Topband: Zo of an individual CAT5 twisted pair

[Tom W8JI]

> For broadband RX antennas you want the transformer to be broadband. For
> isolation from the primary to the secondary you want low capacitance. An
> autotransformer could be used as BALUN, balances input and unbalanced
> output, it could be broadband, but has no isolation.

Technically an autotransformer can only be a balun in the case where it is a 
4:1 RATIO. An autotransformer is a single tapped winding.

A conventional transformer also is not a transmission line transformer. This 
is because to be a transformer, the conductors must have opposing currents 
and opposing voltages between the conductors. While a twisted pair excited 
as a conventional transformer has opposing currents, this occurs because one 
winding is a source and the other is a load. The polarities of a given end 
of the windings are the same, so the conductors do not act in TEM mode and 
they are NOT transmission lines.

A transmission line transformer, by definition, has to be the same 
excitation mode as a transmission line for normal function. Each conductor 
must have opposing polarity at each end between the conductors and 
everywhere along the line (like a transmission line) and opposing currents. 
In that case, TEM excitation takes place.

Without a long book reply:

Balanced lines, by system definition, are where a line has equal and 
opposite voltages from each conductor to "ground" or to space around the 
conductors and between the conductors, along with equal and opposite 
currents in each conductor.

Unbalanced lines, by system definition of a prefect line, is where the line 
still has exactly equal and opposite currents on the center and shield, but 
the voltage from the shield to the outside world around the line is zero.

An isolation transformer, or primary secondary transformer with winding 
isolation, can serve as a balun. It doesn't care if one side is ground 
referenced or not. It is a voltage independent source and load, and just 
follows what the outside world wants. While never perfect, it can be pretty 
good.

A twisted pair transformer, even in transformer mode, often has significant 
capacitance between conductors. Because of the capacitance increase, it no 
longer is independent from primary to secondary for voltages.

A twisted pair transformer fed as a transmission line becomes a normal choke 
balun, normally depending on magnetics from common mode to obtain isolation.

Even a piece of coax, electrically 1/4 wave long on the outside of the 
shield, can act like a very good narrow band balun. This is because it can 
have very high common mode impedance. This is why a dipole fed with line 
hanging in the air, and grounded between 1/8 and 3/8ths wave away (ideally 
1/4 wave, but it has some bandwidth), has a pretty good balun just by that 
cable's common mode impedance.

The entire issue is pretty complicated, but that is the short form of it. 
The main complications are the almost countless ways we use and connect 
things.

Personally, I think common sense is going  a bit backwards. We fail to have 
a gut feeling of how things work, and think the world is always one way. It 
isn't an off-on light switch for almost anything, although sometimes it is 
close enough to be treated as black or white.

73 Tom