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Author Topic: Multi Antenna - single tower  (Read 1681 times)
N8CNJ
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Posts: 11


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« on: February 05, 2012, 06:14:11 PM »

Hi, I'm looking for a solution and hope to get some advice here.  Can I put three yagi's on one tower?  Two multiband HF and a 2m.

I'm trying to cover 40-6m and obviously 2m, but only have the space for a single tower properly installed with yagi's.
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KB4QAA
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Posts: 2241




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« Reply #1 on: February 05, 2012, 07:58:56 PM »

Yes you can if your tower and thrust bearing will support it, and your rotor can handle the wind load.
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K4RVN
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Posts: 757




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« Reply #2 on: February 05, 2012, 09:48:34 PM »

If you do not own the yagi antennas, you may want to consider a 6 band hex beam or cubical quad at a lot less weight and similar performance to  three element trapped yagis. A smaller rotator and wind load would also be possible. A 40 meter directional antenna will have long elements and more wind load. Of course one tower can handle the yagis with the proper thrust bearing, mast, rotator and wind load ratings. Just depends on what you want to do and also depends on ice,snow,
and winds you receive where you have the tower. If you live in Chester,S.C. then a quad would be OK as well as the hex beam. My folks were from Richburg right down the road in Chester County. Here are some links for you if interested.
Hex beam   http://k4kio.com/ :  Quad   http://www.cubex.com/ The hex beam specs show less gain than a 3 element
yagi while the two element quad is about the same as a three el. yagi. I have a 5 band 3 element quad which is a great antenna in my experience, but large footprint.

Frank
« Last Edit: February 06, 2012, 04:38:54 PM by K4RVN » Logged
N8TI
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Posts: 115




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« Reply #3 on: February 07, 2012, 10:06:29 AM »

I agree with the quad recommendation.  I have a two element quad for 10, 12, 15, 17, 20, and 30. (There is no gain for 30).  It also loads up on 40 with the rig's antenna tuner.  It is a "Lightning Bolt" brand.  However, I believe he stopped making them.

I wish is that I would have bought a three element quad while I was going through the trouble of putting up the tower and antenna in the first place. The quad is so light that the tower could have easily handled a three element quad. There is not too much to go wrong with a quad.  No traps or anything like that to worry about.

I always thought of putting up a rotatable dipole for 40 to get some gain out of it. but use an inverted L for 160, 80 and 40 work. The tower is nice for supporting the inverted L or various dipoles or inverted V's.



Joe



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NR9R
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Posts: 147




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« Reply #4 on: February 07, 2012, 11:25:40 AM »

This is a question that I have been considering as well.  The hexbeam or quad can provide 10- 20 m but are not the best choices for stacking other antennas.  If you do some research you will find that achieving a large bandwidth in a small volume is tricky, and almost always comes at the cost of sacrificing gain.  I remember seeing a modeling analysis in the ARRL Antenna Book predicting that a stack of 10 m, 15 m , and 20 m monoband yagis on something like a 20 ft mast will perform similar to an efficient tribander.  If you read the HF tribander comparisons by N0AX, and K7LXC you will also see that for the same boom length, adding more bands decreases gain.  When designing a yagi for a given boom length, once you divide 14 - 30 Mhz into 5 bands without extending the boom you pretty much end up with the forward gain of a LPDA.  Or you can improve f/b by going with the forward staggering design used by Force 12 and Optibeam.  With this knowledge, here are some iterations to consider, assuming you have a single tower and a mast no longer than 15 ft:

1) Steppir yagi for 6 - 40 m and a 2m beam stacked above it (most elegant solution).
 
2) 10/15/20 m yagi + 12/17 m yagi with + 30/40m rotatable dipole kits + vhf beam(s) in the middle.

3) 5 band LPDA or 5 band forward staggered 2 element yagi + 30/40M rotatable dipole + vhf beams

Hope this helps...

