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Roof mount or Chimney mount yagi?

Discussion in 'General Technical Questions and Answers' started by K3STX, Jun 23, 2004.

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  1. K3STX

    K3STX Ham Member QRZ Page

    I am thinking (at least to myself) about getting a small mini-beam and putting it on my roof. Now I know a mini-beam will give me mini-performance, but a) I would like to keep a relatively low profile (just moved into this neighborhood) and b) am interested in learning about beam headings/propagation & long/shortpath openings.

    If I got a small mini-beam, like a Cushcraft MA5B (7 foot boom, 17 foot elements), should I put it on my roof using a tripod (like a Glen Martin) or mount the mast to my chimney. The chimney would be easier (and cheaper), but I don't want to destroy my chimney. THAT would be a problem.

    Anyone with experience/advice with roof mounting yagis? A tower is out for now.

    paul
     
  2. AI4IJ

    AI4IJ Ham Member QRZ Page

    Of what is the chimney made? If it's brick, then I wouldn't think there would be any problem. I would, however, think that mounting a tripod on the roof would introduce the possibility of roof leaks.

    73
    Richard
    KI$DOK
     
  3. AI4IJ

    AI4IJ Ham Member QRZ Page

    Of what is the chimney made? If it's brick, then I wouldn't think there would be any problem. I would, however, think that mounting a tripod on the roof would introduce the possibility of roof leaks.

    73
    Richard
    KI4DOK
     
  4. K9STH

    K9STH Ham Member QRZ Page

    If your chimney is made like mine, it is going to be here a long time after the entire neighborhood is destroyed by an asteroid collision!

    However, if your chimney is used (for a fireplace, water heater, furnace, etc.), then the smoke and fumes are going to greatly hasten the demise of your antenna! Aluminum doesn't like the products of combustion very much. However, the products of combustion definitely like aluminum! That is, they like to "eat" it!

    As such, it is my opinion that a chimney mount should not be used for any long term installation.

    Glen, K9STH
     
  5. K3STX

    K3STX Ham Member QRZ Page

    Brick chimney, we do not burn wood in the fireplace, however we do have a furnace! While I am SOMEWHAT interested in the lifetime of the antenna (a couple hundred bucks), I am far more concerned about the lifetime of the chimney (a couple thousand bucks). I read WIK's comments against chimney mounts once, mentioning if you want a pile of bricks instead of a chimney go for it!

    paul
     
  6. AG3Y

    AG3Y Ham Member QRZ Page

    I've mounted several tripods on several roofs, and never had a leak as a result. However, I would NOT put anthing up on a chimney larger than a good sized long-distance TV antenna or multi-element 2 meter beam !

    My biggest roof installation was a 15 foot self supporting tripod that held a "GemQuad" boomless, full-sized 10-15-20 meter cubical quad antenna. I ran threaded rod through the roof and a set of 2X4s that spanned a couple of the rafters. That setup was so strong that the roof would have been torn away before anything came loose!

    The house was 2 stories plus an attic, and the center of the antenna was 15 feet above all that. I regularly worked phone patches into South America with outstanding results using that setup!

    73 from Jim AG3Y
     
  7. W7DJM

    W7DJM Banned QRZ Page

    As a person that formerly serviced HVAC including gas/propane, oil, and heat pumps, I HAVE seen a couple of chimneys.

    There have been QUITE A FEW that I wouldn't mount ANYTHING on. I've seen old brick chimneys that you could almost tip the exposed portion over and dump it down the roof.

    I would be darn careful, here. I don't know what ice/snow/winds you have in your area, how big/old/strong your roof or chimney is.
     
  8. WD5KCA

    WD5KCA Ham Member QRZ Page

    </span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (K3STX @ June 23 2004,09:55)</td></tr><tr><td id="QUOTE">I am thinking (at least to myself) about getting a small mini-beam and putting it on my roof. Now I know a mini-beam will give me mini-performance, but a) I would like to keep a relatively low profile (just moved into this neighborhood) and b) am interested in learning about beam headings/propagation &amp; long/shortpath openings.

