FW: [Antennas] What is Beldfoil shield?

Tue Sep 5 16:36:04 EDT 2006

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?