steel shed and balanced feed

[David Giles]

Hello All,

Here's another question for the antenna gurus.  I would like to use a balanced
feeder feeder for a multiband dipole.  The problem is that my shack is in a
steel shed and we all know running a balanced feedline near metal objects is
a no-no.  So, is there a way of getting a balanced feedline from my antenna
to my balanced antenna tuner through steel?  Also, my antenna is away from
the shed so the feedline would not approach the shed at a perpendicular
angle.

73 de David VK5DG
K2 S/N 5492
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[Don Wilhelm-3]

David,

The old timers used to use feedthru insulators to make a balanced line
antenna connection thru walls - I don't know where to obtain those old large
ceramic feedthrus anymore, but you can substitute with some other type of
insulator - space your improvised feedthrus 3 to 4 inches apart, use a brass
threaded rod for the conducting and fastening part and put nuts on both
sides - your feedline connects on the outside to the threaded rods and on
the inside, you run from the threaded rod  to the tuner with another length
of feedline.

The other alternative is to go through the wall with shielded balanced line.
You make that from two paralleled lengths of coax - the two center
conductors connect to the balanced feedline and you ground the shields.  Use
low loss coax because the losses due to high SWR are still present, and if
running high power, be certain the voltage rating of the coax is adequate
for the highest RF voltage you will encounter on your balanced feedline.

As an additional note - you can cross a metallic boundary with parallel
feedline with no problem to the RF as long as it occurs at right angles to
the feedline - again be careful that you have adequate insulation where you
cross, the RF voltages can be quite high even with low or moderate power if
the feedline has a high SWR.

73,
Don W3FPR

> -----Original Message-----
>
> Here's another question for the antenna gurus.  I would like to
> use a balanced
> feeder feeder for a multiband dipole.  The problem is that my
> shack is in a
> steel shed and we all know running a balanced feedline near metal
> objects is
> a no-no.  So, is there a way of getting a balanced feedline from
> my antenna
> to my balanced antenna tuner through steel?  Also, my antenna is
> away from
> the shed so the feedline would not approach the shed at a perpendicular
> angle.
>
> 73 de David VK5DG
> K2 S/N 5492
>

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[Chris Kantarjiev K6DBG]

>  So, is there a way of getting a balanced feedline from my antenna
> to my balanced antenna tuner through steel?

I'm about to try something I've heard about, but never done myself: use
parallel lengths of coax with the shields connected but not grounded.
This is supposed to provide 'safe passage' for a balanced signal.

What I don't know about this is how much, if any, of an impedance
bump it causes...

73 de chris K6DBG
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[n6wg]

David
You didn't mention what kind of openings, if any, already exist in
the shed walls.

Two things you can do.
1.  Put a gradual twist in the feedline, say one revolution every
six feet or so.  This evens out the unbalancing due to the nearness
of the metal shed.

2.  Where you do find an opening to pass the line through, come in
directly at right angles to the metal so that the line is only exposed to
the edge of the metal as it passes through.  I think I read somewhere
that the opening should be about three times the width of the feedline,
and the feedline should be centered in it.

I live in a house with aluminum siding.  In the garage, over my station
table,
I put a 2 inch diameter hole in the wall and covered it with an outdoor
plastic entrance box.  It has a hole in the bottom for feed lines to come
through, while keeping rain out.  Absolutely no problems so far, after
five years.

Good luck and 73
Bob N6WG
The Little Station with Attitude


----- Original Message -----
From: "David" <[hidden email]>
To: <[hidden email]>
Sent: Monday, February 19, 2007 10:01 PM
Subject: [Elecraft] steel shed and balanced feed


> Hello All,
>
> Here's another question for the antenna gurus.  I would like to use a
balanced
> feeder feeder for a multiband dipole.  The problem is that my shack is in
a
> steel shed and we all know running a balanced feedline near metal objects
is
> a no-no.  So, is there a way of getting a balanced feedline from my
antenna

> to my balanced antenna tuner through steel?  Also, my antenna is away from
> the shed so the feedline would not approach the shed at a perpendicular
> angle.
>
> 73 de David VK5DG
> K2 S/N 5492
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

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[Stephen W. Kercel]

Don and all:

An assortment of ceramic feedthrus are still available at Surplus
Sales of Nebraska.

(Note: I have no financial interest in the company. I'm merely a
happy customer.)

73,

Steve Kercel
AA4AK


At 09:06 AM 2/20/2007, Don Wilhelm wrote:
>David,
>
>The old timers used to use feedthru insulators to make a balanced line
>antenna connection thru walls - I don't know where to obtain those old large
>ceramic feedthrus anymore, ...


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[Dave, G4AON]

David

I have an upstairs shack with a balanced feeder needing to pass through
a brick wall into the shack. I have a balun mounted on the outside wall
of the shack, hole drilled in the wall to take the feeder and a metre of
feeder to the K2. I use a 4:1 voltage balun in a plastic "project" box,
home wound and connected to the balanced feeder with brass nuts and
bolts. By using a 4:1 voltage balun (not ideal as a balun...) it has the
advantage of being able to provide a DC ground for both halves of the
doublet.

Maybe your antenna/shack configuration doesn't lend itself to this idea,
but if it does it makes life very simple.

73 Dave, G4AON
K1 #1154, K2 #1892
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[Don Wilhelm-3]

Thanks Dave, your comment reminded me of L B Cebik's recommendation - which
is to run coax through the shack wall and place a 1:1 current balun on the
outside to feed the balanced line.

73,
Don W3FPR

> -----Original Message-----
> I have an upstairs shack with a balanced feeder needing to pass through
> a brick wall into the shack. I have a balun mounted on the outside wall
> of the shack, hole drilled in the wall to take the feeder and a metre of
> feeder to the K2. I use a 4:1 voltage balun in a plastic "project" box,
> home wound and connected to the balanced feeder with brass nuts and
> bolts. By using a 4:1 voltage balun (not ideal as a balun...) it has the
> advantage of being able to provide a DC ground for both halves of the
> doublet.
>
> Maybe your antenna/shack configuration doesn't lend itself to this idea,
> but if it does it makes life very simple.
>
> 73 Dave, G4AON
> K1 #1154, K2 #1892
>

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