TopBand: Transformer questions

[w8ji.tom]

I've received about two dozen e-mails from people on the reflector asking
the same basic questions. Unfortunately, there isn't a good book that
describes how to select cores and what correct winding techniques and turns
are. I'm writing a short article on this subject, but here are some brief
notes. 

Anyone with an antenna analyzer or bridge can check out their own cores to
see how many turns to use.  All you need to do is wind a test winding and
measure R, X and Z. 

Here are the technical reasons why one core should be selected over
another.
In this application (low power receiving):

1.) The best core for a broadband transmission line transformer is
generally one with
the highest Z per turn, as long as loss tangent is within acceptable limits
for that core dimension and winding style. 

Loss tangent causes the winding's impedance to look like a resistance, ui
causes it to look like a reactance. Both curves are given in core
manufacturer's ferrite data, and they vary with frequency, temperature, and
flux levels. Avoid dc or ac current through high permeability core
windings, because current magnetizes the core and reduces ui. I've seen
cases where, with very high ui cores, ground loop currents between ground
systems at each end of an antenna, or control voltages for relays,
modulates signals. A reversible loop a friend built had that problem in
it's remote relay scheme, he ran current through the core winding of a high
dc ui core.     

2.) Permeability and loss varies with frequency. An advertised ui of XX at
dc is
NOT necessarily ui of the same XX at other frequencies. ui generally stays
about the same to a certain frequency, and then rapidly drops as loss
tangent
rapidly increases.

3.) Loss tangent is less important when the windings couple via
transmission line modes. If windings couple that way, a Faraday shield is a
waste of time. You might call it a Faraday shield, but it won't be one. (A
shielded wire wound in a transformer, a shield over a loop antenna, or over
a single wire hanging in the air actually becomes the antenna or winding.
The effect is almost like the shield isn't there at all, except the shield
mostly changes balance. That's why you can receive on a piece of coax if
the shield isn't properly connected at BOTH ends.)

4.) Core dimensions are just as important as ui in determining impedance at
the operating frequency.
No one can say how many turns are needed based only on an "X" material
core, because
it depends on winding style and core style. 

You need to measure a test winding, and shoot for a minimum "Z" several
times (four or so is often enough) source or load impedance. If the low Z
winding is correct, that will also take care of all the other windings.

73 material with a dc ui of 2000 (and 43 material) has nearly TWICE the
impedance for a given number of turns as dc 10,000 ui core when cores are
identical in dimensions at 2 MHz. More of the impedance is reactance than
resistance when compared to other cores.

5.) If the windings are separated, and the coupling mode is magnetic
instead of transmission line mode, loss tangent becomes important because
the magnetic path through the core can become so lossy the transformer
loses mutual coupling. Minimize loss tangent, or a transformer that truly
couples via the core will have very high loss!

6.) Impedance ratio goes up by the square of the turns ratio IF loss
tangent is reasonable for the winding style. (Bifilar or other transmission
line transformers or transformers that have the windings all paced together
tightly are much less dependent on core effects. Even audio transformers
are wound with interleaved or parallel wire windings, to make the core less
of a factor in design.)

7.) Common mode coupling is almost always a concern, because it lets the
feedline act like part of the antenna. To minimize common mode coupling:

a.) Use windings with small dimensions that are parallel and isolate the
primary and secondary with an intentional air gap so only the core couples
energy. Using a shielded wire for one winding will NOT work at all, because
the shield acts like the winding a and couples via capacitance to the other
winding. If you use a shield, the windings must be separated and a small
low impedance shield placed between the two windings. This is sometimes
done in directional couplers by using a single short eyelet between
multiple turn windings.

b.) Another way to isolate windings is with a common mode choke or a series
of common mode chokes (called current baluns or choke baluns) after the
transformation has taken place. If the antenna has a high common mode
impedance (like a small loop) balancing voltages is easier than choking or
isolating via shields. That's what the shield over a loop really does, it
acts as the antenna and allows a ground at the exact electrical middle of
the loop. The winding inside can be unbalanced, because the real loop
antenna, the shield, is forced into balance by a common point ground. (I've
designed loops for military and medical applications where balance was a
major issue, contrary to what is published the shield over a loop does NOT
prevent E-field pickup.)

I think balance is the toughest problem to cure for any form of loop or
stub vertical that is ground independent or has a poor ground. 

73 Tom

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