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Author Topic: Why are insulators used at wire antenna ends?  (Read 7277 times)
AF8F
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Posts: 32




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« on: January 09, 2012, 07:36:32 PM »

Wire antennas are usually supported by plastic ropes or stapled to wood, which are not conductive.  So why do people use plastic (or other non-conductive) insulators at the ends?

Is it because some people are stupid enough to use cotton rope that might get wet?

Is it because sparks tend to fly off the end of antenna wires and catch things on fire?

Is it because parts suppliers want to sell insulators?

I don't understand the need for end insulators.  Does anyone know?

Thanks for your help.
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N4JTE
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« Reply #1 on: January 09, 2012, 07:49:50 PM »

First of all, don't know of anyone who staples wire antenna ends to anything, but there is very high voltage at the ends of most wire antennas, best to seperate from nearby supports.
Bob
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K0ZN
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« Reply #2 on: January 09, 2012, 08:57:13 PM »

That is kind of a surprising question from someone who holds an Amateur Extra Class license.....??

As you should know/be aware of, the ends of a dipole and most types of wire antennas are high voltage points, even with 100 watts. Even loop antennas
have very high voltage points on them. At legal limit, the voltages go into the thousands of volts. At those voltages, leakage can be significant and that
can have several negative effects such as reduced Q, greater R losses, unstable feedpoint impedance, etc. A good end insulator is a cheap and very necessary
part of a wire antenna. There can also be mechanical advantages too that make the wire termination more reliable by reduced stress on the wire.

Obviously, there are a lot of materials/items that can be used for an end insulator. If you use nylon rope, etc. it collects dirt from the atmosphere and when wet can be fairly conductive at high voltage. Insulator losses in the end can reduce radiation efficiency, detune the system, etc.  The main need for good insulators is driven by the effects of rain, ice and snow.

Dry wood is a modestly good insulator, but wet wood is not.  There are documented cases of antennas touching wood and causing fires. You NEVER want an antenna
in contact with wood if at all possible, especially at high power.

Bottomline:  an antenna must be very well insulated to reduce R losses.  The engineers that designed all those broadcast, commercial HF, marine and military HF antenna systems were not just trying to run up the bill by installing insulators in the system. They put them there for a REASON: they are needed and work.

73, K0ZN
« Last Edit: January 09, 2012, 09:08:39 PM by K0ZN » Logged
WB6BYU
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Posts: 13225




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« Reply #3 on: January 09, 2012, 10:03:15 PM »

Insulators are traditional.  They look interesting, especially the old ceramic or glass ones.
If you were building a large wire antenna array in the days before Kevlar and Phillystran,
you didn't have much choice but to use steel cables to string it up.  So, of course, you
needed to put insulators between the wires - sometimes several of them, with special
cones to lower the voltage gradient across each.  Especially for 10kW or 100kW of AM.

Do you need them in a modern station with synthetic ropes?  There may be cases
when you do if you are running maximum legal power (or more), especially into a
shortened antenna.  Some commercial antennas have been known to set fire to their
end insulators in that situation.  (Voltages are higher at the end of a shortened antenna
than a full-sized one, unless the efficiency is much lower.)

I have lots of insulators around of various types, both plastic and ceramic, compression
and "dog bone" style.  Even some nice glass ones.  But I rarely use them on the ends
of dipoles or other wire antennas, especially since I only run 100W maximum.  Usually
I'm tying the ends off with baling twine or nylon rope:  why bother with 2" or 4" of
additional plastic insulator when I am already using 20' of a similar material?  I just
tie a loop in the end of my antenna wire (generally stranded, insulated hookup wire)
and tie the support rope to it.  Easy to do, easy to undo later. 

I suppose I should point out that all my antennas are temporary - even the one that
stayed up 6 years. I'm always taking them down and trying something else.  The hookup
wire lasts about 2 years outdoors before the insulation cracks from the sunlight, and then
the wire breaks because the insulation provides much of the strength. If I did happen to
set a rope on fire it would be just another occasion to go out and fix it - no big deal.

