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Author Topic: What effect does tying a knot in an antenna wire have?  (Read 4081 times)
N8TI
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Posts: 115




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« on: March 15, 2012, 09:00:32 PM »

When I put together a dipole or some othe wire antenna, I might tie the wire in a knot, say to affix a spreader in a multiband dipole or perhaps attach the wire to the center conductor or the end conductor. Do a few knots make any difference?

Joe
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WA9ZOH
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« Reply #1 on: March 15, 2012, 10:12:26 PM »

Assuming you are using a non insulated wire, I would say the length of the wire would be subtracted from the beginning to the end of the knot.
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N8TI
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« Reply #2 on: March 16, 2012, 02:03:31 AM »

With insulated wire?
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W1JKA
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« Reply #3 on: March 16, 2012, 02:29:31 AM »

    I use single overhand knots in my homebrew dipole  insulated wire connections to prevent pull through, never had any problems.
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KA5N
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« Reply #4 on: March 16, 2012, 03:48:11 AM »

If you tie a knot with your antenna, that means you are married to that antenna till death
 do you  part.   Or maybe you just have a knotty antenna.

Allen

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W8GP
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« Reply #5 on: March 16, 2012, 05:45:31 AM »

I've been doing that for years,it will have no measurable effect on antenna performance.
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WX7G
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« Reply #6 on: March 16, 2012, 07:49:29 AM »

If you tie a knot with your antenna, that means you are married to that antenna till death
 do you  part.   Or maybe you just have a knotty antenna.

Allen



So you're saying that a knot is knot knot knot a problem?.......knot!
« Last Edit: March 16, 2012, 08:27:13 AM by WX7G » Logged
AA4PB
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« Reply #7 on: March 16, 2012, 08:03:08 AM »

It sounds to me like you are all knots  Grin
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N2EY
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« Reply #8 on: March 16, 2012, 08:04:20 AM »

It's not good for the wire.

Western Union splice or half-splice is better.

73 de Jim, N2EY
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SWMAN
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« Reply #9 on: March 16, 2012, 08:46:56 AM »

 It would KNOT affect it at all!
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AD4U
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« Reply #10 on: March 16, 2012, 09:03:25 AM »

A typical "granny knot" tied in antenna wire is definitely a weak point where the wire is most likely to break.  As far as electrical performance, it probably does not matter.

Dick  AD4U
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WB6BYU
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« Reply #11 on: March 16, 2012, 09:31:59 AM »

It depends on what type of knots you tie, and whether you measure the wire before or
after tying the knots.

I regularly make my dipoles and other antennas using stranded, insulated wire.  At the
feedpoint I tie a loop through the hole in the center insulator, with enough of an end to
reach the connection point.  At the ends I tie loops to attach the insulator or rope, often
with more wire hanging down so I can prune the antenna without retying the knots. For
multi-band antennas there may be multiple wire sections each with loops tied at each end.

Does it make a difference?  Yes, it certainly does:  tying knots in the wire changes the
resonant frequency.  If I cut the wire to length first, then I always come up short by
an amount that varies with where in the antenna I put the knot.

The reason is that the doubled wire section of the loop acts like a short linear-loading
stub.  For a given stub inductance, the effective amount of loading is greatest near
the feedpoint and least at the end of the wire (for a half wave dipole.)  The loop at
the feedpoint doesn't make a lot of difference in the resonant length - as a guess, the
correction would be 1/2 the length of the loop.  At the end of the antenna the added
inductance has little effect, so the correction is nearly twice the length of the stub.
Depending on the application, my loops are often 4" to 8" long (8" to 16" of wire, and
probably somewhat more considering the rest of the knot) so the overall shortening
effect can be as much as 3 feet on the total dipole length.  Obviously if you tie
smaller knots there is less of an impact.

For just an overhand knot in the wire there is little impact (other than weakening the
wire.)  Even with several such knots, the total shift is will within the expected variation
due to environment, installation, wire insulation, etc.  For loops there will be a bit of
a difference depending if you use a bowline or a figure-8 follow-through because of the
required amount of wire for the same size loop.

Other than weakening the wire and shifting the resonant frequency (which simply requires
using more wire) there doesn't appear to be any impact on the performance of the antenna.
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N8TI
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Posts: 115




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« Reply #12 on: March 16, 2012, 09:49:24 AM »

I read up on "Western Union Splices.". NASA determined that such a splice is stronger than the wire! 

I think the conclusion is (other than Hams have a weird sense of humor and should knot ever be left alone with scissors given their ability to quickly mentally regress), is that you have to check the antenna with an analyzer after you put it up.

Joe
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KJ4ANC
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« Reply #13 on: March 16, 2012, 10:39:31 AM »


You have to check the antenna with an analyzer after you put it up.

Joe

Hmm, Analyzer...   Oh yeah, I have one of those..  (mine says Radio on the front panel)

This is an important question none the less.
I have tied many a long wire. Always have been impressed with the signals my short wave can capture. I will add an analyzer to my list of hamfest search and rescue missions.       
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WB6BYU
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« Reply #14 on: March 16, 2012, 10:47:03 AM »

Quote from: N8TI

...you have to check the antenna with an analyzer after you put it up.



Or at least check the SWR using your rig. 
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