FW: [Antennas] What is Beldfoil shield?

Tue Sep 5 16:49:56 EDT 2006

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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