But if you are going to run an amp and aren't in a position to run out and fix the antenna
when it happens to catch fire, then a proper insulator might be worthwhile insurance.
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KA8OCN
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« Reply #4 on: January 09, 2012, 11:52:43 PM »

I think that was a great question, I usually use them (homemade out of PVC pipe scraps) but my 75 meter antenna at this time does not have them.

My plastic support poles broke in the high wind and without the plastic support I had to quit using my windom. until I have time to put up a telephone pole to hold it up. So I took a old 40 meter dipole from a past field day and all I had was some 6 conductor phone wire, I added some phone wire on my 40 meter wire and tied it up almost 10 full feet off the ground and 5 ft at the ends. I just looped the phone wire on the ends and tied some nylon cord on to that. The crazy part is Its working GREAT not doing much DX but I rag chew with the local group almost every night.

The moral of the story is any antenna is better that no antenna so just put something up and have a ball.

Do not let anyone give you a hard time about being a extra and asking what they think is a dumb question. I have been a Amateur Radio operator for 30 years and I am a extra but I ask lots of questions.

I dont always use it but I have a AL-1200 Amp and when I use it I normally run 1000 watts and the only time I every had a problem with antennas that could be dangerous (except having them fall on me) is when I had to move my vertical at my cabin because of construction and I did it in a hurry and only had some electrical tape to seal the PL connector, I forgot to bring some better sealer up to the cabin, it went a entire year and got some moisture into the PL and shorted the PL connector, it BURNT the center conductor on LMR400 WOW. MY tuner detected the problem and shut the amp down with no damage to the amp at all! The Palstar AT-AUTO tuner paid for itself that night!

My wire antennas always have insulation on the wire, some say its good some say its bad I seem to have the best luck with the insulated wire.
 
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WX7G
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« Reply #5 on: January 10, 2012, 12:12:48 AM »

I don't use insulators, I just tie a poly rope to the wire.

I suspect insulators are left over from the days when antennas were built like so: Antenna wire, insulator, support wire.

As to the risk of corona at the wire ends burning the poly rope, I don't find many ceramic insulators for sale, just plastic such as MFJ sells and that can certainly burn. 
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VE7RF
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Posts: 212




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« Reply #6 on: January 11, 2012, 01:50:26 AM »

I don't use insulators, I just tie a poly rope to the wire.

I suspect insulators are left over from the days when antennas were built like so: Antenna wire, insulator, support wire.

As to the risk of corona at the wire ends burning the poly rope, I don't find many ceramic insulators for sale, just plastic such as MFJ sells and that can certainly burn. 

##  It's not the corona [which is just stripping electrons out of the air] it's the HV you have to worry about.  You can solve the corona problem just by looping the wire back onto itself, like u would with any egg insulator.  Even dacron rope, soaking wet,used at HV points, with enough power into the ant, will create problems.  So egg's  and also the better end insulators are used. The ones with the serrations on the sides of the aprx 7" long  x 1.25"  diam types, like made by Hy-gain,etc.

Later... Jim  VE7RF 
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K1CJS
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« Reply #7 on: January 11, 2012, 05:16:53 AM »

Don't believe for a second that insulators are simply 'traditional'.  They're necessary--for the reasons pointed out already.  Keep in mind that kevlar or poly rope is also an insulator, but an insulator that absorbs water--and can therefore become a conductor, lowering antenna efficiency.  I've yet to see a ceramic or plastic egg type insulator that can cause that.
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AK4KZ
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« Reply #8 on: January 11, 2012, 05:54:48 AM »

First of all, don't know of anyone who staples wire antenna ends to anything, but there is very high voltage at the ends of most wire antennas, best to seperate from nearby supports.
Bob

Ha.. actually, my dipole's stapled along the length of it to a wooden fence. Only 100 watts (usually about 70) though so no fires to date.

When I do hang it up though, which is coming soon, I hope to get different color insulators so I can talk further.  Wink

73,
Chris
AK4KZ
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N3OX
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« Reply #9 on: January 11, 2012, 07:23:07 AM »

Ha.. actually, my dipole's stapled along the length of it to a wooden fence. Only 100 watts (usually about 70) though

And also a fair amount of power being used to heat the soil and some amount going to heat the fence, unless your fence is exceptionally tall and absolutely bone dry... pressure treated wood may be more of a conductor/more lossy dielectric than ordinary wood too. 

