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Author Topic: Ridge Vent question  (Read 5082 times)
KD8TZC
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Posts: 67




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« on: May 30, 2013, 09:42:39 AM »

I live in a community where there is a grey line on outside antenna's.  It doesn't matter though as the XYL said she doesn't want to see anything so placing a typical dipole outside is not really an option.

One hair brained idea that I have come up with though to try and get better results than what I am getting with my attic mounted dipole is to route the dipole wires out the ridge vent on the peak of my house and either run them along the ridge vent, or make it a fan dipole and add a second band to my antenna and make it a sort of inverted V that runs at a 45 degree angle to the ridge.  In both instances, I would need to fasten the wire to the shingles (asphalt) somehow to keep them from flying around.

Has anyone tried this or can anyone comment on any issues I might have?  I have a DX Engineering 1:1 Balun that I am looking to mount right up (or as close to) in the ridge vent, and then just connect the wires and run them out the ridge vent opening.  I don't know if this will help isolate me from the wires from electrical connections and other things in the attic or not.  I would think it should improve things somewhat.

John - KD8TZC
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John - KD8TZC
W0FM
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Posts: 2055




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« Reply #1 on: May 30, 2013, 01:23:32 PM »

Hi John,

I did something similar to what you are describing at a condo I once owned.  I ran a dipole along the length of the ridge of the roof.  I had no ridge vent, so I simply strung the dipole along the length of the ridge a couple inches to the back side of the peak to keep in out of sight from the front.  I used dollops of colored (black to match my roofing shingles) silicone to secure the element wires to the asphalt singles every couple of feet.  I ran the coax down the back face of the roof, along a chimney and to the ground, where it connected to a lightning arrestor and eventually into the basement shack.

It worked ok, and was all but invisible.  My next idea was to use wooden spools or dowel rod to support the wire elements an inch or two off the roof, but I never got around to it.  I'm not sure that you'll see a big difference between your attic dipole and one laying on the roof, but it's worth a shot.

The dime size dabs of colored silicone I used to secure the wire elements to the roof proved to last a long time through all kinds of weather.  The condo cops never knew it was there.

73,

Terry, WØFM
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KD8TZC
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Posts: 67




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« Reply #2 on: May 30, 2013, 01:42:44 PM »

Thanks Terry.  One of the things I was also looking to do is to put a 40 meter 1/2 wave dipole up there.  Since my ridgeline is not long enough, one thing I was thinking was making V where one side of the dipole goes on the West side of the ridge and the other goes on the East side.  My question though is how far apart do these wires need to be so they don't interfere with each other (or will they since they are part of the same dipole)?  If I can keep them fairly close to the ridgeline, they east one shouldn't be too visible from the street unless you were really looking for it.

John - KD8TZC
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John - KD8TZC
W0FM
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Posts: 2055




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« Reply #3 on: May 30, 2013, 02:34:24 PM »

Unfortunately, John, the answer is "it depends".  I ran the longest elements of my dipole the length of the ridge line then 90 degrees downward along the opposite edges of the roof (upside down "U").  You can bend and zig zag a dipoles elements usually with little interaction unless you fold them back on themselves.  The ARRL Antenna Handbook has some drawings of various ways you can eat up the length of a dipole in a restricted space.  I have two fan-dipoles (actually, parallel dipoles) in my attic and their parallel elements are less than a foot apart.  Took some fiddling but both antennas (10-15-20 & 40M and a 30-17 & 12M) do the job.  Do they equal the same antenna erected out in the open?  No.  But they beat a sharp stick in the eye.

I still get enjoyment out of experimenting with less-than-optimum antennas.  Have fun!

Terry, WØFM
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WX7G
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Posts: 6039




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« Reply #4 on: May 30, 2013, 03:07:58 PM »

Configured as a VEE the dipole wires "interact" approximately as:

Z2 = Z1[COS(A)]^2, where Z1 is the straight dipole radiation resistance, Z2 is the Vee radiation resistance, and A is the angle between the dipole wires.

For example, in free space Z1 is 72 ohms. Given an angle of 90 degrees the formula gives a radiation resistance of 36 ohms.

Why? Because the effective current-area of the antenna has been reduced to 0.71X of the straight length and radiation resistance is proportional to current-area squared. Note this is approximately valid for an antenna one-half wavelength or shorter. Why "approximately?" It is exact for antennas having a triangular current distribution, such as a short dipole, but not exact for a 1/2 wavelength dipole where the current distribution is close to a sine function.

The VSWR bandwidth, normalized to the resonant impedance of the antenna, is reduced to approximately [COS(A)]^2 that of the straight dipole. In the example above the VSWR bandwidth is approximately one-half that of a straight dipole.

For your hidden VEE dipole you can achieve all band performance by feeding it with TV Twin Lead or ladder line and using a manual tuner. You then do not have to worry too much about all of the above. This antenna will be effective from about 2/3 the design frequency (where it is 1/2 wavelength) on up.
« Last Edit: May 30, 2013, 03:16:36 PM by WX7G » Logged
W0FM
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Posts: 2055




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« Reply #5 on: May 30, 2013, 04:49:11 PM »

Yeah.  What Dave said.  Exactly.

Terry, WØFM
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WN2C
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Posts: 447




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« Reply #6 on: May 30, 2013, 08:02:16 PM »

My wife told me that I couldn't put up a tower in the back yard.  So what did I do?  I put up a tower in the back yard! I told her that I own this house too and that I don't tell her how to furnish it or decorate the inside, don't tell me how to decorate the outside. 

Rick  wn2c
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K1CJS
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Posts: 6034




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« Reply #7 on: June 02, 2013, 07:56:50 AM »

Not exactly on what was asked, but you may want to check the ridge vent to make sure there isn't any metal flashing or strengtheners running near where you're going to be running your wires.  They could well make your whole project an exercise in futility.  73!
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W0FM
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Posts: 2055




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« Reply #8 on: June 03, 2013, 07:33:14 AM »

Chris' post reminded me of an article I read years ago.  If your ridge vent is plastic or fiberglass see if it can be replaced with a metal vent (aluminum).  This ham had an aluminum ridge vent, and he cut in in half replacing about 2" in the middle with a piece of plastic ridge vent making a "center insulator".  He sealed up the patch in the center and fed the two aluminum sides of the vent as a dipole.

I had done something similar to that once with a long run of aluminum guttering along the back of my house.  It was a simple project and didn't work bad at all.

Good luck.

Terry, WØFM
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K4FMH
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Posts: 254




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« Reply #9 on: June 18, 2013, 11:13:34 AM »

My wife told me that I couldn't put up a tower in the back yard.  So what did I do?  I put up a tower in the back yard! I told her that I own this house too and that I don't tell her how to furnish it or decorate the inside, don't tell me how to decorate the outside. 

Rick  wn2c

Happy Father's Day, Rick! Wink
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N1UK
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Posts: 1436




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« Reply #10 on: June 18, 2013, 03:12:25 PM »

Quote
My wife told me that I couldn't put up a tower in the back yard.  So what did I do?  I put up a tower in the back yard! I told her that I own this house too and that I don't tell her how to furnish it or decorate the inside, don't tell me how to decorate the outside.

Well done Rick..finally someone who isn't afraid to do what he wants.

Mark N1UK
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