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W9EEE
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Posts: 8




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« Reply #5 on: February 08, 2012, 10:23:53 PM »

Yes,  it is possible.   The most you can stack is determined by the lower of any of the following:
  a.  the wind load of the tower.
  b.  the loading of the rotor.
  c.  the bending strength of the mast.

The more square feet of windload high on the mast the more the stress due to the moment arms.
In other words the largest antenna should be closest to the top of the tower with progressively smaller square feet of surface area higher up.   Use a thrust bearing at the top of the tower.    Move the rotor down somewhat
inside of the tower.   Do not exceed any of the above specifications.
I have a US Tower HDX 572 (72 foot crank up) with T2X rotor and a 24 foot mast length (total);  the mast is aluminum with about 1/2 inch wall thickness.  On it mounted the following:
1.  A 5 element (homebrew extended) Mosley 20,15, 10 tribander at 74 feet with a 40M add on-kit.  It is trussed vertically with Kevlar and horizontally with Dacron lines.    It also has an enlarged boom plate with additional
mast to boom clamps and saddles.   Boom length is 29 feet.
2. Above this at 85 feet is a 2 element 30, 17, and 12 tribander.   Boom is 11 feet.
3. At ~ 90 feet is a 19 element 2M yagi with a 25' boom (very lightweight elements/boom)
4. At the top is a 2M/440  J-pole with its apex ~ 100 feet.
Has been through 80-88 mph winds several times fully extended.   Mast will flex but not set.
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NR9R
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Posts: 147




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« Reply #6 on: February 09, 2012, 06:58:08 AM »

Probably, the most factors, as W9EEE points out, are the structural design components.  When you start stacking big yagis on a long mast you have to take great care in engineering the system.  Aside from the rotator and thrust bearing, you must be able to calculate the total bending moment applied to the mast and be able to ascertain whether the load will exceed the specs of the tower.  In addition, the right material must be used for the mast.  No guesstimating allowed in this kind of set-up.   
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WI8A
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Posts: 6




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« Reply #7 on: February 10, 2012, 06:17:08 AM »

HELLO WILLIAM- OVER A LOT OF YEARS I PUT UP SEVERAL MULTIPLE BEAM ON THE SAME MAST INSTALLATIONS.. A FEW SUGGESTIIONS ..  YOUR MAST IS A INVESTMENT SO GET A GOOD ONE !  INSTALLING YOUR ROTOR BELOW THE TOP OF THE TOWER IS A MUST -- I LIKED MINE DOWN ABOUT 8 FEET INTO THE TOWER AND BELOW TWO THRUST BEARINGS SO YOU CAN PULL THE ROTOR WITHOUT THE MAST TILTING AND YOU MUST BE ABLE TO BLOCK OFF THE MAST FROM TURNING WHILE THE ROTOR IS BEING REPLACED ETC .. MAINTAINENCE IS ALWAYS GOING TO BE NECESSARY SO MAKE THINGS AS EASY FOR YOURSELF AS POSSIBLE  .. INTERACTION BETWEEN THE ANTENNAS IS ALSO A CONSIDERATION AND A PROBLEM I FOUND  .. SPACING ETC IS IMPORTANT ...NOW AFTER DOING THAT STUFF I NOW AM USING A STEPPIR  IT RESOLVES SO MANY ISSUES I HAVE TO RECOMMEND THEM!  ESPECIALLY IF YOU DO NOT ALREADY OWN THE YAGIS .. BESIDES THEIR EXCELLANT  PERFORMANCE IT ALLOWS YOU TO USE  SMALLER TOWER AND SMALLER ROTOR  ONE FEEDLINE   AND THEY WORK WELL  --THEY HAVE SEVERAL MODELS TO PICK FROM ...EVERY PART OF YOUR DESIGN CHANGES IF YOU ARE GOING TO USE A FIXED TOWER OR CRANK OVER ALSO  -- MINE WERE FIXED GUYED TOWERS

ROD    WI8A       
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