[Elecraft] PAR/LNR EFHW antenna

[Jim Brown]

On 1/8/2018 8:19 PM, K9MA wrote:
> I would be cautious about using a transformer above 40 meters, as the 
> stray capacitance may be too large.  I haven't tried it, though, so it 
> might work with some transformers.

For years, we've been winding our transformers wrong. The windings 
should be on opposite sides of the core, so that capacitive coupling is 
minimized, NOT laying on top of one another. In a ferrite materialthat 
is SUITABLE for use as a transformer, the mu is high enough that a VERY 
high fraction of the flux in  toroid is contained within the core, with 
very little leakage flux. Windings that lay on top of each other 
maximize capacitive coupling, and thus maximize common mode current. 
Windings on opposite sides of the core minimize common mode current.

Ferrites are semiconductors, so they act as a dielectric. When we place 
windings on opposite sides of a core, we maximize magnetic coupling and 
minimize capacitive coupling, which is from one winding to the core and 
from the core to the other winding. That capacitance is VERY small if 
the windings are widely spaced.

73, Jim K9YC