FW: [Antennas] What is Beldfoil shield?

[David J. Ring, Jr.]

Thank you Martin, yes that's what I am thinking about.

Thanks for your additional information - it makes me think of how big a 
subject this actually could be - I could think about making a list of things 
that unbalance a line.

One thing is running a parallel feed so that one lead is always nearer to a 
surface while the other lead is further away with no surface.

Twinax will elminate this problem entirely as will running two shielded coax 
lines side by side as a "balanced feedline".

Now there is a list of things that happen between the antenna feedpoint and 
the antenna that can unbalance the feedline - the type of matching network 
(delta match that is not adjusted symetrically, etc.) and the near by 
objects in the near field of the antenna.

I remember a fellow who used a r.f. probe on a balanced feedline and 
adjusted a Tee-match (double gamma) system so that the currents were equal 
and out of phase.  I thought he was doing too much work for so little 
benefit - but since he could hear more than me and work more than me - I no 
longer make any opinion because he did something to make those wires work!

73

David Ring, N1EA

----- Original Message ----- 
From: "Martin AA6E" <aa6e at ewing.homedns.org>
To: <antennas at mailman.qth.net>
Sent: Tuesday, September 05, 2006 4:36 PM
Subject: Re: FW: [Antennas] What is Beldfoil shield?


Couple of notes:

Maybe David is thinking of "twinax"?

Shielding the feed line is usually less important that insuring that you
have a balanced antenna system.  A well balanced open wire line would
normally be better than a poorly balanced coax feed wire -- except for
noise sources very close to the line.  A balanced dipole is symmetric -
no major conductors, trees, earth, etc. near either leg.  Then it should
not matter very much whether you use coax+balun or open wire line.

The problem with an unbalanced dipole is that currents will flow on the
outside of the coax - or "common mode" on an open wire pair.  Turning
that around, a noise emitter in the environment can get coupled onto the
feed line, travel up to the antenna and back into the receiver --> noisy
reception.

(Thanks to Chuck W1HIS for this perspective.)

73, Martin AA6E

David J. Ring, Jr. wrote:
> Feedlines in order of less noise pickup:
>
> Single wire feedline
> parallel wire feedline
> coaxial feedline
> parallel conductor coaxial feedline - I think this was called "twinplex" 
> or
> something like that.
> two coaxial feedlines in parallel
>
> With the parallel feedlines one wire is fed in phase and the other is fed
> antiphase or 180 degrees out of phase.
>
> 73
>
> David Ring
> N1EA
>
> ----- Original Message ----- 
> From: "Sam Morgan" <ka5oai at cox.net>
> To: <antennas at mailman.qth.net>
> Sent: Tuesday, September 05, 2006 8:24 AM
> Subject: Re: FW: [Antennas] What is Beldfoil shield?
>
>
> Eric Lemmon wrote:
>> Sam,
>>
>> The answer is found in the glossary in the back of a Belden catalog:
>> "BELDFOIL- A Belden trademark for a highly effective electrostatic shield
>> of
>> reinforced metallic foil."  It comprises a very thin aluminum foil bonded
>> to
>> polyester tape.  When properly applied, it provides 100% shielding.  Most
>> Beldfoil-shield cables include a stranded bonding wire that is in
>> continuous
>> contact with the aluminum foil.  Due to the possibility of
>> dissimilar-metal
>> corrosion, Beldfoil-insulated cables should be used with caution in moist
>> environments.
>>
> Well I guess the use of that cable outside is not the way to go. It 
> doesn't
> rain
> much here in Texas, but sometimes it does.
>
> I have heard of running 2 coax lines in parallel to form a shielded
> feedline,
> but I guess just a balun at the dipole feedline might just take care of 
> the
> thing I had read about unbalanced feedlines being more noisy? Or would it,
> or is
> that even anything I need to address?

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