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TopBand: More on transformers

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Subject: TopBand: More on transformers
From: (w8ji.tom)
Date: Thu, 17 Sep 1998 09:43:23 -0400
Hi Peter and all,

I'd like to clarify a few points.

> Why beverages need good transformers
> ------------------------------------
> The requirements for a beverage transformer are less onerous than for a
> pennant or flag, because the antenna already operates in the unbalanced
> mode. Therefore there is no other mode of operation to suppress, in which
> the antenna might receive signals from all directions, and fill in the
> pattern nulls.

It would be better if we pictured the antenna differently. 

That is true ONLY IF we assume the Beverage has a PERFECT ground, but
virtually all Beverages (and some verticals, especially those with small
elevated systems ) have less than perfect grounds.

A few six foot rods at the feedpoint of a Beverage do not present a zero
ohm impedance, and so the antenna is less than perfectly UNbalanced. It is,
like most antennas,  an antenna in limbo. It is somewhere in the twilight
between balanced and unbalanced. Although much more unbalanced than
balanced, it
is far from perfect since we usually depend on a very marginal ground.   

As such, it is susceptible to common mode ingress from the feedline or any
other conductor attached directly or indirectly to the ground system. That
includes unwanted signals that radiate in, as well as noise.

With multiple Beverages to one switch box, that can be a particularly bad
problem requiring all antennas to be isolated to separate grounds, or
installation of a VERY good ground at the common point.

While you won't always see a difference, an ounce of prevention is worth it
with low noise antennas.

> I believe baluns and clip-on ferrites are ineffective simply because so
> much inductance is needed at low frequencies. If one is unlucky, they
> even turn the coax into a resonant length, making the problem worse. What
> is needed is complete isolation between the antenna system and the
> coax/shack system, and a transformer with a separate primary and
> is ideal. To maintain this isolation, as Larry says it is essential to
> ground the outer conductor of the coax at the antenna, but leave it

Common mode impedance of a cable laying on the ground is pretty low,
generally well under a few hundred ohms. Resonance effects (standing waves
the outside of the cable) are all but gone after a few hundred feet. I
doubt you could add any reactance that would substantially increase common
mode coupling, once the cable is laid along or near ground any distance.
I recommend is adding a series common mode choke using a very high ui
material, making the choke more like a large resistor than an inductor. I
add a ground rod at ONE end of the choke, to provide a shunt impedance. 

This will do a lot of good if you have an ingress or common mode problem,
especially since the
cable can radiate unwanted signals directly through the air and into the
antenna despite the fact you obtain perfect feedpoint shield decoupling.
That was a point I tried to make with the Flag and Pennant, coupling will
occur from induction and radiation fields even if perfect transformer
isolation is obtained. Just because we call it a feedline, it doesn't mean
it doesn't act like an antenna, one plate of a capacitor, or an inductor
coupling through space to the antenna. 
By the way, as Peter points out these problems exist with transmitting
antennas. While I agree with him that most antennas would benefit from
common mode isolation (especially elevated radial verticals). 

I disagree with the need for link coupling. 

In the real world, the feedline is virtually always common mode coupled to
other lossy "stuff" that saps energy from the outside of the feedline. That
coupling makes it very difficult to resonate (and de-resonate) the feedline
no matter what you do with length or distributed reactances. A common mode
choke from ferrites is quite effective, especially if you select materials
that have high impedance (high ui cores). Since a feedline laying on, in,
or near earth is pretty much non-resonant, any large series reactance helps
a lot. 

For example, ground isolating the feedpoint of a four elevated radial
vertical made one dB measured FS improvement here (in the far field) over a
base connection directly to grounded coax! Of course with 30 or more
radials, the isolation made no difference at all. 


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