[TowerTalk] Balun for Force 12 6 Element 20 Meter

[jimlux]

On 7/21/20 9:59 AM, Jim Thomson wrote:
> Date: Tue, 21 Jul 2020 09:10:30 +0200
> From: <dj7ww at t-online.de>
> To: 'M?ximo EA1DDO_HK1H' <ea1ddo at hotmail.com>,
> <towertalk at 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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