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Author Topic: hanging a102ft doublet?  (Read 4249 times)
AK4YA
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Posts: 106




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« on: September 29, 2012, 10:57:25 PM »

Got the MFJ model in this week.  Can I just string it between a 30ft tree and my 30ft chimney?  And drop the ladderline to the ground with no center support?  If so i plan to use a pulley on the chimney end with a weight to handle sway.  And dacron from the trees to insulators.

How much weight will be required on the pulley end?  I believe it requires infinite weight to get it 100% completely horizontal straight, but in our world, how much droop is acceptable?

Thanks everyone, hope to be on the air soon
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K9ZMD
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Posts: 173




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« Reply #1 on: September 29, 2012, 11:35:53 PM »

I wouldn't worry about a little sag in the middle of your antenna.  I've never seen a dipole yet that was pulled so tight that the wire was completely horizontal.  Here's another consideration: tight wire + vibrations can equal a multi-buck repair bill for a masonry chimney.  If your chimney isn't masonry, then just don't pull the wire so tight that the eye bolt pulls out of the wood.

You probably already know this, but please pardon me for asking if you intend to "drop the ladderline to the ground", then run it on the ground back to the radio shack?  If so, that is a non-starter.  You need to get the ladder line elevated from the ground and avoid long runs close to things (like metal rain gutters, the kid's metal swing set, and wet privet hedge leaves) that can upset the feed line balance or increase loss.  Three times the width of your balanced line is generally adequate spacing to avoid problems.

Gary, K9ZMD/7
Ridgefield, WA
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AK4YA
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« Reply #2 on: September 30, 2012, 12:36:19 AM »

Yes it will drop to a wood fence to run to qth.  I know its impractical to wire it straight as a razor, i was just speaking in engineering terms so as so start a conversation about weight requirements.  My main concerns are do I need a center support, and how much weight on the pulley end will I need to appropriately support the weight of the antenna as straight as needed for Ham radio requirements?

I definitely don't want to put much strain on a 30 ft bricked chimney
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GW3OQK
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« Reply #3 on: September 30, 2012, 04:17:13 AM »

The worst is that ladderline will lash about furiously in a gale and is heavy. I centre-support my doublet with a fibreglass telescopic pole and even run the line down inside it. When I had a tree-hung doublet the tensioning weights either end were about 6 Kg
Andrew
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WA8JNM
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« Reply #4 on: September 30, 2012, 04:56:42 AM »

Why the comment about not running ladder line on the grass? Can you do so do long as there is no metal along the path? I just set this up. No danger; it's thru a thick woods that no one walks. Am I missing something?
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K5LXP
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« Reply #5 on: September 30, 2012, 06:40:28 AM »

... i plan to use a pulley on the chimney end with a weight to handle sway.  ... How much weight will be required on the pulley end?  I believe it requires infinite weight to get it 100% completely horizontal straight, but in our world, how much droop is acceptable?

You can use this calculator to figure the sag of a wire vs the tension on it.  That will give you an idea how much tension you'll need for your chimney mount.

http://www.spaceagecontrol.com/calccabl.htm

Quote
Why the comment about not running ladder line on the grass?

Balanced line has a magnetic field surrounding a few widths of itself.  If you run a significant length of it along an RF lossy medium, you'll have greater signal losses.  Crossing over say, a tree branch, a metal gutter or going through a wall at a right angle is just a very small "blip" and isn't a problem.  Running 50ft in the dirt would noticeably, if not significantly affect the impedance and loss of the line.  If you're running it through the woods perhaps you can suspend it from tree to tree or whatever foliage is there.  Just enough to keep it off the ground will do it.

Mark K5LXP
Albuquerque, NM
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WB6BYU
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« Reply #6 on: September 30, 2012, 08:16:38 AM »

If you have deer wandering through the woods, you'll want to put it above
antler height.

You can weave a rope through the line and stretch that between the trees to
support the feedline without putting tension on it.


Sag in an antenna is usually more of an aesthetic issue:  the antenna will still
radiate at 25'.  The real question is how much the antenna + feedline weigh,
and how far it is from the antenna to the chimney.

The tension is much more than the weight of the antenna - probably 10 or 20
times that in many cases.  Passing webbing or a metal strap around your
chimney is probably better than drilling a hole and using a screw eye in line
with the antenna, which won't take as much pull.   (Though that would have
the advantage of pulling out before the stress was high enough to otherwise
damage the chimney - they usually are NOT as strong as one might think
when dealing with horizontal forces.)