    If I got a small mini-beam, like a Cushcraft MA5B (7 foot boom, 17 foot elements), should I put it on my roof using a tripod (like a Glen Martin) or mount the mast to my chimney. The chimney would be easier (and cheaper), but I don't want to destroy my chimney. THAT would be a problem.

    Anyone with experience/advice with roof mounting yagis? A tower is out for now.

    paul[/QUOTE]<span id='postcolor'>
    I did exactly what you describe here with a Mini-Quad. During a wind storm it came down. It sheared off at the mortar about one foot from the top.

    The mortar just won't take much shear force caused by wind pushing the antenna.
     
  9. K4JSR

    K4JSR Ham Member QRZ Page

    Paul, I have used ten and fifteen foot tall tripods at three
    different QTH's for mounting a Mosely TA-33, rotor, and
    two meter beams (stacked 11's) with a Ringo Ranger on top.
    Unfortunately the tripod manufacturers I had used are not in business any longer. I will say that I never had a bit of problem with roof leaks or structure damage. The fifteen foot tripod required some carpentry to get all three legs onto a roof joist, but that only involved using some 2 X 4 and
    a few 16p nails. The Glen Martin roof towers are built a lot
    stronger than the Tripods I used and should be more than
    sufficient for your use. Stay away from chimney mounts as
    smoke and heat are corrosive over time.
    73, Cal K4JSR
     
  10. K7UNZ

    K7UNZ Ham Member QRZ Page

    Hi Paul!

    Just a few comments concerning your antenna mounting situation.

    First off, none of us can tell you if your chimmney is sound.  I suggest you have it inspected by a licensed contractor if you are serious about that type installation.

    I can tell you from my own experience that mounting it on the chimmney is NOT the best thing to do.  While living in Norway, I had mounted a HQ-1 Mini-Quad  that way.  While it made it thru the winter, I had to replace vituually everything that held it together, due to corrosion and heat from the chimmney. Yes, I burned a lot of wood, but even without that, anything you burn and send up the spout will have an effect on the antenna.  It spent the next two years on a deck mounted tri-pod (hi).

    I would suggest that you give consideration to a good, ground mounted vertical antenna rather that the smaller beam.  You can mount it in an out-of-the-way location, which will conceal it from view.  AND, you do not have the added expense of a rotor system, and the extra control cable.

    The beam you mention will set you back as much as a good vertical, and give you less band coverage.  And a set of 17' elements, to me at least, are really not that stealthy on your rooftop!

    You might also consider a Delta Loop, fed with open wire line, for multi-band coverage.  They work well, are relative inexpensive, and can also be installed &quot;out of sight&quot; in most locations.

    Whatever way you go, good luck!  

    73, Jim/k7unz
     
  11. K3STX

    K3STX Ham Member QRZ Page

    Thanks for the tips guys, sounds like a chimney mount is out of the question and a Glen Martin roof mount sounds good. Interesting, the reviews of the HQ-2 mini-Quad on eHam are pretty good, maybe I will think of that.

    I certainly plan to keep my existing vertical system (a 40 M 1/4 wavelength and soon an 80 M 1/4 wavelength), maybe I will ground mount an all purpose one too (I love verticals). I just would like to try an antenna that has some directivity: I have never had one and suspect I would learn alot (and get some new ones on 10 meters, where I need them most).

    Thanks again, the last thing I want is a destroyed chimney (and I am not worried about roof leaks. It is nice the know that roof tower will not tear off my roof&#33[​IMG].

    paul
     
  12. K8CQ

    K8CQ Ham Member QRZ Page

    Paul,

    If you are looking for directivity, you might want to read up on phasing verticals. I live in a CCR community and the gutter dipole I've been using has managed 180 countries, but it lacks the directivity I need for certain areas of the globe. Thus I'm in the process of putting at least 2 phased verticals up, and maybe 3. I'm not sure how well they will play, but I love to experiment. There are some good sections in the ARRL Antenna book on phasing verticals, and also ON4UN's Low Band DXing will give you some good ideas, too. Your experience with single verticals will give you a good basis to start from in going on to the next step of phasing multiple verticals. If you do, let me know; I'd love to exchange notes on successes &amp; failures. I'm planning on getting mine up the week of the 4th; so we'll see how they play.

