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Re: [TowerTalk] Dipole center insulator strain releif

To: Bob K6UJ <>
Subject: Re: [TowerTalk] Dipole center insulator strain releif
From: K8RI <>
Date: Sun, 22 Apr 2012 02:41:03 -0400
List-post: <">>
On 4/21/2012 11:03 PM, Bob K6UJ wrote:
> Roger,
> I like your design,  I have made a similar arrangement, including the #31 
> cores but am not that satisfied with my
> scheme.  I let the RF choke hang below unsupported.

In the link The 
Plexiglass, which is two 1/4" thick pieces epoxied together, supports 
the coax which comes though a hole on one side of the ceramic insulator, 
curves around and goes through the plexiglass again where it comes out 
on the opposite side of the ceramic insulator.  The ceramic insulator 
takes all of the tension of the antenna and also supports the weight of 
the plexiglass which supports the coax. So although the current balun is 
supported by the coax it's done so in such a manner that no stress is 
put on any coax connectors. shows the 
3.5' spreaders at the ends of the antenna.  I need to shorten the 
antenna by several feet and make the legs about a half a foot difference 
in length.  I'm guessing at the changes needed for dimensions.  The 
antenna was originally cut to formula minus about 3' over all and was 
still resonant about 30-40 KHz below the bottom edge of the band. After 
cutting off about 4 feet it's still resonant at the bottom end of the 
band.  I may try tying the ends together across the spreaders to see if 
that makes any difference in bandwidth.  The next step is to add a 
second wire to the 160 half sloper with a spreader at the far end 
(Opposite the feed point) as well.

>     If you can do a picture that would be great.
The sun, as you can see by the shadows and the left side being brightly 
lit, was in the wrong place for me to get a shot of the other side of 
the center insulator, but it's supposed to be clear tomorrow so if I can 
get out there before noon I might be able to get a shot of the long as the sun isn't directly behind it.  The antenna 
runs about 240 to 250 degrees from the tower so it's only a bit South of 

BTW those photos were shot very late in the afternoon, yesterday.

One more thing.  The top end of the antenna shown with the spreader it 
held up with 5/16" double braid line through a pulley near the top of 
the tower. (Almost impossible to see)  The 160 half sloper can be seen 
if you look closely. It runs to the right ( Straight South) and is just 
above the Phillystran guy.  The feed point looks like a tiny white dot 
at the end of the top cross brace on the right side of the tower.

> Do you know what type of plastic the white cutting boards are ?

High density Polyethylene.  It makes good thrust bearings when multiple 
layers are used, but it's pretty soft for insulators used in wire under 
tension.  OTOH the way I use Plexiglass or Lexan it's not under tension 
I much prefer either Plexiglass  or Lexan which are both available at 
the local hardware store.  Unfortunately the maximum thickness they 
carry is only 1/4 inch.  They used to carry a small quantity of 1/2 inch 
but there is very little call for it.  My preference is Lexan, but the 
stuff is pricey.  They used to give me pieces of Plexiglass the size of 
the center insulator, but they even charge for that now drays  I epoxy 
two sheets together, under pressure which works very well, BUT epoxy 
does not hold up well under UV.  It'll be interesting to see how long 
the bond lasts.

>    They sure look like teflon although I'm sure they are not, hihi.

Teflon is far cheaper than it used to be, but it's no where near that 
> I am thinking of getting one and trying it for a dipole center support panel 
> like we are discussing.  I guess the first thing to do is to see if they
> are immune to RF.  Putting one in the microwave for a little bit to see it 
> gets warm might be a good test.  The ones I have seen are about 1/2" thick,
> and they sure look like they are begging to be used on antennas, hihi

As to the links, there are also hot links from the two small photos at 
the bottom of "My ham History"


Roger (K8RI)
> Bob
> K6UJ
> On Apr 21, 2012, at 5:18 PM, K8RI wrote:
>> On 4/21/2012 6:17 PM, Dick wrote:
>>> I just got a rope over a tree after 4 years of trying. I need a dipole 
>>> center insulator that has strain relief for the coax. To clear the 
>>> branches, the coax will be pulling against the center support rope. All the 
>>> center insulators I've seen have an SO-239, it doesn't seem like putting 
>>> that strain on a PL-259 is a good idea... Do any have a strain relief??
>> I make my own out of 1/2" Plexiglass, or two 1/4" sheets laminated
>> together with epoxy.  I drill a hole in the center slightly larger than
>> the coax and round the edges. Typically I make a RF Choke of 5 or 6 2.4"
>> # 31 cores with 5 or 6 turns of BuryFlex(TM) coax. I typically make the
>> choke part of the feed line so no PL-259s are under tension.  It takes
>> very little (relatively speaking) to pull the coax out of one even if
>> it's thoroughly soldered. I will often put a connector on the antenna
>> side of the choke and run a coax pigtail from it to aid in water
>> proofing.  To take up the antenna strain I use a 4 or 6" ceramic
>> insulator  TieWraped to the Plexiglass. I drill holes just large enough
>> for the tiewraps.  Remember that nylon does not stand up well to UV or
>> out in the elements nor do cheap cable ties.
>> So I end up with the coax going through the Plexiglass, then the choke
>> on the other side and finally to the antenna ends at the ends of the
>> ceramic insulator.  In some cases I epoxy the coax in the hole through
>> the Plexiglass.  I have some of these supporting about a 100' of
>> BuryFlex(TM) back to the tower while the antenna may be under between
>> 100 and 200# of tension such as the 75 meter fan dipole.
>> I hope to have a photo up yet this evening.
>> 73
>> Roger (K8RI)
>>> Thank,
>>> Dick NY1E
>>> _______________________________________________
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