I use light wire for my antennas, and a single brick is usually enough.  One wire
runs from the barn to a tree, and even that was too much:  the wire would hum
in the wind and the barn acted as a sounding board.  I ended up putting the brick
on the ground so the wire is under very little tension until the tree sways and
lifts the brick up off the ground.
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WX7G
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« Reply #7 on: October 01, 2012, 08:53:43 AM »

The first question is "how much weight will be required on the pulley end?"

MFJ does not say what the antenna wire is so let's be conservative and say it is only #14 hard drawn copper. The breaking strength is 200 lbs and if you derate it for a working load of 10% you can use a 20 lb weight at the end.

The droop will be what it is.
« Last Edit: October 01, 2012, 09:08:06 AM by WX7G » Logged
WB2WIK
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Posts: 20636




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« Reply #8 on: October 01, 2012, 08:58:11 AM »

I wouldn't run the ladder line on the ground, either.

If it's gently laying on dry earth, that's not bad.  However, it will get covered by "stuff," and get wet and potentially be laying in water, and if it ever snows or freezes there that's another issue.

Also, laying on the ground it is subject to "critters" chewing on it and potentially damaging or breaking it.  I know around here, nothing "chewable" can lay on the ground for long before it's destroyed by squirrels, mice, possums or whatever is roaming around -- even my dogs!

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AK4YA
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Posts: 106




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« Reply #9 on: October 02, 2012, 09:06:22 AM »

<SNIP>
Sag in an antenna is usually more of an aesthetic issue:  the antenna will still
radiate at 25'.  The real question is how much the antenna + feedline weigh,
and how far it is from the antenna to the chimney.
<SNIP>


antenna/feedline is about 10lbs. distance from the tree/chimney is 130ft, since the antenna is 102ft I have a fairly decent range to pick from to go between the antenna end and the chimney.  Why is this important (not an engineering major)

Thanks!
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WB6BYU
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Posts: 13486




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« Reply #10 on: October 02, 2012, 09:39:10 AM »

Quote from: AK4YA

...Why is this important?



The sag is a function of the total length between supports, not just the antenna length.
If you had a 300' span between supports, it probably would be hard to keep the antenna
off the ground without using excessive tension, even though the antenna length is the
same as in your case.

At a very rough calculation, assuming all the weight of the antenna is at the feedpoint
in the middle of the span, you can make an estimate of the required tension (or sag) by
assuming that the ratio of the sag to half the span equals the ratio of half the antenna
weight to the rope tension.

As an example, if your antenna weights 10 pounds and you pull up 50 pounds tension on the
end ropes (ratio = 5/50 = 1/10).  If your span between supports is 120', then 1/10 * 60 = 6'
of sag in the middle.

Certainly you should use one of the more accurate calculators if you are considering anything
related to safety:  any simple formula like this makes assumptions that don't hold in the real
world.  But it will at least give you a sense of how sag and rope tension are related.
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AK4YA
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Posts: 106




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« Reply #11 on: October 02, 2012, 01:50:21 PM »

Quote from: AK4YA

...Why is this important?

<SNIP>
As an example, if your antenna weights 10 pounds and you pull up 50 pounds tension on the
end ropes (ratio = 5/50 = 1/10).  If your span between supports is 120', then 1/10 * 60 = 6'
of sag in the middle.
<SNIP>

Very interesting.  I suppose that "5" in the "5/50" is the weight of the antenna (10) divided by the number of ends supporting it (2)?
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KG6YV
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« Reply #12 on: October 02, 2012, 02:28:13 PM »

If you use pulleys buy a marine pulley not the kind sold at Home Depot or hardware stores.  The cheaper pulleys from general stores have lots of "slop" between the wheel and housing and are perfect candidates to pinch your rope and make retreival
futile.

Greg
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AK4YA
Member

Posts: 106




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« Reply #13 on: October 02, 2012, 02:33:50 PM »

If you use pulleys buy a marine pulley not the kind sold at Home Depot or hardware stores.  The cheaper pulleys from general stores have lots of "slop" between the wheel and housing and are perfect candidates to pinch your rope and make retreival
futile.

Greg


so I should use pulleys at both ends?  why wouldnt just at the chimney end be sufficient.  they chimney wont sway, its the tree that will sway. Huh
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WX7G
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« Reply #14 on: October 02, 2012, 03:13:34 PM »

One pulley.

What kind of wire is the antenna made of, #14 copper?

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