REFLECTOR: ELT

[Sid Knox]

----- Original Message ----- 
From: <MMurp16900 at aol.com>
To: <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


-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.tvbf.org/mailman/private/reflector/attachments/20060201/dc325bc3/attachment.htm