Topband: Beverage Transformers

[Tom Rauch]

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