu 857D all band radio and also have the Yaesu FC-30 antenna tuner attached. I just got everything set up and turned on this weekend. I have a 100 foot dipole that I got from MFJ, model MFJ 1777. . When I attempt to tune, the only band that will tune up is the 20meter, which I realize is the ideal band for that size antenna. All the other bands either say they are out of band for tuning, or have a HSWR. As is often the case in these things, you ask 4 experts and you will get at least 5 answers, two or three of which are in direct contradiction to each other. I called MFJ and the technician there said that he had been in HAM radio since 1960 and had never gotten a balun to work, (thought that was a little odd since I bought an MFJ balun) that he just went directly from the ladder line to the coax. I asked him how you do that and he said, "You just do." That's great for a Nike slogan but did not help me much with my antenna. I then called Yaseu and the technician there told me I needed to get rid of the dipole, was wasting my time, was not a good antennae, was poorly resonant etc. He gave me the number of a couple of antenna companies to check with. That seemed odd since I have been reading about dipole, G5RV (or whatever they are) antennas a bunch and everybody sells them. I would prefer not to get rid of my dipole. Did not cost much but I have invested a lot of time in getting it on top of my house, fabricated stand offs to keep it off the roof, put on my climbing spurs and ascended two trees up near the top to install my pulleys. Would very much like to get my dipole to work. I would appreciate any advice.

John Lewis
AG5AL
Oct 21st 2015, 02:38

WA0CBW

Joined: Apr 4th 1998, 00:00
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That antenna is basically a random length dipole fed with ladder line and requires a wide range balanced tuner to be able to tune all bands. The length of ladder line and length of coax will determine what bands you will be able to tune with the FC-30 tuner. You didn't say what the ratio of the balun was or where it is in relationship to the antenna and coax so it may be that the SWR is out of range of the capabilities of the tuner.
BB
Oct 21st 2015, 16:56

AG5AL

Joined: Sep 16th 2015, 14:25
Total Topics: 0
Total Posts: 0
The balun is 4:1. There is about 40 feet of ladder line going to the balun and then 50 feet of coax going to the radio. What are my option?
Nov 9th 2015, 19:36

AG5AL

Joined: Sep 16th 2015, 14:25
Total Topics: 0
Total Posts: 0
More questions: I still cannot get my dipole to tune on several of the bands. I have tried lengthening the ladder line, shortening the ladder line, having the 4:1 MFJ balun, not using the 4:1 balun and going directly from the ladder line to the coax. Will not tune on 160, 80, 40, 30. 17. Best setup I have found so far is probably about 60 feet of ladder line, in the balun and 50 feet of coax to the tuner, which is a Yeasu FC-30 automatic tuner and I have a Yeasu 857D radio. The wire is 100 feet long. That will tune the 20, 10, and 6 almost all the time, Will tune the 60 and 15 some of the time. Never has tuned the 160, 80, 40 30 or the 17. Is my next step to try to do away with the ladder line all together and go directly from the wire to the coax, or do I need a more powerful tuner. Of course it seems the band that I hear the best is the 80. I would appreciate any suggestions.

John AG5AL
Nov 9th 2015, 20:50

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The FC-30 tuner is only designed to handle a 16.5 to 150 ohm impedance range--3:1 SWR. A better tuner is your best option if you need to cover more bands and don't want to change the antenna. Direct wiring coax only works if the SWR is low--if the SWR is high and you are using 100 ft of coax, signals will get weak.
An increasingly popular option is to purchase an autotuner that can be mounted outside and placed between the ladderline/balun and the coax. But, you can run into situations in which the balun will overheat and the SWR will drift--which will cause the autotuner to re-tune. You may find the antenna effectively unusable on that band as the tuner constantly re-tunes.

http://w5dxp.com/ZS6BKW80/ZS6BKW80.HTM
The ZS6BKW antenna will cover 40, 20, 17, and 10 meters with a limited range tuner.

Zack W1VT

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