Oh yeah - another bugabbo that sneaks in is that frequently the balance
point of a yagi is NOT at its rotational center...
...after assembling the yagi take a string and run it from the tip of
the reflector acros the boom to fartherest director's tip - where the
string crosses the boom is your rotational center.
It is best to mount the antenna here, not at the balance as you will
have common distances when rotating about the tower this way versus one
side of "center" being a different radius than the other...
SO, if you mount it here and the balnce is off what do you do?
I have adopted a really simple procedure to counter balance an antenna
to move the CG to where the rotational center is.
Since booms are typically 2 or 3 inch OD if you slip something 2 or 3
inch ID over them you will cause very little difference in wind
loading...conveniently, gray PVC used for running electrical wiring is
readily available in these sizes...
How do I know how much to use...you can tape the piece onto the boom
initially to determine how weight (and hence length) you will
need...once you learn the magic comnbination of length and location pull
off the farthest director (or the reflector if the back end is light)
and slide the pvc over the boom into position.
Reassemble the antenna and fine tune the balancing pieces
location...once you have it simple run a small self tapping sheet metal
screw throught the tubing and the boom and you are done...
This method is inexpensive and while it makes the overall antenna weight
higher you probably are trussing the boom if it is worth stacking anyhow
and now that it is balanced at the mounting point it seems to eigh less
anyhow since it is in balance!
Next...
73,
Jim, K4OJ
Mike Gilmer, N2MG wrote:
>You'll find in this case that since your antenna height is close to, but
>under a top guy, and the turning radius is rather large, that the guy
>anchors must be very far away (relative to "standard designs") from the
>tower to achieve your goal.
>
>The math (no, you don't need any a^2 + b^2!)
>You can calculate the "similar" triangles: they have the same angles, so the
>sides scale. Sketch it to see.
>The first (larger) triangle 1 is the full tower height (75') for one side,
>tower-to-guy distance (length along the ground) as the other side, and the
>guy wire itself as the hypotenuse. The other (much smaller) triangle 2 is
>the beam-to-tower height (15') as one side, the beam-to-guy wire distance
>(IOW, your turning radius = 25') as the other side, and the guy wire as the
>hypotenuse. For clearance calcs, we don't even care about the hypotenuse
>lengths.
>
>The ratio of one side of triangle 1 to the other side of the same triangle
>is the same as the ratio of one side of triangle 2 to the other side of
>triangle 2.
>
>75' : x = 15' : 25'
>
>This makes guy anchor distance x = 125' for zero clearance. To get 1 foot
>of clearance we change the 25' turning radius to 26' and get: 130' Then
>there's ground irregularities and guy wire sag so you may opt for more
>minimum clearance. Lower the antenna 5 feet and the 1-foot clearance anchor
>spacing is only 98 feet Big difference. Of course, if you have the room
>and the EHS...
>
>Note that typical towers are guyed at ~80% of tower height which is about 67
>feet in your case. Your original spec nearly doubled it.
>
>Mike N2MG
>
>
>
>
>>I am looking for a formula to determine in advance how far my guy anchors
>>need to be for a side mounted antenna at any given height to clear the
>>guys. Example, a 75' rotating tower with guy wires attached at 75' and an
>>antenna with a turning radius of 25' at say 60'. I'm sure I could graph
>>
>>
>it
>
>
>>out, but I would prefer to know exactly before sinking anchors (I want to
>>do things once).
>>
>>
>
>
>
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>
>
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