REFLECTOR: ELT

michalk michalk at awpi.com
Thu Feb 2 09:07:49 CST 2006


Excellent description.
Shorter answer?  The toroids strip currents common to the center and 
outside conductor.

This is at RF frequencies.

What I don't understand is how such a simple antenna can become unbalanced.

Sid Knox wrote:
> 
> ----- Original Message -----
> From: <MMurp16900 at aol.com <mailto:MMurp16900 at aol.com>>
> To: <reflector at tvbf.org <mailto:reflector at tvbf.org>>
> Sent: Wednesday, February 01, 2006 9:52 PM
> Subject: Re: REFLECTOR: ELT
> 
> 
>  > Will someone explained why ferrites are required?
>  > Mack
>  >
> The dipole is a "balanced" (symetrical) antenna. Coax cable in un-balanced.
> Connecting a coax directly to a dipole causes an upset in the impedance
> match and if used as a transmitting antenna can allow rf current to 
> "escape"
> down the outside of the coax (so-called "standing waves") which can get 
> into
> audio circuits and cause problems, etc. If used as a receiving antenna, the
> unbalance probably is of little consequence considering the generally
> wretched rf environment in which most (internally-mounted) aircraft 
> antennas
> operate. A device that allows one to properly connect a coax to a balanced
> antenna is called a balun (from BALanced-to-UNbalanced). In ancient times,
> a balun was constructed with lengths of coax connected in a non-intuitave
> manner. These baluns were un-wieldly and had a rather restricted bandwidth.
> A few years ago, someone discovered that inserting an impedance in the
> shield of the coax at the antenna, blocked these standing wave currents.
> This evolved into simply using (several) ferrite beads (toroids) at the
> antenna feed point. This method is cheap, easy, effective, and has a wide
> bandwidth.
> 
> Sid Knox
> Oklahoma
> 
> Velocity N199RS
> Starduster N666SK
> KR2 N24TC
> W7QJQ
> 
> 
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