Stacking Question?

[KT4XP]

I have a two element Triband Yagi (TH2MK3) , It has a 6 foot Boom.

I want to stack a 4 element 6 meter Yagi with a 10 foot boom, about 5 - 7 feet above it.


Will the longer boom of the 6m Yagi cut into the Capture Area of the HF Yagi?

I cant separate the antennas any further.Would using a 3 El 6 meter Yagi,which has the same boom length(6') be a good option?I hate to loose the gain of the longer boom 6m Yagi, but I don't know what else I could do?

                                thanks in advance, Peter KT4XP

[G3RZP]

I doubt you'll see any difference in the TH2  performance.

[K3VAT]

... Will the longer boom of the 6m Yagi cut into the Capture Area of the HF Yagi?                                  thanks in advance, Peter KT4XP

Hi Peter,
Adding the 6M at the separation distance you have should be OK.  I believe a better and more precise wording to express your concern may be: " will the new yagi significantly interact with the existing TH2MK3?"  Don't think about 'capture area' in this situation - it's related but not of concern.

Significant interaction (primarily the mutual coupling of the metal elements & to a lesser extent the boom of one antenna [or metal object such as close by rain gutters] to the other antenna)  can cause problems in antenna matching as well as pattern distortion (low performance).  I believe that there are plenty of stations that have similar spacing between their HF (10-20) yagi and the VHF (6M) yagi and still do fine on both.

GL, 73, Rich, K3VAT

[KT4XP]

Thanks for the replies

I hope  my rotator can handle it.

[WA3SKN]

The wind forces on the bearings will be high.  This is the reason you see most spaced about 2-3 ft apart.  A six foot spacing will provide quite a lever.  Are you planning a thrust bearing or direct rotator connection?

-Mike.

[KT4XP]

It will be mast mounted on a roof Tripod, with the thrust bearing between the Rotator and the first antenna, and guyed at four points on the thrust bearing.

I know this is pushing the envelope, so maybe 3 feet separation would be less strain on the rotator.

[K3VAT]

It will be mast mounted on a roof Tripod, with the thrust bearing between the Rotator and the first antenna, and guyed at four points on the thrust bearing.  I know this is pushing the envelope, so maybe 3 feet separation would be less strain on the rotator. 

How do you know that you're pushing the envelope with the proposed setup?  I usually recommend that builders gain some understanding on the mechanics of their antenna/mast/wind load/mast material and the relative positions on the tower of these parameters by using WD9P's instructional mast calculator http://www.math.niu.edu/KARC/mast/.  You'll need the wind load specifications of your two antennas (from the manufacturer), the load ratings from your roof tower manufacturer and the type of mast material that you are using (aluminum or steel; wall thickness).  I believe that you are in a 70mph region.

Using roof towers with multiple yagis can be risky.  If you the least bit unsure of the safety factor then I would seek out a qualified professional engineer to run your numbers.

The wind forces on the bearings will be high.  ... -Mike.

That's true Mike, but how 'high' is high? Two times, Three time as high?  Without the above tool, it's just guesswork) - a more important engineering consideration may be how well the roof tower is affixed to the roof and how much lateral force is exerted at the tower feet - roof interface.

... This is the reason you see most spaced about 2-3 ft apart.  ... -Mike.

 

I'm only familiar with the several dozen multiple HF (I'm including 6M here) yagi configurations that I've personally seen, and probably another several dozen that I've seen pictures of (on QSL cards, etc.) and the vast majority  of these had stacking distances between dissimilar yagis of between 5 and 10 feet.  I've only see one or two situations that had three HF antennas (example: M-square 20M4, 15M4, and 10M4) all on 6 feet of mast.  So I guess it depends where you look.

GL, 73, Rich, K3VAT

[KT4XP]

Thanks for all your comments and that website.

Sorry for the delay in responding as I have a few questions out there in other forums.

By pushing the envelope I am referring to the wind load of this stack is right at the limit of the
Rotator's specifications.(Yaesu G-450A).

Would adding more mast length  between the roof tripod (which is secured to braced rafters).
and the rotator (secured with guy ropes) transfer some of the torque when rotation is stopped, from the rotator to be dissipated in the mast as it twists until the antenna comes to a full stop?

I was thinking 16 GA steel masts?

                       73 and Thanks Peter KT4XP

[K3VAT]

By pushing the envelope I am referring to the wind load of this stack is right at the limit of the
Rotator's specifications.(Yaesu G-450A).  Would adding more mast length  between the roof tripod (which is secured to braced rafters). and the rotator (secured with guy ropes) transfer some of the torque when rotation is stopped, from the rotator to be dissipated in the mast as it twists until the antenna comes to a full stop?  I was thinking 16 GA steel masts?  73 and Thanks Peter KT4XP 

Peter,
If you could send a link to a photograph of your current setup I think it would help us get a better understanding of what exactly you have so we can more carefully and accurately respond to your proposal.    What is important here is that your roof-mounted tower (even though mounted to the roof rafters) probably is an inferior setup for mounting HF yagi antennas.  Why?  First, it is a tripod; a quadpod is much more stable and for windloads of > a few square feet up on a tall mast (along with the rotator) one doesn't want to have the roof tower swaying in the wind.  Second, and may be more importantly for heavier loads with greater wind loads, it does not have the provision for nesting the rotator within the tower.  It would be helpful to know the physical dimensions and the load ratings of this roof-mount tripod.

As I mentioned in my previous post: If you the least bit unsure of the safety factor then I would seek out a qualified professional engineer to run your numbers.  One of your local clubs may have a PE who would be willing to come out and look over your setup and review with you the concerns about this proposal.

73, Rich, K3VAT

[KT4XP]

Thanks Rich,

                   I am about to get the Rotator back from Yaesu Service in California.
I will be sure to post a picture ASAP.

                          73, Peter KT4XP