If the antenna is in a somewhat lossy environment that reduces the strength of the electric field available to pile electrons up on the ends of the dipole.  Probably doesn't really matter, and I've never arced or burned anything with 100W.  But that doesn't mean there aren't losses in all kinds of situations where you don't put an excellent dielectric in the strongest electric field region of an antenna.

Here's some model pictures of the electric field around a short loaded dipole :

http://n3ox.net/files/coil_80_fields.jpg

(on 40m it's not resonant... it's not actually a dual band resonant antenna in the model configuration)

If you have wet rope in that dark red region, it's going to have more effect than if you have ceramic or smooth plastic.  Fires are easy to notice, but loss isn't.  Is wet rope, even synthetic rope, significant in terms of loss?  Only way to really know for sure is to set up a field strength measurement setup and measure the difference.  I very much doubt it's worth worrying about but I've never seen any real data. 

The fact that things light on fire is not totally irrelevant to this question, but once you've struck an arc, it doesn't take much power can be used to sustain it and burn stuff. 

I do think this is the kind of thing that we can talk about for a million years and still not really know the answer until someone tries several ropes and a couple kind of end insulators while doing FS measurements Cheesy
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AK4KZ
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« Reply #10 on: January 11, 2012, 07:58:28 AM »

Yeah, no question about it, Dan. I don't think I could have a much worse setup unless I actually laid my dipole on the ground. (Although.. I did do that for a day or two before I put it all the way up those 8 ft.). It's an HOA neighborhood so I can't get away with much. And.. it's a temporary one so I didn't want to spend a lot of money on a stealthier antenna.

But.. it's enough to get me on the air and any improvement will be just that. Sometime in the next week or two, we'll be signing on a house. No HOA and an adequate backyard. Let the games begin! Of course, now I'm going to have to think about lightening protection.. something I didn't have to worry about TOO much when everything else in the neighborhood was taller than my antenna.

But, at least now, Bob can't say he never heard of anyone who stapled wire antenna ends to anything. At the moment... I'm that guy.  Grin

73,
Chris
AK4KZ
« Last Edit: January 11, 2012, 08:10:35 AM by AK4KZ » Logged
W8JI
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« Reply #11 on: January 11, 2012, 08:06:11 AM »

I've melted nylon ropes with 1500 watts on wire antennas, when the rope gets damp.

I use insulators to avoid having the ends of the antenna fall down.

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N3QE
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« Reply #12 on: January 11, 2012, 08:20:17 AM »

Although there are electrical reasons for the insulators, there are also very good mechanical considerations as well to use them in anything except the most temporary installations.

Stapling may work for some temporary situations with lightweight wire and low powers (I've done it myself!), but it ain't gonna cut it for a 130 foot doublet with 20-40 pounds of tension on the ends.

Wires and ropes don't like being bent at tight angles. Such bends guarantee failure under flexing in wind and rain and ice. Insulators serve as mechanical tie-off points that (when correctly used see next paragraph) result in much longer mechanical life.

Look in an ARRL handbook from anywhere in the last century for good advice on how to terminate wires and ropes onto insulators, in ways that don't concentrate all the stress and result in flexural breakage of wires.

In antenna installation, all but the most trivial wire antennas are built on the ground and lifted into the air via the support ropes. The insulators at the end is a very good demarcation between antenna itself and support system. Yes in some cases, for economy and lightweight and temporary usage, you might have an antenna without such a demarcation and actually have wires stapled to buildings or trees. But that's hardly desirable.
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KC9TNH
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« Reply #13 on: January 11, 2012, 09:34:50 AM »

I've melted nylon ropes with 1500 watts on wire antennas, when the rope gets damp.

I use insulators to avoid having the ends of the antenna fall down.
For the WIN!

For those who eschew plastic some may have success with ceramic materials at a farm supply store instead of a ham vendor. (Yes, they still sell 'em - besides they're heavy enough you can toss them over a tree limb)
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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
KB4QAA
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« Reply #14 on: January 11, 2012, 01:10:39 PM »

Bottom line:  Insulators are manly!  Especially ceramic or glass.
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