Hi Jim, pretty good question. And since I played a bit with the idea for
another
reason, I'll chime in.
You can stagger the lower 10 out in front of the tower and compensate for the
"early" phase of the wave by making its feedline longer than the upper center
mounted Yagi. How long would be the number of meters that its director is in
front of the others director multiplied by the Velocity Factor of the coax.
Make sure you mount it VERY securely. I had a long TV antennas on a tower and
just like you want to do with your 10. The truss worked very well in the
horizontal but the winds that came up from underneath the beam blew it up and
bent
the Aluminum extension I put on the beam. So you might consider trussing it
down
as well. Winds really whip the tower around too.
I have three 4 ele 10m monos on that tower now as well as 3 4 ele 15s, I dont
see much detuning of the 10s as they rotate around the tower on swing arms. So I
would not worry about tower interaction, and its sure much better to not torque
the tower by having that antenna way out in front.
And yes folks I do use StackMatches, many times the lower antennas are the best
over the top one. So the ability to chose them is and disable the top is great.
Jay, WX0B
K4OJ@aol.com wrote:
> Most yagi installations @ HF have the attachment of the beam to the mast at
> approximately the middle - or - center of gravity along the boom.
>
> When you get up to the VHF and UHF stuff you see end mounting of yagis as it
> is physically feasible with such little tiny weights involved.
>
> My Question to the unwashed masses of towertalkians:
>
> Somewhere down the road I plan on a stack of beams for 10 meters....the
> sunspots are rumoured to be coming back, and ten will be a DX band again for
> a lil while.
>
> I am considering using 4 element yagis on about a 15 foot boom.....why not
> make that boom 17 feet long, extending it behind the reflector and mounting
> the boom to the tower face fixed at EU (of course)......with a Philystran
> truss to a turnbuckle running up from the 2nd director end of the boom?
>
> By making the tower be "behind" the reflector it (I think) will have a
> negligible effect on the antenna's pattern....whenever I do the side mount on
> the leg/face of the tower it ends up near an element and although it is not re
> sonant I think the tower MUST do some degrading of the beam's pattern.
>
> How about it, you yagi sages....at 10 meters these antennas will weigh in at
> about 20 lbs. (including the coaxial feed line)....I am not worried about
> upsetting the "balance" of the physical load on the Rohn 55 they are mounting
> to...if they were bigger beams at a higher frequency I could see being
> worried but these are overgrown TV antennas.
>
> (They also won't make climbing up the tower as awkward at their
> heights....ever had your climbing belt catch on a gamma match?)
>
> How bout it?
>
> AND, the very top antenna of the stack will be mounted in traditional (center
> of gravity/boom) fashion, at the top of the Christmas Tree...above another
> yagi for another band.
>
> DOES THE FACT THAT THE OTHER ANTENNAS BELOW IT ARE NOT "directly" BELOW CAUSE
> ANY POTENTIAL QFU? (Seem that when you use swinging gate side mounts and
> ringrotors you are never directly underneath the top beam, anyhow.
>
> Pretty good question, huh!
>
> What do you think.....LB are you out there?
>
> Jim, K4OJ
>
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