It is a mistake to think the only problem caused by not using
isolated windings is noise. While that probably is a common problem,
feedline common mode can also cause a loss of directivity without
adding noticeable "noise".
Using a common ground for the coax shield and the antenna is a
problem because the ground rod forms a "T" attenuator with the feed
resistances of the Beverage and the coax shield isolated only by the
ground rod resistance to "perfect earth".
A typical single rod driven several feet deep has a RF impedance of
about 100 ohms. With a Beverage resistance of 500 ohms and a coax
shield resistance of 200 ohms, a feed system with a ground resistance
of 100 ohms has less than 20dB of attenuation between anything
carried on the cable shield and the path to the INSIDE of the
feedline.
While we might not notice that effect without an A-B test, I'd wager
most of us would never *intentionally* lay a long wire parallel with
the feedline brought right into the house and couple that wire into
the feeder with a 15 to 20dB matching pad!
A more direct analogy would be we could just use a cable with about
18dB of shielding, and have nearly the same effect. Sound like a good
idea? Not to me!
As for spacing the windings or using a Faraday shield, neither are
necessary or useful. I have typically measured about 10pF or less of
capacitance with one winding laid directly over the other. That is
more than 9k-ohms of leakage reactance, and that would easily put any
common mode well into the Beverage's noise floor.
If you are worried about feedline pickup despite using an isolated
ground for the antenna, you'd be far better off to add a ground rod
several feet from the Beverage ground; grounding the feedline shield
to that separate ground while just using a winding-over-a-winding
type transformer. A conventional style transformer would have less
loss and better bandwidth.
73, Tom W8JI
W8JI@contesting.com
|