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Author Topic: Rope Length for Supporting Dipole Legs  (Read 2062 times)
KL3HY
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Posts: 117




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« on: September 30, 2011, 04:32:53 PM »

I just put up my first transmitting antenna, an off-center fed dipole, and it dawned on me that the rope lengths I used to secure the legs might be a little short.  I did some google searches on this but came up empty.

The masts I'm using are steel.  I don't think the power I'll be running (100 watts) will be enough to make the ends arc, but I'm not really sure.  I'm also unsure if I'd be harming my transmit and/or receive performance if the ends are too close to metal.  Currently I'm using about 2' of rope to tie the insulators to the masts.

Is there some rule of thumb that deals with this, like "always use a minimum of X feet"?

Thanks,
Mike
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N4NYY
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« Reply #1 on: September 30, 2011, 05:05:11 PM »

Mike,

If you are running an inverted V, than use wood posts (if you think the ends will interact) and install a weighted pulley system that allows the legs to move with wind.
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AC5UP
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« Reply #2 on: September 30, 2011, 05:14:50 PM »

There are very few perfect antenna installs and yet somehow people make Q's.

Sure, the farther from any metal objects the better, but at least the wire is perpendicular to the mast. Parallel will give maximum coupling, perpendicular minimum. You chose well. If you're concerned that 2' of rope might be bridged by an RF corona you're assuming the wire has some serious moxie and the insulator is there for show.

Very slight risk the rope will burn, arc, or melt. If it does concern you, avoid transmitting on rainy days and be sure to eyeball the rope for signs of discoloration whenever you're out in the yard.
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N4JTE
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« Reply #3 on: September 30, 2011, 05:17:48 PM »

Your fine unless running 1500 watts off frequency.
Bob
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W5FYI
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« Reply #4 on: September 30, 2011, 05:25:23 PM »

One rule-of-thumb is to have the ends of the antenna where no one can accidentally come in contact with them--not even a basketball player who wants to check out your handiwork. It is the ends of your antenna that have the highest voltage on them, whether it's fed off-center or not.

The center of the antenna only needs to be a few inches from the mast. GL
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KL3HY
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Posts: 117




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« Reply #5 on: September 30, 2011, 05:29:54 PM »

Great--this is exactly what I was looking for.

I had read that the leg ends have the highest voltage, which was the basis of my concern about arcing.  I do have the entire length of the antenna at least 15' high (balun is at 30', short leg mast is 18' tall and long leg mast is 15' tall), so I think I should be okay there.

Thanks guys!
Mike
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WB2WIK
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« Reply #6 on: September 30, 2011, 05:37:27 PM »

The rope length should be very accurately calculated by this formula:

L = 2dx^2/4pi*tan(area)

 Cheesy

Just do what everyone has done since the beginning of the radio age, which I must say is now "officially" 102 years old, and "try stuff."  When the first stuff doesn't work well, try other stuff.
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N4JTE
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« Reply #7 on: September 30, 2011, 06:23:14 PM »

Wow just reread your post, 15 ft high?, worry less about your original concen about arcing, any possible way you can get that sucker higher??
Best regards,
Bob
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KL3HY
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Posts: 117




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« Reply #8 on: September 30, 2011, 10:16:22 PM »

Well, funny you should bring that up--earlier this evening I got another section of steel conduit and raised the end of the longer leg up to about 20'.  I'm not sure I can get it much higher than that though.  It did make a noticeable difference in what I could hear.

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N4NYY
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« Reply #9 on: October 01, 2011, 07:51:29 AM »

Quote
The rope length should be very accurately calculated by this formula:

L = 2dx^2/4pi*tan(area)

 Cheesy

Oh no, no, no!

LOL Grin Grin Grin Grin Grin Grin
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KA5N
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« Reply #10 on: October 01, 2011, 08:01:17 AM »



Can't anybody get a straight answer about an inverted V any more??

Allen
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N4NYY
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« Reply #11 on: October 01, 2011, 08:03:21 AM »

Quote
Can't anybody get a straight answer about an inverted V any more??

Don't worry. When I put mine up in a few weeks, I will ask a question and they 100 different responses.  Wink Wink Wink Wink Wink
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WB6BYU
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Posts: 13172




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« Reply #12 on: October 01, 2011, 09:04:01 AM »

The rope should be long enough to reach from the end of the antenna through a pulley or ring on the mast
and down to ground level, or wherever you can tie it off.  Even better is to have enough rope so that you can
untie it and lower the end of the antenna to the ground for adjustment without having to splice on additional
rope (so the knot doesn't get stuck in the pulley), but sometimes that isn't practical.

As to the spacing between the end of an antenna and nearby metal (whether it is used to support the
antenna or just happens to be there) two feet is plenty of distance to avoid arcing as long as the antenna
is full sized (at least 1/2 wave long for a dipole.)  Depending on the size of the object, it may shift the
resonant frequency of the antenna somewhat.  If the antenna can wave in the wind so that the distance
varies, you may see the SWR shifting.  I've never seen any problems with nearby metal affecting the
transmitted signal, but if you have multiple pieces of metal in a strong RF field (such as metal shingles
on a roof) then you can get harmonic generation from poor connections.

Problems can be worse with shortened antennas - not only is the end voltage higher, but the tuning is
much more critical.  We had a Hamstick Dipole for 80m where a branch blowing in the wind 5' from one
element shifted the SWR more than the radio was happy with.  But you shouldn't have that problem
with your antenna.
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G0GQK
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Posts: 634




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« Reply #13 on: October 01, 2011, 01:45:50 PM »

Google doesn't have a formula for extrapolating the length of a piece of cord for your dipole. Think, if its up 30 ft and it needs to be lowered to the ground on occcasions, how long should it be ? Er.... dunno, do a Google ?

G0GQK
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WB6BYU
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Posts: 13172




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« Reply #14 on: October 01, 2011, 03:53:32 PM »

But a bit of geometry will give the answer.  Clearly it has to be more than twice the height of the mast.

If that wire is 50' long and the center is at 30', it would reach the ground at 40' out from the center.
If the end support is also 50' away from the center (approximate, given that the end of the wire is 2'
short of the support, but at a lower height), then you have another triangle 10' on the base and 20'
high (or whatever that end happened to be.)  Pythagoras tells us it would take about 23 feet to reach
the top of the support, and another 20' to come down to base.  So about 43'.  I'd use 50'.
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