    73 and best DX, Jeff K8CQ
     
  13. K8CQ

    K8CQ Ham Member QRZ Page

    Paul,

    I forgot to comment on the chimney mount vs. roof mount question. I've used both methods in the past. The chimney mount is ok if your chimney is relatively new and well-constructed. Unfortunately, it isn't always clear what it's structural integrity is like inside. So it's a risk. Chimneys are not normally designed to take large side thrust forces and torques, other than normal wind load forces and vibrations from the earth (like earth quakes). Roof monting is denitely superior. And the comments about smoke ingredients loving aluminum are right on. I've seen TV antennas that were literally falling apart in a relatively short time if close-mounted to the top of a chimney at locations where fire places are routinely used.

    73, Jeff K8CQ
     
  14. K3STX

    K3STX Ham Member QRZ Page

    Jeff, I too have considered phased verticals, but I live on a pretty small suburban lot. I also would have to ground mount the vertical elements for a phased system, and I am not sure in my area, with all the houses around, a regular ground mounted vertical would do well. Currently my vertical is up about 20 feet in the air and it is great. Another point against phased verticals is I will be limited in directivity.

    paul
     
  15. K9STH

    K9STH Ham Member QRZ Page

    STX:

    On 40 meters all you need between the verticals is 35 feet for 90 degree phasing. That gives a cardiod pattern with about 4.5 dB gain in the &quot;desired&quot; direction and a 20 dB &quot;null&quot; off the back. But, off the sides of the antenna you still have about 3 dB gain.

    It is very easy to be able to &quot;flip&quot; the pattern so that the 4.5 dB gain and 20 dB null are reversed. However, you still have the 3 dB gain off of the sides.

    I have done this using my HyTower and a full sized 40 meter vertical. It works great! The &quot;trick&quot; for 90 degree phasing is to add 1/4 wavelength of feedline in the feedline to one of the verticals. You do have to have equal length lines to each of the verticals.

    Now, for the higher bands you can also use the 35 foot spacing and use various lengths of a phasing line to change the pattern. You will get gain, low angle of radiation, etc. The absolute pattern changes with the amount of phasing line that is used.

    W5AH has published several articles on this in QST, etc. Bob's method actually uses a control box made with various lengths of coax being switched into the feedlines to actually rotate the pattern on several bands. Do a &quot;google&quot; on &quot;phased antennas&quot;, &quot;W5AH&quot;, and maybe &quot;Bob Alexander&quot;. I have known Bob for going on 30 years! He used to work for Electrospace Incorporated which started as a &quot;spin off&quot; from the old Collins Radio Antenna Group.

    Electrospace was doing all sorts of various phasing arrays including multiple yagis and log periodics for the United States military. I could see their antenna range, which was located right behind their building from my rooftop, about 2 miles away.

    As for &quot;stealth&quot;, a vertical is definitly less obvious than a beam (of any type) mounted on your roof!

    Glen, K9STH
     
  16. K4JSR

    K4JSR Ham Member QRZ Page

    Aw shucks Glen,
      I was going to tell Paul to put a 1.8 MHz thru 30 MHz
    LPDA on top of his roof tripod.  At least then he could come
    home drunk and have no problem finding his house!  [​IMG]

    73,  Cal   K4JSR

    PS. Vertical antenna designers never die...
    They just phase array. (Take that, FT&#33[​IMG]
     
  17. KA3RFE

    KA3RFE QRZ Member QRZ Page

    I have a Create roof tower on my roof. It's the biggest one they sell and I forget how tall it is. The tower is support by guy wires and there's nothing under the tower itself. but the feet of the tower. The guys do all the work. I had to take it down to put new shingles on the roof and haven't put all the antennas back on it that it had before.

    The roof tower supported an Explorer 14 multiband HF beam, a 2 meter 4 element beam, and a three band Diamond vertical on a mast that I think is 20 feet. It is supported by a heavy duty rotor at the bottom and the tower needed a thrust bearing to keep the mast still.

    I did not directly install the eye bolts that the guys are hooked to the roof but that's another story itself.

    73, Pete KA3RFE
     
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