Topband: Silent Feedline for RX antennas

[Tom Rauch]

> The single coax shield with twin center conductors would be excellent,
> if available.  A multitude of problems now faced with the darn
> unbalanced feedlines would be solved.  Conversion from balanced to
> unbalanced, impedance matching & further isolation, would all be
> accomplished right at the receiver with a single toroid transformer. 
> This would reduce the length of unbalanced line in the system to a few
> feet, from transformer to receiver, and eliminate the inherent receive
> problems that come with it.

Actually using fully balanced line won't fix any problem, the 
problems will remain and likely increase significantly.

The problem with our antennas and feedlines are that they are never 
completely perfectly balanced, or perfectly unbalanced.

Perfectly unbalanced requires exactly equal and opposite currents, 
with one terminal common mode impedance zero and the other terminal 
at the load impedance value. This means one terminal has zero voltage 
to the rest of the world, and the other has the full feedline voltage 
while currents flowing into and out of each terminal are equal and 
opposite. An example of this is a monopole with the feedline exiting 
below an infinite zero ohm impedance groundplane.

The standard 4-radial groundplane has considerable radiation from the 
feedline unless the feedline shield presents a very high common mode 
impedance, because the antenna is not perfectly UNbalanced with a 
poor four radial ground...even when many wavelengths above earth.     

On the other extreme perfectly balanced requires exactly equal common 
mode impedances, exactly equal (and opposite phase) terminal 
voltages, and exactly equal and opposite phase currents into each 
terminal of the feeder and antenna. Most antennas won't come close to 
that condition either.
  
The Flag or Pennant would be a good example of antennas we call 
balanced that are really in that limbo land between balanced and 
unbalanced, like the groundplane antenna. In the Flag or Pennant, one 
side of the antenna is closer to earth and other objects than the 
other side. That means the common mode impedance of each antenna 
terminal is different.

Using a shielded balanced line would most likely actually make things 
much worse than using a standard unbalanced feed system with an 
isolated secondary and primary winding in a transformer at the 
feedpoint. 

73, Tom W8JI
W8JI@contesting.com