On 7/21/20 9:59 AM, Jim Thomson wrote:
Date: Tue, 21 Jul 2020 09:10:30 +0200
From: <dj7ww@t-online.de>
To: 'M?ximo EA1DDO_HK1H' <ea1ddo@hotmail.com>,
<towertalk@contesting.com>
Subject: Re: [TowerTalk] Balun for Force 12 6 Element 20 Meter
<I don?t think so, he is not even using feed through insulators.
<73
<Peter
## speaking of feed through insulators, who makes em, that have a .25-20 or
.25-28 brass thread through the middle ?? Looked everywhere, and cant find
any.
Or is a feed through insulator even required on a nema plastic / fiberglass
box ?
A divider wall glued or rtved into place, midway between the 2 threaded
lugs should suffice to increase V breakdown from
water, rain, debris, gunk build up. I dont see any feed through insulators
used on steve, ve6wz, nema boxes, but the outputs
are well spaced apart.
Typically one would use 3x free space breakdown voltage as the required
"creep along surface" distance.
As for what voltage should it handle? 1500W is about 400V peak at 50 ohms.
Breakdown in air is about 70kV/inch (for uniform field, etc.) - needle
gap (worst case) is significantly lower - let's take 10kV/inch.
So an inch of creepage is probably fine.
## I have tried making hb feed through insulators using 1 inch solid
teflon rod, with a .25 hole down
the middle of each rod...done in drill press. This is on the outside of a
nema box. .25-20 brass threaded rod was then silver plated,
and inserted through each hb insulator, with a 1 inch diameter rubber gasket,
between nema box and bottom of teflon rod.
2 x more hb insulators, but a lot shorter, used on inside of box. I found 3
inch long, solid brass, .25-20 bolts locally, plus mating
brass nuts, washers etc.
I think teflon is massive overkill if you're using a plastic box. I run
20kV with something similar to banana jacks on a 4x4" Carlon PVC box
with 2" spacing.
The dielectric losses are going to be small in this kind of situation.
Silicone sealant will stick nicely to PVC and metal.
I don't know that copper or brass is needed here. The short run of bolt
isn't going to be a big loss thing - regular old galvanized steel or
stainless steel bolts would probably work. However, brass and copper
have nice corrosion properties.
By the way, some brass has terrible conductivity (worse than steel and
iron). That might make the resistance a few milliohms instead of tens of
microohms <grin>
Skin depth in silver at 10Mhz is 0.6 microns - I don't know how thick
the plating is, but I think the advantage of silver is more about
corrosion - silver oxide is a better conductor than copper oxide.
(an inch of AWG10, which is 0.1 inch diameter copper, is 83 microohms -
a quarter inch bolt is about 5 times the area)
## works, but nothing sticks to teflon for water proofing. I have not tried
white delrin rod, it might succumb to UV.
I was just trying to use what I had available around the shop.
Jim VE7RF
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